C Programming Code Examples

Close file. Closes the file associated with the stream and disassociates it. All internal buffers associated with the stream are disassociated from it and flushed: the content of any unwritten output buffer is written and the content of any unread input buffer is discarded. Even if the call fails, the stream passed as parameter will no longer be associated with the file nor its buffers. Pointer to a FILE object that specifies the stream to be closed. The fclose() function shall cause the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream shall be written to the file; any unread buffered data shall be discarded. Whether or not the call succeeds, the stream shall be disassociated from the file and any buffer set by the setbuf() or setvbuf() function shall be disassociated from the stream. If the associated buffer was automatically allocated, it shall be deallocated. After the call to fclose(), any use of stream results in undefined behavior. If the stream is successfully closed, a zero value is returned. On failure, EOF is returned.

Get character from stream. Returns the character currently pointed by the internal file position indicator of the specified stream. The internal file position indicator is then advanced to the next character. If the stream is at the end-of-file when called, the function returns EOF and sets the end-of-file indicator for the stream (feof). If a read error occurs, the function returns EOF and sets the error indicator for the stream (ferror). getc and fgetc are equivalent, except that getc may be implemented as a macro in some libraries. See getchar for a similar function that reads directly from stdin. Pointer to a FILE object that identifies an input stream. Because some libraries may implement this function as a macro, and this may evaluate the stream expression more than once, this should be an expression without side effects. On success, the character read is returned (promoted to an int value). The return type is int to accommodate for the special value EOF, which indicates failure: If the position indicator was at the end-of-file, the function returns EOF and sets the eof indicator (feof) of stream. If some other reading error happens, the function also returns EOF, but sets its error indicator (ferror) instead. The getc function in C reads the next character from any input stream and returns an integer value. It is a standard function in C and can be used by including the <stdio.h> header file. The getc() function can be implemented as a macro whereas fgetc() function can not be used as macro. Also getc() function is highly optimized and hence calls to fgetc() probably take longer than calls to getc(). So, getc() is preferred in most situations.

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. 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.

Read formatted data from stdin. Reads data from stdin and stores them according to the parameter format into the locations pointed by the additional arguments. The additional arguments should point to already allocated objects of the type specified by their corresponding format specifier within the format string. In C programming, scanf() is one of the commonly used function to take input from the user. The scanf() function reads formatted input from the standard input such as keyboards. The scanf() function enables the programmer to accept formatted inputs to the application or production code. Moreover, by using this function, the users can provide dynamic input values to the application. C string that contains a sequence of characters that control how characters extracted from the stream are treated: • Whitespace character: the function will read and ignore any whitespace characters encountered before the next non-whitespace character (whitespace characters include spaces, newline and tab characters -- see isspace). A single whitespace in the format string validates any quantity of whitespace characters extracted from the stream (including none). • Non-whitespace character, except format specifier (%): Any character that is not either a whitespace character (blank, newline or tab) or part of a format specifier (which begin with a % character) causes the function to read the next character from the stream, compare it to this non-whitespace character and if it matches, it is discarded and the function continues with the next character of format. If the character does not match, the function fails, returning and leaving subsequent characters of the stream unread. • Format specifiers: A sequence formed by an initial percentage sign (%) indicates a format specifier, which is used to specify the type and format of the data to be retrieved from the stream and stored into the locations pointed by the additional arguments. A format specifier for scanf follows this prototype: %[*][width][length]specifier Where the specifier character at the end is the most significant component, since it defines which characters are extracted, their interpretation and the type of its corresponding argument: Any number of digits, optionally preceded by a sign (+ or -). Decimal digits assumed by default (0-9), but a 0 prefix introduces octal digits (0-7), and 0x hexadecimal digits (0-f). Signed argument. Any number of decimal digits (0-9), optionally preceded by a sign (+ or -). d is for a signed argument, and u for an unsigned. Any number of octal digits (0-7), optionally preceded by a sign (+ or -). Unsigned argument. Any number of hexadecimal digits (0-9, a-f, A-F), optionally preceded by 0x or 0X, and all optionally preceded by a sign (+ or -). Unsigned argument. A series of decimal digits, optionally containing a decimal point, optionally preceeded by a sign (+ or -) and optionally followed by the e or E character and a decimal integer (or some of the other sequences supported by strtod). Implementations complying with C99 also support hexadecimal floating-point format when preceded by 0x or 0X. The next character. If a width other than 1 is specified, the function reads exactly width characters and stores them in the successive locations of the array passed as argument. No null character is appended at the end. Any number of non-whitespace characters, stopping at the first whitespace character found. A terminating null character is automatically added at the end of the stored sequence. A sequence of characters representing a pointer. The particular format used depends on the system and library implementation, but it is the same as the one used to format %p in fprintf. Any number of the characters specified between the brackets. A dash (-) that is not the first character may produce non-portable behavior in some library implementations. Any number of characters none of them specified as characters between the brackets. No input is consumed. The number of characters read so far from stdin is stored in the pointed location. A % followed by another % matches a single %. Except for n, at least one character shall be consumed by any specifier. Otherwise the match fails, and the scan ends there. The format specifier can also contain sub-specifiers: asterisk (*), width and length (in that order), which are optional and follow these specifications: An optional starting asterisk indicates that the data is to be read from the stream but ignored (i.e. it is not stored in the location pointed by an argument). Specifies the maximum number of characters to be read in the current reading operation (optional). One of hh, h, l, ll, j, z, t, L (optional). This alters the expected type of the storage pointed by the corresponding argument (see below). Depending on the format string, the function may expect a sequence of additional arguments, each containing a pointer to allocated storage where the interpretation of the extracted characters is stored with the appropriate type. There should be at least as many of these arguments as the number of values stored by the format specifiers. Additional arguments are ignored by the function. These arguments are expected to be pointers: to store the result of a scanf operation on a regular variable, its name should be preceded by the reference operator (&) (see example). On success, the function returns the number of items of the argument list successfully filled. This count can match the expected number of items or be less (even zero) due to a matching failure, a reading error, or the reach of the end-of-file. If a reading error happens or the end-of-file is reached while reading, the proper indicator is set (feof or ferror). And, if either happens before any data could be successfully read, EOF is returned. If an encoding error happens interpreting wide characters, the function sets errno to EILSEQ.

Get current position in stream. Returns the current value of the position indicator of the stream. For binary streams, this is the number of bytes from the beginning of the file. For text streams, the numerical value may not be meaningful but can still be used to restore the position to the same position later using fseek (if there are characters put back using ungetc still pending of being read, the behavior is undefined). Pointer to a FILE object that identifies the stream. On success, the current value of the position indicator is returned. On failure, -1L is returned, and errno is set to a system-specific positive value. The ftell() function returns the current file position of the specified stream. We can use ftell() function to get the total size of a file after moving file pointer at the end of file. We can use SEEK_END constant to move the file pointer at the end of file.

Write string to stdout. Writes the C string pointed by str to the standard output (stdout) and appends a newline character ('\n'). The function begins copying from the address specified (str) until it reaches the terminating null character ('\0'). This terminating null-character is not copied to the stream. Notice that puts not only differs from fputs in that it uses stdout as destination, but it also appends a newline character at the end automatically (which fputs does not). The puts() function is very much similar to printf() function. The puts() function is used to print the string on the console which is previously read by using gets() or scanf() function. The puts() function returns an integer value representing the number of characters being printed on the console. Since, it prints an additional newline character with the string, which moves the cursor to the new line on the console, the integer value returned by puts() will always be equal to the number of characters present in the string plus 1. C string to be printed. On success, a non-negative value is returned. On error, the function returns EOF and sets the error indicator (ferror).

Write character to stdout. Writes a character to the standard output (stdout). It is equivalent to calling putc with stdout as second argument. putchar() function is a file handling function in C programming language which is used to write a character on standard output/screen. The putchar() function is used for printing character to a screen at current cursor location. It is unformatted character output functions. It is defined in header file stdio.h. The int promotion of the character to be written. The value is internally converted to an unsigned char when written. The character which is read is an unsigned char which is converted to an integer value. In the case of file handling, it returns EOF when end-of-file is encountered. If there is an error then it also returns EOF. On success, the character written is returned. If a writing error occurs, EOF is returned and the error indicator (ferror) is set.

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. 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).

Get character from stream. Returns the character currently pointed by the internal file position indicator of the specified stream. The internal file position indicator is then advanced to the next character. If the stream is at the end-of-file when called, the function returns EOF and sets the end-of-file indicator for the stream (feof). If a read error occurs, the function returns EOF and sets the error indicator for the stream (ferror). fgetc and getc are equivalent, except that getc may be implemented as a macro in some libraries. Pointer to a FILE object that identifies an input stream. fgetc() is used to obtain input from a file single character at a time. This function returns the ASCII code of the character read by the function. It returns the character present at position indicated by file pointer. After reading the character, the file pointer is advanced to next character. If pointer is at end of file or if an error occurs EOF file is returned by this function. On success, the character read is returned (promoted to an int value). The return type is int to accommodate for the special value EOF, which indicates failure: If the position indicator was at the end-of-file, the function returns EOF and sets the eof indicator (feof) of stream. If some other reading error happens, the function also returns EOF, but sets its error indicator (ferror) instead.

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. path to file which you want to use 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: Open for reading only. Open for writing only. 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: If set, the file offset shall be set to the end of the file prior to each write. 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. Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. 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. If set and path identifies a terminal device, open() shall not cause the terminal device to become the controlling terminal for the process. 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. 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. Write I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. 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: 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. O_CREAT and O_EXCL are set, and the named file exists. A signal was caught during open(). The implementation does not support synchronized I/O for this file. The path argument names a STREAMS file and a hangup or error occurred during the open(). The named file is a directory and oflag includes O_WRONLY or O_RDWR. A loop exists in symbolic links encountered during resolution of the path argument. {OPEN_MAX} file descriptors are currently open in the calling process. The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}. The maximum allowable number of files is currently open in the system. 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. The path argument names a STREAMS-based file and the system is unable to allocate a STREAM. The directory or file system that would contain the new file cannot be expanded, the file does not exist, and O_CREAT is specified. A component of the path prefix is not a directory. O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set, and no process has the file open for reading. The named file is a character special or block special file, and the device associated with this special file does not exist. The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t. 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: The path argument names the slave side of a pseudo-terminal device that is locked. The value of the oflag argument is not valid. More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument. As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}. The path argument names a STREAMS file and the system is unable to allocate resources. The file is a pure procedure (shared text) file that is being executed and oflag is O_WRONLY or O_RDWR.

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. 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: a conversion format specifier. Signed decimal integer Unsigned decimal integer Unsigned octal Unsigned hexadecimal integer Unsigned hexadecimal integer (uppercase) Decimal floating point, lowercase Decimal floating point, uppercase Scientific notation (mantissa/exponent), lowercase Scientific notation (mantissa/exponent), uppercase Use the shortest representation: %e or %f Use the shortest representation: %E or %F Hexadecimal floating point, lowercase Hexadecimal floating point, uppercase Character String of characters Pointer address 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: 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. 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. Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier). an optional * or integer value used to specify minimum width field. 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. an optional field consisting of a . followed by * or integer or nothing to specify the precision. 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. an optional length modifier that specifies the size of the argument. 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.

#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. 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. 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.

Reposition stream position indicator. Sets the position indicator associated with the stream to a new position. For streams open in binary mode, the new position is defined by adding offset to a reference position specified by origin. For streams open in text mode, offset shall either be zero or a value returned by a previous call to ftell, and origin shall necessarily be SEEK_SET. If the function is called with other values for these arguments, support depends on the particular system and library implementation (non-portable). The end-of-file internal indicator of the stream is cleared after a successful call to this function, and all effects from previous calls to ungetc on this stream are dropped. On streams open for update (read+write), a call to fseek allows to switch between reading and writing. The fseek() function is used to move the cursor in the file to the desired position. Pointer to a FILE object that identifies the stream. Binary files: Number of bytes to offset from origin. Text files: Either zero, or a value returned by ftell. Position used as reference for the offset. It is specified by one of the following constants defined in exclusively to be used as arguments for this function:

SEEK_SET

Beginning of file

SEEK_CUR

Current position of the file pointer

SEEK_END

End of file * * Library implementations are allowed to not meaningfully support SEEK_END (therefore, code using it has no real standard portability). If successful, the function returns zero. Otherwise, it returns non-zero value. If a read or write error occurs, the error indicator (ferror) is set.

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. • 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).

Open file. Opens the file whose name is specified in the parameter filename and associates it with a stream that can be identified in future operations by the FILE pointer returned. The operations that are allowed on the stream and how these are performed are defined by the mode parameter. The returned stream is fully buffered by default if it is known to not refer to an interactive device (see setbuf). The returned pointer can be disassociated from the file by calling fclose or freopen. All opened files are automatically closed on normal program termination. The running environment supports at least FOPEN_MAX files open simultaneously. C string containing the name of the file to be opened. Its value shall follow the file name specifications of the running environment and can include a path (if supported by the system). C string containing a file access mode. It can be: Open file for input operations. The file must exist. Create an empty file for output operations. If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file. Open file for output at the end of a file. Output operations always write data at the end of the file, expanding it. Repositioning operations (fseek, fsetpos, rewind) are ignored. The file is created if it does not exist. Open a file for update (both for input and output). The file must exist. Create an empty file and open it for update (both for input and output). If a file with the same name already exists its contents are discarded and the file is treated as a new empty file. Open a file for update (both for input and output) with all output operations writing data at the end of the file. Repositioning operations (fseek, fsetpos, rewind) affects the next input operations, but output operations move the position back to the end of file. The file is created if it does not exist. With the mode specifiers above the file is open as a text file. In order to open a file as a binary file, a "b" character has to be included in the mode string. This additional "b" character can either be appended at the end of the string (thus making the following compound modes: "rb", "wb", "ab", "r+b", "w+b", "a+b") or be inserted between the letter and the "+" sign for the mixed modes ("rb+", "wb+", "ab+"). The new C standard (C2011, which is not part of C++) adds a new standard subspecifier ("x"), that can be appended to any "w" specifier (to form "wx", "wbx", "w+x" or "w+bx"/"wb+x"). This subspecifier forces the function to fail if the file exists, instead of overwriting it. If additional characters follow the sequence, the behavior depends on the library implementation: some implementations may ignore additional characters so that for example an additional "t" (sometimes used to explicitly state a text file) is accepted. On some library implementations, opening or creating a text file with update mode may treat the stream instead as a binary file. Text files are files containing sequences of lines of text. Depending on the environment where the application runs, some special character conversion may occur in input/output operations in text mode to adapt them to a system-specific text file format. Although on some environments no conversions occur and both text files and binary files are treated the same way, using the appropriate mode improves portability. For files open for update (those which include a "+" sign), on which both input and output operations are allowed, the stream shall be flushed (fflush) or repositioned (fseek, fsetpos, rewind) before a reading operation that follows a writing operation. The stream shall be repositioned (fseek, fsetpos, rewind) before a writing operation that follows a reading operation (whenever that operation did not reach the end-of-file). If the file is successfully opened, the function returns a pointer to a FILE object that can be used to identify the stream on future operations. Otherwise, a null pointer is returned. On most library implementations, the errno variable is also set to a system-specific error code on failure.

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. 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. 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: 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. fildes is not a valid open file descriptor, or the socket parameter is not a valid socket descriptor. The file cannot be closed because it is blocked. close() was interrupted by a signal. The file may or may not be closed. Added for XPG4.2: An I/O error occurred while reading from or writing to the file system. fildes does not exist. The minor number for the file is incorrect.

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. 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. 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.