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C Programming Code Examples

C > Miscellaneous Code Examples

Create 1 MB test garbage file

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/* Create 1 MB test garbage file */ #include <stdio.h> #include <string.h> #include <stdlib.h> #include <unistd.h> #include <getopt.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #define PACKAGE "garfile" #define MB 1024 * 1024 void print_help(int exval); int main(int argc, char *argv[]) { int opt = 0; char *mptr; int fd = -1; int megabyte = MB; if(argc == 1) print_help(1); /* option parser */ while((opt = getopt(argc, argv, "h")) != -1) { switch(opt) { case 'h': print_help(0); case '?': /* no such option */ fprintf(stderr, "%s: Error - No such option: `%c'\n\n", PACKAGE, optopt); print_help(1); } } if((argc - optind) == 0) print_help(1); for(; optind < argc; optind++) { if((mptr = malloc(megabyte)) == NULL) { fprintf(stderr, "%s: Error - malloc(%d)\n", PACKAGE, megabyte); return 1; } memset(mptr, '\0', sizeof(mptr)); if((fd = open(argv[optind], O_RDWR|O_CREAT|O_TRUNC, 0666)) == -1) { fprintf(stderr, "%s: Error - open(%s)\n", PACKAGE, argv[optind]); return 1; } write(fd, mptr, megabyte); close(fd); } free(mptr); return 0; } /* status epilepticus, print help and exit with exval */ void print_help(int exval) { printf("%s create a test file filled with `\\0' of 1 Megabyte in size\n", PACKAGE); printf("Usage: %s [-h] FILE1 FILE2 FILE3...\n\n", PACKAGE); printf(" -h print this help and exit\n\n"); exit(exval); }
close() Function in C
Closes a file descriptor, fildes. This frees the file descriptor to be returned by future open() calls and other calls that create file descriptors. The fildes argument must represent a hierarchical file system (HFS) file. When the last open file descriptor for a file is closed, the file itself is closed. If the file's link count is 0 at that time, its space is freed and the file becomes inaccessible. When the last open file descriptor for a pipe or FIFO file is closed, any data remaining in the pipe or FIFO file is discarded. close() unlocks (removes) all outstanding record locks that a process has on the associated file.
Syntax for close() Function in C
#include <unistd.h> int close(int fildes);
fildes
The descriptor of the socket to be closed. Behavior for sockets: close() call shuts down the socket associated with the socket descriptor socket, and frees resources allocated to the socket. If socket refers to an open TCP connection, the connection is closed. If a stream socket is closed when there is input data queued, the TCP connection is reset rather than being cleanly closed. All sockets should be closed before the end of your process. You should issue a shutdown() call before you issue a close() call for a socket. For AF_INET and AF_INET6 stream sockets (SOCK_STREAM) using SO_LINGER socket option, the socket does not immediately end if data is still present when a close is issued. The following structure is used to set or unset this option, and it can be found in sys/socket.h.
struct linger { int l_onoff; /* zero=off, nonzero=on */ int l_linger; /* time is seconds to linger */ };
If the l_onoff switch is nonzero, the system attempts to deliver any unsent messages. If a linger time is specified, the system waits for n seconds before flushing the data and terminating the socket. For AF_UNIX, when closing sockets that were bound, you should also use unlink() to delete the file created at bind() time. Special behavior for XPG4.2: If a STREAMS-based fildes is closed and the calling process was previously registered to receive a SIGPOLL signal for events associated with that STREAM, the calling process will be unregistered for events associated with the STREAM. The last close() for a STREAM causes the STREAM associated with fildes to be dismantled. If O_NONBLOCK is not set and there have been no signals posted for the STREAM, and if there is data on the module's write queue, close() waits for an unspecified time (for each module and driver) for any output to drain before dismantling the STREAM. The time delay can be changed using an I_SETCLTIME ioctl() request. If the O_NONBLOCK flag is set, or if there are any pending signals, close() does not wait for output to drain, and dismantles the STREAM immediately. Note: z/OS® UNIX services do not supply any STREAMS devices or pseudodevices. See open() - Open a file for more information. If fildes refers to the master side of a pseudoterminal, a SIGHUP signal is sent to the process group, if any, for which the slave side of the pseudoterminal is the controlling terminal. If fildes refers to the slave side of a pseudoterminal, a zero-length message will be sent to the master. If fildes refers to a socket, close() causes the socket to be destroyed. If the socket is connection-oriented and the SO_LINGER option is set for the socket and the socket has untransmitted data, then close() will block for up to the current linger interval until all data is transmitted. If successful, close() returns 0. If unsuccessful, close() returns -1 and sets errno to one of the following values:
EAGAIN
The call did not complete because the specified socket descriptor is currently being used by another thread in the same process. For example, in a multithreaded environment, close() fails and returns EAGAIN when the following sequence of events occurs (1) thread is blocked in a read() or select() call on a given file or socket descriptor and (2) another thread issues a simultaneous close() call for the same descriptor.
EBADF
fildes is not a valid open file descriptor, or the socket parameter is not a valid socket descriptor.
EBUSY
The file cannot be closed because it is blocked.
EINTR
close() was interrupted by a signal. The file may or may not be closed.
EIO
Added for XPG4.2: An I/O error occurred while reading from or writing to the file system.
ENXIO
fildes does not exist. The minor number for the file is incorrect.
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/* close a file descriptor by close() function code example */ #include <fcntl.h> #include <unistd.h> #include <stdlib.h> int main( void ) { int filedes; filedes = open( "file", O_RDONLY ); if( filedes != -1 ) { /* process file */ close( filedes ); return EXIT_SUCCESS; } return EXIT_FAILURE; }
#define Directive in C
In the C Programming Language, the #define directive allows the definition of macros within your source code. These macro definitions allow constant values to be declared for use throughout your code. Macro definitions are not variables and cannot be changed by your program code like variables. You generally use this syntax when creating constants that represent numbers, strings or expressions.
Syntax for #define Directive in C
#define NAME value /* this syntax creates a constant using define*/ // Or #define NAME (expression) /* this syntax creates a constant using define*/
NAME
is the name of a particular constant. It can either be defined in smaller case or upper case or both. Most of the developers prefer the constant names to be in the upper case to find the differences.
value
defines the value of the constant.
Expression
is the value that is assigned to that constant which is defined. The expression should always be enclosed within the brackets if it has any operators. In the C programming language, the preprocessor directive acts an important role within which the #define directive is present that is used to define the constant or the micro substitution. The #define directive can use any of the basic data types present in the C standard. The #define preprocessor directive lets a programmer or a developer define the macros within the source code. This macro definition will allow the constant value that should be declared for the usage. Macro definitions cannot be changed within the program's code as one does with other variables, as macros are not variables. The #define is usually used in syntax that created a constant that is used to represent numbers, strings, or other expressions. The #define directive should not be enclosed with the semicolon(;). It is a common mistake done, and one should always treat this directive as any other header file. Enclosing it with a semicolon will generate an error. The #define creates a macro, which is in association with an identifier or is parameterized identifier along with a token string. After the macro is defined, then the compiler can substitute the token string for each occurrence of the identifier within the source file.
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/* #define directive allows the definition of macros within your source code. These macro definitions allow constant values to be declared for use throughout your code. */ #include <stdio.h> #include <string.h> typedef struct Books { char title[50]; char author[50]; char subject[100]; int book_id; } Book; int main( ) { Book book; strcpy( book.title, "C Programming"); strcpy( book.author, "XCoder"); strcpy( book.subject, "C Programming Tutorial"); book.book_id = 6495407; printf( "Book title : %s\n", book.title); printf( "Book author : %s\n", book.author); printf( "Book subject : %s\n", book.subject); printf( "Book book_id : %d\n", book.book_id); return 0; }
sizeof() Operator in C
The sizeof() operator is commonly used in C. It determines the size of the expression or the data type specified in the number of char-sized storage units. The sizeof() operator contains a single operand which can be either an expression or a data typecast where the cast is data type enclosed within parenthesis. The data type cannot only be primitive data types such as integer or floating data types, but it can also be pointer data types and compound data types such as unions and structs.
Syntax for sizeof() Operator in C
#include <stdio.h> sizeof (data type)
data type
Where data type is the desired data type including classes, structures, unions and any other user defined data type. Mainly, programs know the storage size of the primitive data types. Though the storage size of the data type is constant, it varies when implemented in different platforms. For example, we dynamically allocate the array space by using sizeof() operator:
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/* return the size of a variable by sizeof() operator example */ int main( int argc, char* argv[] ) { printf("sizeof(char) = %d\n", sizeof(char) ); printf("sizeof(short) = %d\n", sizeof(short) ); printf("sizeof(int) = %d\n", sizeof(int) ); printf("sizeof(long) = %d\n", sizeof(long) ); printf("sizeof(long long) = %d\n", sizeof(long long) ); printf("\n"); printf("sizeof(unsigned char) = %d\n", sizeof(unsigned char) ); printf("sizeof(unsigned short) = %d\n", sizeof(unsigned short) ); printf("sizeof(unsigned int) = %d\n", sizeof(unsigned int) ); printf("sizeof(unsigned long) = %d\n", sizeof(unsigned long) ); printf("\n"); printf("sizeof(float) = %d\n", sizeof(float) ); printf("sizeof(double) = %d\n", sizeof(double) ); printf("sizeof(long double) = %d\n", sizeof(long double) ); printf("\n"); int x; printf("sizeof(x) = %d\n", sizeof(x) ); }
open() Function in C
The open() function shall establish the connection between a file and a file descriptor. It shall create an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file. The open() function shall return a file descriptor for the named file that is the lowest file descriptor not currently open for that process. The open file description is new, and therefore the file descriptor shall not share it with any other process in the system. The FD_CLOEXEC file descriptor flag associated with the new file descriptor shall be cleared.
Syntax for open() Function in C
#include <fcntl.h> int open(const char *path, int oflag, ... );
path
path to file which you want to use
oflag
How you like to use The file offset used to mark the current position within the file shall be set to the beginning of the file. The file status flags and file access modes of the open file description shall be set according to the value of oflag. Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>. Applications shall specify exactly one of the first three values (file access modes) below in the value of oflag:
O_RDONLY
Open for reading only.
O_WRONLY
Open for writing only.
O_RDWR
Open for reading and writing. The result is undefined if this flag is applied to a FIFO. Any combination of the following may be used:
O_APPEND
If set, the file offset shall be set to the end of the file prior to each write.
O_CREAT
If the file exists, this flag has no effect except as noted under O_EXCL below. Otherwise, the file shall be created; the user ID of the file shall be set to the effective user ID of the process; the group ID of the file shall be set to the group ID of the file's parent directory or to the effective group ID of the process; and the access permission bits (see <sys/stat.h>) of the file mode shall be set to the value of the third argument taken as type mode_t modified as follows: a bitwise AND is performed on the file-mode bits and the corresponding bits in the complement of the process' file mode creation mask. Thus, all bits in the file mode whose corresponding bit in the file mode creation mask is set are cleared. When bits other than the file permission bits are set, the effect is unspecified. The third argument does not affect whether the file is open for reading, writing, or for both. Implementations shall provide a way to initialize the file's group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the file's group ID to the effective group ID of the calling process.
O_DSYNC
Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion.
O_EXCL
If O_CREAT and O_EXCL are set, open() shall fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executing open() naming the same filename in the same directory with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are set, and path names a symbolic link, open() shall fail and set errno to [EEXIST], regardless of the contents of the symbolic link. If O_EXCL is set and O_CREAT is not set, the result is undefined.
O_NOCTTY
If set and path identifies a terminal device, open() shall not cause the terminal device to become the controlling terminal for the process.
O_NONBLOCK
When opening a FIFO with O_RDONLY or O_WRONLY set: • If O_NONBLOCK is set, an open() for reading-only shall return without delay. An open() for writing-only shall return an error if no process currently has the file open for reading. • If O_NONBLOCK is clear, an open() for reading-only shall block the calling thread until a thread opens the file for writing. An open() for writing-only shall block the calling thread until a thread opens the file for reading. When opening a block special or character special file that supports non-blocking opens: • If O_NONBLOCK is set, the open() function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific. • If O_NONBLOCK is clear, the open() function shall block the calling thread until the device is ready or available before returning. Otherwise, the behavior of O_NONBLOCK is unspecified.
O_RSYNC
Read I/O operations on the file descriptor shall complete at the same level of integrity as specified by the O_DSYNC and O_SYNC flags. If both O_DSYNC and O_RSYNC are set in oflag, all I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If both O_SYNC and O_RSYNC are set in flags, all I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion.
O_SYNC
Write I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion.
O_TRUNC
If the file exists and is a regular file, and the file is successfully opened O_RDWR or O_WRONLY, its length shall be truncated to 0, and the mode and owner shall be unchanged. It shall have no effect on FIFO special files or terminal device files. Its effect on other file types is implementation-defined. The result of using O_TRUNC with O_RDONLY is undefined. If O_CREAT is set and the file did not previously exist, upon successful completion, open() shall mark for update the st_atime, st_ctime, and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory. If O_TRUNC is set and the file did previously exist, upon successful completion, open() shall mark for update the st_ctime and st_mtime fields of the file. If both the O_SYNC and O_DSYNC flags are set, the effect is as if only the O_SYNC flag was set. If path refers to a STREAMS file, oflag may be constructed from O_NONBLOCK OR'ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values are not applicable to STREAMS devices and shall have no effect on them. The value O_NONBLOCK affects the operation of STREAMS drivers and certain functions applied to file descriptors associated with STREAMS files. For STREAMS drivers, the implementation of O_NONBLOCK is device-specific. If path names the master side of a pseudo-terminal device, then it is unspecified whether open() locks the slave side so that it cannot be opened. Conforming applications shall call unlockpt() before opening the slave side. The largest value that can be represented correctly in an object of type off_t shall be established as the offset maximum in the open file description. Upon successful completion, the function shall open the file and return a non-negative integer representing the lowest numbered unused file descriptor. Otherwise, -1 shall be returned and errno set to indicate the error. No files shall be created or modified if the function returns -1. The open() function shall fail if:
EACCES
Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by oflag are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created, or O_TRUNC is specified and write permission is denied.
EEXIST
O_CREAT and O_EXCL are set, and the named file exists.
EINTR
A signal was caught during open().
EINVAL
The implementation does not support synchronized I/O for this file.
EIO
The path argument names a STREAMS file and a hangup or error occurred during the open().
EISDIR
The named file is a directory and oflag includes O_WRONLY or O_RDWR.
ELOOP
A loop exists in symbolic links encountered during resolution of the path argument.
EMFILE
{OPEN_MAX} file descriptors are currently open in the calling process.
ENAMETOOLONG
The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
ENFILE
The maximum allowable number of files is currently open in the system.
ENOENT
O_CREAT is not set and the named file does not exist; or O_CREAT is set and either the path prefix does not exist or the path argument points to an empty string.
ENOSR
The path argument names a STREAMS-based file and the system is unable to allocate a STREAM.
ENOSPC
The directory or file system that would contain the new file cannot be expanded, the file does not exist, and O_CREAT is specified.
ENOTDIR
A component of the path prefix is not a directory.
ENXIO
O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set, and no process has the file open for reading.
ENXIO
The named file is a character special or block special file, and the device associated with this special file does not exist.
EOVERFLOW
The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t.
EROFS
The named file resides on a read-only file system and either O_WRONLY, O_RDWR, O_CREAT (if the file does not exist), or O_TRUNC is set in the oflag argument. The open() function may fail if:
EAGAIN
The path argument names the slave side of a pseudo-terminal device that is locked.
EINVAL
The value of the oflag argument is not valid.
ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}.
ENOMEM
The path argument names a STREAMS file and the system is unable to allocate resources.
ETXTBSY
The file is a pure procedure (shared text) file that is being executed and oflag is O_WRONLY or O_RDWR.
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/* open or create a file for reading, writing or executing by open() function code example */ // C program to illustrate // open system call #include<stdio.h> #include<fcntl.h> #include<errno.h> extern int errno; int main() { // if file does not have in directory // then file foo.txt is created. int fd = open("foo.txt", O_RDONLY | O_CREAT); printf("fd = %d/n", fd); if (fd ==-1) { // print which type of error have in a code printf("Error Number % d\n", errno); // print program detail "Success or failure" perror("Program"); } return 0; }
main() Function in C
In C, the "main" function is treated the same as every function, it has a return type (and in some cases accepts inputs via parameters). The only difference is that the main function is "called" by the operating system when the user runs the program. Thus the main function is always the first code executed when a program starts. main() function is a user defined, body of the function is defined by the programmer or we can say main() is programmer/user implemented function, whose prototype is predefined in the compiler. Hence we can say that main() in c programming is user defined as well as predefined because it's prototype is predefined. main() is a system (compiler) declared function whose defined by the user, which is invoked automatically by the operating system when program is being executed. Its first function or entry point of the program from where program start executed, program's execution starts from the main. So main is an important function in c , c++ programming language.
Syntax for main() Function in C
void main() { ......... // codes start from here ......... }
void
is a keyword in C language, void means nothing, whenever we use void as a function return type then that function nothing return. here main() function no return any value. In place of void we can also use int return type of main() function, at that time main() return integer type value.
main
is a name of function which is predefined function in C library. • An operating system always calls the main() function when a programmers or users execute their programming code. • It is responsible for starting and ends of the program. • It is a universally accepted keyword in programming language and cannot change its meaning and name. • A main() function is a user-defined function in C that means we can pass parameters to the main() function according to the requirement of a program. • A main() function is used to invoke the programming code at the run time, not at the compile time of a program. • A main() function is followed by opening and closing parenthesis brackets.
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/* basic c program by main() function example */ #include <stdio.h> #include <conio.h> main() { printf (" It is a main() function "); int fun2(); // jump to void fun1() function printf ("\n Finally exit from the main() function. "); } void fun1() { printf (" It is a second function. "); printf (" Exit from the void fun1() function. "); } int fun2() { void fun1(); // jump to the int fun1() function printf (" It is a third function. "); printf (" Exit from the int fun2() function. "); return 0; }
While Loop Statement in C
While loop is also known as a pre-tested loop. In general, a while loop allows a part of the code to be executed multiple times depending upon a given boolean condition. It can be viewed as a repeating if statement. The while loop is mostly used in the case where the number of iterations is not known in advance. The while loop evaluates the test expression inside the parentheses (). If test expression is true, statements inside the body of while loop are executed. Then, test expression is evaluated again. The process goes on until test expression is evaluated to false. If test expression is false, the loop terminates.
Syntax of While Loop Statement in C
while (testExpression) { // the body of the loop }
• The while loop evaluates the testExpression inside the parentheses (). • If testExpression is true, statements inside the body of while loop are executed. Then, testExpression is evaluated again. • The process goes on until testExpression is evaluated to false. • If testExpression is false, the loop terminates (ends).
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/* while loop statement in C language */ #include<stdio.h> int main() { int n, num, sum = 0, remainder; printf("Enter a number: "); scanf("%d", &n); num = n; // keep looping while n > 0 while( n > 0 ) { remainder = n % 10; // get the last digit of n sum += remainder; // add the remainder to the sum n /= 10; // remove the last digit from n } printf("Sum of digits of %d is %d", num, sum); // signal to operating system everything works fine return 0; }
Switch Case Statement in C
Switch statement in C tests the value of a variable and compares it with multiple cases. Once the case match is found, a block of statements associated with that particular case is executed. Each case in a block of a switch has a different name/number which is referred to as an identifier. The value provided by the user is compared with all the cases inside the switch block until the match is found. If a case match is NOT found, then the default statement is executed, and the control goes out of the switch block.
Syntax for Switch Case Statement in C
switch(expression) { case constant-expression : statement(s); break; /* optional */ case constant-expression : statement(s); break; /* optional */ /* you can have any number of case statements */ default : /* Optional */ statement(s); }
• The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type. • You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon. • The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal. • When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached. • When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement. • Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached. • A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.
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/* switch case statement in C language*/ // Program to create a simple calculator #include <stdio.h> int main() { char operation; double n1, n2; printf("Enter an operator (+, -, *, /): "); scanf("%c", &operation); printf("Enter two operands: "); scanf("%lf %lf",&n1, &n2); switch(operation) { case '+': printf("%.1lf + %.1lf = %.1lf",n1, n2, n1+n2); break; case '-': printf("%.1lf - %.1lf = %.1lf",n1, n2, n1-n2); break; case '*': printf("%.1lf * %.1lf = %.1lf",n1, n2, n1*n2); break; case '/': printf("%.1lf / %.1lf = %.1lf",n1, n2, n1/n2); break; // operator doesn't match any case constant +, -, *, / default: printf("Error! operator is not correct"); } return 0; }
printf() Function in C
Writes the C string pointed by format to the standard output (stdout). If format includes format specifiers (subsequences beginning with %), the additional arguments following format are formatted and inserted in the resulting string replacing their respective specifiers. printf format string refers to a control parameter used by a class of functions in the input/output libraries of C programming language. The string is written in a simple template language: characters are usually copied literally into the function's output, but format specifiers, which start with a % character, indicate the location and method to translate a piece of data (such as a number) to characters. "printf" is the name of one of the main C output functions, and stands for "print formatted". printf format strings are complementary to scanf format strings, which provide formatted input (parsing). In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible template engines or parsers, but are sufficient for many purposes.
Syntax for printf() function in C
#include <stdio.h> int printf ( const char * format, ... );
format
C string that contains the text to be written to stdout. It can optionally contain embedded format specifiers that are replaced by the values specified in subsequent additional arguments and formatted as requested. A format specifier follows this prototype: [see compatibility note below] %[flags][width][.precision][length]specifier Where the specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:
specifier
a conversion format specifier.
d or i
Signed decimal integer
u
Unsigned decimal integer
o
Unsigned octal
x
Unsigned hexadecimal integer
X
Unsigned hexadecimal integer (uppercase)
f
Decimal floating point, lowercase
F
Decimal floating point, uppercase
e
Scientific notation (mantissa/exponent), lowercase
E
Scientific notation (mantissa/exponent), uppercase
g
Use the shortest representation: %e or %f
G
Use the shortest representation: %E or %F
a
Hexadecimal floating point, lowercase
A
Hexadecimal floating point, uppercase
c
Character
s
String of characters
p
Pointer address
n
Nothing printed. The corresponding argument must be a pointer to a signed int. The number of characters written so far is stored in the pointed location.
%
A % followed by another % character will write a single % to the stream. The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:
flags
one or more flags that modifies the conversion behavior (optional)
-
Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+
Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign.
(space)
If no sign is going to be written, a blank space is inserted before the value.
#
Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.
0
Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).
width
an optional * or integer value used to specify minimum width field.
(number)
Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*
The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
.precision
an optional field consisting of a . followed by * or integer or nothing to specify the precision.
.number
For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For a, A, e, E, f and F specifiers: this is the number of digits to be printed after the decimal point (by default, this is 6). For g and G specifiers: This is the maximum number of significant digits to be printed. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. If the period is specified without an explicit value for precision, 0 is assumed.
.*
The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
length
an optional length modifier that specifies the size of the argument.
... (additional arguments)
Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n). There should be at least as many of these arguments as the number of values specified in the format specifiers. Additional arguments are ignored by the function. If a writing error occurs, the error indicator (ferror) is set and a negative number is returned. If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.
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/* print formatted data to stdout by printf() function example */ #include <stdio.h> int main() { char ch; char str[100]; int a; float b; printf("Enter any character \n"); scanf("%c", &ch); printf("Entered character is %c \n", ch); printf("Enter any string ( upto 100 character ) \n"); scanf("%s", &str); printf("Entered string is %s \n", str); printf("Enter integer and then a float: "); // Taking multiple inputs scanf("%d%f", &a, &b); printf("You entered %d and %f", a, b); return 0; }
malloc() Function in C
Allocate memory block. Allocates a block of size bytes of memory, returning a pointer to the beginning of the block. The content of the newly allocated block of memory is not initialized, remaining with indeterminate values. If size is zero, the return value depends on the particular library implementation (it may or may not be a null pointer), but the returned pointer shall not be dereferenced. The "malloc" or "memory allocation" method in C is used to dynamically allocate a single large block of memory with the specified size. It returns a pointer of type void which can be cast into a pointer of any form. It doesn't Iniatialize memory at execution time so that it has initializes each block with the default garbage value initially.
Syntax for malloc() Function in C
#include <stdlib.h> void* malloc (size_t size);
size
Size of the memory block, in bytes. size_t is an unsigned integral type. On success, function returns a pointer to the memory block allocated by the function. The type of this pointer is always void*, which can be cast to the desired type of data pointer in order to be dereferenceable. If the function failed to allocate the requested block of memory, a null pointer is returned.
Data races
Only the storage referenced by the returned pointer is modified. No other storage locations are accessed by the call. If the function reuses the same unit of storage released by a deallocation function (such as free or realloc), the functions are synchronized in such a way that the deallocation happens entirely before the next allocation.
Exceptions
No-throw guarantee: this function never throws exceptions.
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/* allocate memory block by malloc() function example */ // Program to calculate the sum of n numbers entered by the user #include <stdio.h> #include <stdlib.h> int main() { int n, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &n); ptr = (int*) malloc(n * sizeof(int)); // if memory cannot be allocated if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); } printf("Enter elements: "); for(i = 0; i < n; ++i) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); // deallocating the memory free(ptr); return 0; }
For Loop Statement in C
The for loop is used in the case where we need to execute some part of the code until the given condition is satisfied. The for loop is also called as a per-tested loop. It is better to use for loop if the number of iteration is known in advance. The for-loop statement is a very specialized while loop, which increases the readability of a program. It is frequently used to traverse the data structures like the array and linked list.
Syntax of For Loop Statement in C
for (initialization; condition test; increment or decrement) { //Statements to be executed repeatedly }
Step 1
First initialization happens and the counter variable gets initialized.
Step 2
In the second step the condition is checked, where the counter variable is tested for the given condition, if the condition returns true then the C statements inside the body of for loop gets executed, if the condition returns false then the for loop gets terminated and the control comes out of the loop.
Step 3
After successful execution of statements inside the body of loop, the counter variable is incremented or decremented, depending on the operation (++ or --).
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/* for loop statement in C language */ // Program to calculate the sum of first n natural numbers // Positive integers 1,2,3...n are known as natural numbers #include <stdio.h> int main() { int num, count, sum = 0; printf("Enter a positive integer: "); scanf("%d", &num); // for loop terminates when num is less than count for(count = 1; count <= num; ++count) { sum += count; } printf("Sum = %d", sum); return 0; }
memset() Function in C
Fill block of memory. Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char). The C library function copies the character value (an unsigned char) to the first num characters of the string pointed to, by the argument ptr. This function returns a pointer to the memory area ptr.
Syntax for memset() Function in C
#include <string.h> void * memset ( void * ptr, int value, size_t num );
ptr
Pointer to the block of memory to fill.
value
Value to be set. The value is passed as an int, but the function fills the block of memory using the unsigned char conversion of this value.
num
Number of bytes to be set to the value. size_t is an unsigned integral type. This function returns a pointer to the memory area str. The function memset("memory setter") is a standard library function in C language that sets or fills a memory block semantically with a value.
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/* fill a block of memory with a particular value by memset() function code example */ #include <stdio.h> #include <string.h> #define LEN 10 int main(void) { char arr[LEN]; int loop; printf("Array elements are (before memset()): \n"); for(loop=0; loop<LEN; loop++) printf("%d ",arr[loop]); printf("\n"); //filling all blocks with 0 memset(arr,0,LEN); printf("Array elements are (after memset()): \n"); for(loop=0; loop<LEN; loop++) printf("%d ",arr[loop]); printf("\n"); //filling first 3 blocks with -1 //and second 3 blocks with -2 //and then 3 blocks with -3 memset(arr,-1,3); memset(arr+3,-2,3); memset(arr+6,-3,3); printf("Array elements are (after memset()): \n"); for(loop=0; loop<LEN; loop++) printf("%d ",arr[loop]); printf("\n"); return 0; }
write() Function in C
Write to a file descriptor. write() writes up to count bytes from the buffer pointed buf to the file referred to by the file descriptor fd. The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered, or the call was interrupted by a signal handler after having written less than count bytes. For a seekable file (i.e., one to which lseek may be applied, for example, a regular file) writing takes place at the current file offset, and the file offset is incremented by the number of bytes actually written. If the file was opened with O_APPEND, the file offset is first set to the end of the file before writing. The adjustment of the file offset and the write operation are performed as an atomic step.
Syntax for write() Function in C
#include <unistd.h> ssize_t write(int fd, const void *buf, size_t count);
fd
It is the file descriptor which has been obtained from the call to open. It is an integer value. The values 0, 1, 2 can also be given, for standard input, standard output & standard error, respectively .
buf
It points to a character array, which can be used to store content obtained from the file pointed to by fd.
count
It specifies the number of bytes to be written from the file into the character array. POSIX requires that a read which can be proved to occur after a write() has returned returns the new data. Note that not all file systems are POSIX conforming. On success, the number of bytes written is returned (zero indicates nothing was written). On error, -1 is returned, and errno is set appropriately. If count is zero and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If count is zero and fd refers to a file other than a regular file, the results are not specified.
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/* write to a file descriptor by write() function code example */ // C program to illustrate // I/O system Calls #include<stdio.h> #include<string.h> #include<unistd.h> #include<fcntl.h> int main (void) { int fd[2]; char buf1[12] = "hello world"; char buf2[12]; // assume foobar.txt is already created fd[0] = open("foobar.txt", O_RDWR); fd[1] = open("foobar.txt", O_RDWR); write(fd[0], buf1, strlen(buf1)); write(1, buf2, read(fd[1], buf2, 12)); close(fd[0]); close(fd[1]); return 0; }
#include Directive in C
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program. Here are the two types of file that can be included using #include: • Header File or Standard files: This is a file which contains C/C++ function declarations and macro definitions to be shared between several source files. Functions like the printf(), scanf(), cout, cin and various other input-output or other standard functions are contained within different header files. So to utilise those functions, the users need to import a few header files which define the required functions. • User-defined files: These files resembles the header files, except for the fact that they are written and defined by the user itself. This saves the user from writing a particular function multiple times. Once a user-defined file is written, it can be imported anywhere in the program using the #include preprocessor.
Syntax for #include Directive in C
#include "user-defined_file"
Including using " ": When using the double quotes(" "), the preprocessor access the current directory in which the source "header_file" is located. This type is mainly used to access any header files of the user's program or user-defined files.
#include <header_file>
Including using <>: While importing file using angular brackets(<>), the the preprocessor uses a predetermined directory path to access the file. It is mainly used to access system header files located in the standard system directories.
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/* #include directive tells the preprocessor to insert the contents of another file into the source code at the point where the #include directive is found. */ // C program to illustrate file inclusion // <> used to import system header file #include <stdio.h> // " " used to import user-defined file #include "process.h" // main function int main() { // add function defined in process.h add(10, 20); // mult function defined in process.h multiply(10, 20); // printf defined in stdio.h printf("Process completed"); return 0; }
getopt() Function in C
The getopt() function is a builtin function in C and is used to parse command line arguments. The getopt() function is a command-line parser that shall follow Utility Syntax Guidelines 3, 4, 5, 6, 7, 9, and 10 in the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines. The parameters argc and argv are the argument count and argument array as passed to main() (see exec() ). The argument optstring is a string of recognized option characters; if a character is followed by a colon, the option takes an argument. All option characters allowed by Utility Syntax Guideline 3 are allowed in optstring. The implementation may accept other characters as an extension.
Syntax for getopt() Function in C
#include <unistd.h> int getopt(int argc, char * const argv[], const char *optstring); extern char *optarg; extern int optind, opterr, optopt;
argc
The argument count that was passed to main().
argv
The argument array that was passed to main().
optstring
A string of recognized option letters; if a letter is followed by a colon, the option takes an argument. Valid option characters for optstring consist of a single alphanumeric character (i.e. a letter or digit). The variable optind is the index of the next element of the argv[] vector to be processed. It shall be initialized to 1 by the system, and getopt() shall update it when it finishes with each element of argv[]. When an element of argv[] contains multiple option characters, it is unspecified how getopt() determines which options have already been processed. The getopt() function shall return the next option character (if one is found) from argv that matches a character in optstring, if there is one that matches. If the option takes an argument, getopt() shall set the variable optarg to point to the option-argument as follows: 1- If the option was the last character in the string pointed to by an element of argv, then optarg shall contain the next element of argv, and optind shall be incremented by 2. If the resulting value of optind is greater than argc, this indicates a missing option-argument, and getopt() shall return an error indication. 2- Otherwise, optarg shall point to the string following the option character in that element of argv, and optind shall be incremented by 1. If, when getopt() is called:
argv[optind] is a null pointer *argv[optind] is not the character - argv[optind] points to the string "-"
getopt() shall return -1 without changing optind. If:
argv[optind] points to the string "--"
getopt() shall return -1 after incrementing optind. If getopt() encounters an option character that is not contained in optstring, it shall return the question-mark ( '?' ) character. If it detects a missing option-argument, it shall return the colon character ( ':' ) if the first character of optstring was a colon, or a question-mark character ( '?' ) otherwise. In either case, getopt() shall set the variable optopt to the option character that caused the error. If the application has not set the variable opterr to 0 and the first character of optstring is not a colon, getopt() shall also print a diagnostic message to stderr in the format specified for the getopts utility. The getopt() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe. The getopt() function shall return the next option character specified on the command line. A colon ( ':' ) shall be returned if getopt() detects a missing argument and the first character of optstring was a colon ( ':' ). A question mark ( '?' ) shall be returned if getopt() encounters an option character not in optstring or detects a missing argument and the first character of optstring was not a colon ( ':' ). Otherwise, getopt() shall return -1 when all command line options are parsed.
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/* parse command line arguments by getopt() function code example */ // Program to illustrate the getopt() // function in C #include <stdio.h> #include <unistd.h> int main(int argc, char *argv[]) { int opt; // put ':' in the starting of the // string so that program can //distinguish between '?' and ':' while((opt = getopt(argc, argv, ":if:lrx")) != -1) { switch(opt) { case 'i': case 'l': case 'r': printf("option: %c\n", opt); break; case 'f': printf("filename: %s\n", optarg); break; case ':': printf("option needs a value\n"); break; case '?': printf("unknown option: %c\n", optopt); break; } } // optind is for the extra arguments // which are not parsed for(; optind < argc; optind++){ printf("extra arguments: %s\n", argv[optind]); } return 0; }
exit() Function in C
The exit() function is used to terminate a process or function calling immediately in the program. It means any open file or function belonging to the process is closed immediately as the exit() function occurred in the program. The exit() function is the standard library function of the C, which is defined in the stdlib.h header file. So, we can say it is the function that forcefully terminates the current program and transfers the control to the operating system to exit the program. The exit(0) function determines the program terminates without any error message, and then the exit(1) function determines the program forcefully terminates the execution process.
Syntax for exit() Function in C
#include <stdlib.h> void exit(int status)
status
Status code. If this is 0 or EXIT_SUCCESS, it indicates success. If it is EXIT_FAILURE, it indicates failure. The exit function does not return anything. • We must include the stdlib.h header file while using the exit () function. • It is used to terminate the normal execution of the program while encountered the exit () function. • The exit () function calls the registered atexit() function in the reverse order of their registration. • We can use the exit() function to flush or clean all open stream data like read or write with unwritten buffered data. • It closed all opened files linked with a parent or another function or file and can remove all files created by the tmpfile function. • The program's behaviour is undefined if the user calls the exit function more than one time or calls the exit and quick_exit function. • The exit function is categorized into two parts: exit(0) and exit(1).
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/* call all functions registered with atexit and terminates the program by exit() function example */ #include <stdio.h> #include <stdlib.h> int main () { // declaration of the variables int i, num; printf ( " Enter the last number: "); scanf ( " %d", &num); for ( i = 1; i<num; i++) { // use if statement to check the condition if ( i == 6 ) /* use exit () statement with passing 0 argument to show termination of the program without any error message. */ exit(0); else printf (" \n Number is %d", i); } return 0; }
fprintf() Function in C
Write formatted data to stream. Writes the C string pointed by format to the stream. If format includes format specifiers (subsequences beginning with %), the additional arguments following format are formatted and inserted in the resulting string replacing their respective specifiers. After the format parameter, the function expects at least as many additional arguments as specified by format.
Syntax for fprintf() Function in C
#include <stdio.h> int fprintf ( FILE * stream, const char * format, ... );
stream
Pointer to a FILE object that identifies an output stream.
format
C string that contains the text to be written to the stream. It can optionally contain embedded format specifiers that are replaced by the values specified in subsequent additional arguments and formatted as requested. A format specifier follows this prototype: %[flags][width][.precision][length]specifier Where the specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:
specifier
a conversion format specifier.
d or i
Signed decimal integer
u
Unsigned decimal integer
o
Unsigned octal
x
Unsigned hexadecimal integer
X
Unsigned hexadecimal integer (uppercase)
f
Decimal floating point, lowercase
F
Decimal floating point, uppercase
e
Scientific notation (mantissa/exponent), lowercase
E
Scientific notation (mantissa/exponent), uppercase
g
Use the shortest representation: %e or %f
G
Use the shortest representation: %E or %F
a
Hexadecimal floating point, lowercase
A
Hexadecimal floating point, uppercase
c
Character
s
String of characters
p
Pointer address
n
Nothing printed. The corresponding argument must be a pointer to a signed int. The number of characters written so far is stored in the pointed location.
%
A % followed by another % character will write a single % to the stream. The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:
flags
one or more flags that modifies the conversion behavior (optional)
-
Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+
Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign.
(space)
If no sign is going to be written, a blank space is inserted before the value.
#
Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.
0
Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).
width
an optional * or integer value used to specify minimum width field.
(number)
Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*
The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
.precision
an optional field consisting of a . followed by * or integer or nothing to specify the precision.
.number
For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For a, A, e, E, f and F specifiers: this is the number of digits to be printed after the decimal point (by default, this is 6). For g and G specifiers: This is the maximum number of significant digits to be printed. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. If the period is specified without an explicit value for precision, 0 is assumed.
.*
The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
length
an optional length modifier that specifies the size of the argument.
h
The argument is interpreted as a short int or unsigned short int (only applies to integer specifiers: i, d, o, u, x and X).
l
The argument is interpreted as a long int or unsigned long int for integer specifiers (i, d, o, u, x and X), and as a wide character or wide character string for specifiers c and s.
L
The argument is interpreted as a long double (only applies to floating point specifiers - e, E, f, g and G).
... (additional arguments)
Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n). There should be at least as many of these arguments as the number of values specified in the format specifiers. Additional arguments are ignored by the function. On success, the total number of characters written is returned. If a writing error occurs, the error indicator (ferror) is set and a negative number is returned. If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.
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/* write the C string pointed by format to the stream by fprintf() function example */ #include <stdio.h> void main() { FILE *fptr; int id; char name[30]; float salary; fptr = fopen("emp.txt", "w+");/* open for writing */ if (fptr == NULL) { printf("File does not exists \n"); return; } printf("Enter the id\n"); scanf("%d", &id); fprintf(fptr, "Id= %d\n", id); printf("Enter the name \n"); scanf("%s", name); fprintf(fptr, "Name= %s\n", name); printf("Enter the salary\n"); scanf("%f", &salary); fprintf(fptr, "Salary= %.2f\n", salary); fclose(fptr); }
If Else Statement in C
The if-else statement is used to perform two operations for a single condition. The if-else statement is an extension to the if statement using which, we can perform two different operations, i.e., one is for the correctness of that condition, and the other is for the incorrectness of the condition. Here, we must notice that if and else block cannot be executed simiulteneously. Using if-else statement is always preferable since it always invokes an otherwise case with every if condition.
Syntax for if-else Statement in C
if (test expression) { // run code if test expression is true } else { // run code if test expression is false }
If the test expression is evaluated to true, • statements inside the body of if are executed. • statements inside the body of else are skipped from execution. If the test expression is evaluated to false, • statements inside the body of else are executed • statements inside the body of if are skipped from execution.
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/* if else statement in C language */ // Check whether an integer is odd or even #include <stdio.h> int main() { int number; printf("Enter an integer: "); scanf("%d", &number); // True if the remainder is 0 if (number%2 == 0) { printf("%d is an even integer.",number); } else { printf("%d is an odd integer.",number); } return 0; }
free() Function in C
The free() function in C library allows you to release or deallocate the memory blocks which are previously allocated by calloc(), malloc() or realloc() functions. It frees up the memory blocks and returns the memory to heap. It helps freeing the memory in your program which will be available for later use. In C, the memory for variables is automatically deallocated at compile time. For dynamic memory allocation in C, you have to deallocate the memory explicitly. If not done, you may encounter out of memory error.
Syntax for free() Function in C
#include<stdlib.h> void free(void *ptr).
ptr
This is the pointer to a memory block previously allocated with malloc, calloc or realloc to be deallocated. If a null pointer is passed as argument, no action occurs. This function does not return any value.
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/* deallocate memory block by free() function example */ #include <stdio.h> #include <stdlib.h> #include <string.h> int main () { char *str; /* Initial memory allocation */ str = (char *) malloc(15); strcpy(str, "HappyCodings"); printf("String = %s, Address = %u\n", str, str); /* Reallocating memory */ str = (char *) realloc(str, 25); strcat(str, ".com"); printf("String = %s, Address = %u\n", str, str); /* Deallocate allocated memory */ free(str); return(0); }


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