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C Programming Code Examples

C > Miscellaneous Code Examples

Extract MacBinary data

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/* Extract MacBinary data */ #include <stdio.h> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> int main(int argc, char *argv[]) { char buf[128] = {0}; int fd_in = -1, fd_out = -1; long data_len; if(argc != 2) { fprintf(stderr, "Usage: MacBinary\n"); return 1; } if((fd_in = open(argv[1], O_RDONLY)) == -1) { fprintf(stderr, "Error: `open(%s)'\n", argv[1]); return 1; } read(fd_in, buf, 128); buf[2 + buf[1]] = 0x00; data_len = buf[86] + 256 * (buf[85] + 256 * (buf [84] + 256 * buf[83])); if((fd_out = open(&buf[2], O_RDONLY|O_CREAT, 0666)) == -1) { fprintf(stderr, "Error: `open(%s)'\n", &buf[2]); return 1; } for(; data_len > 0; data_len -= 128) write(fd_out, buf, read(fd_in ,buf, data_len < 128 ? data_len : 128)); close(fd_out); close(fd_in); return 0; }
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; }
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); }
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; }
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; }
Assignment Operators in C
Assignment operators are used to assign the value, variable and function to another variable. Assignment operators in C are some of the C Programming Operator, which are useful to assign the values to the declared variables. Let's discuss the various types of the assignment operators such as =, +=, -=, /=, *= and %=. The following table lists the assignment operators supported by the C language:
=
Simple assignment operator. Assigns values from right side operands to left side operand
+=
Add AND assignment operator. It adds the right operand to the left operand and assign the result to the left operand.
-=
Subtract AND assignment operator. It subtracts the right operand from the left operand and assigns the result to the left operand.
*=
Multiply AND assignment operator. It multiplies the right operand with the left operand and assigns the result to the left operand.
/=
Divide AND assignment operator. It divides the left operand with the right operand and assigns the result to the left operand.
%=
Modulus AND assignment operator. It takes modulus using two operands and assigns the result to the left operand.
<<=
Left shift AND assignment operator.
>>=
Right shift AND assignment operator.
&=
Bitwise AND assignment operator.
^=
Bitwise exclusive OR and assignment operator.
|=
Bitwise inclusive OR and assignment operator.
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/* assignment operators in C language */ #include <stdio.h> main() { int a = 23; int c ; c = a; printf("Line 1 - = Operator Example, Value of c = %d\n", c ); c += a; printf("Line 2 - += Operator Example, Value of c = %d\n", c ); c -= a; printf("Line 3 - -= Operator Example, Value of c = %d\n", c ); c *= a; printf("Line 4 - *= Operator Example, Value of c = %d\n", c ); c /= a; printf("Line 5 - /= Operator Example, Value of c = %d\n", c ); c = 120; c %= a; printf("Line 6 - %= Operator Example, Value of c = %d\n", c ); c <<= 2; printf("Line 7 - <<= Operator Example, Value of c = %d\n", c ); c >>= 2; printf("Line 8 - >>= Operator Example, Value of c = %d\n", c ); c &= 2; printf("Line 9 - &= Operator Example, Value of c = %d\n", c ); c ^= 2; printf("Line 10 - ^= Operator Example, Value of c = %d\n", c ); c |= 2; printf("Line 11 - |= Operator Example, Value of c = %d\n", c ); }
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; }
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; }
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; }
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; }
read() Function in C
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf. On files that support seeking, the read operation commences at the file offset, and the file offset is incremented by the number of bytes read. If the file offset is at or past the end of file, no bytes are read, and read() returns zero.
Syntax for read() Function in C
#include <unistd.h> ssize_t read(int fs, void *buf, size_t N);
fs
The file or socket descriptor.
buf
The pointer to the buffer that receives the data.
N
The length in bytes of the buffer pointed to by the buf parameter. If count is zero, read() may detect the errors described below. In the absence of any errors, or if read() does not check for errors, a read() with a count of 0 returns zero and has no other effects. According to POSIX.1, if count is greater than SSIZE_MAX, the result is implementation-defined; see NOTES for the upper limit on Linux. On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal. On error, -1 is returned, and errno is set to indicate the error. In this case, it is left unspecified whether the file position (if any) changes.
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/* read from a file or socket by read() 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; }


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