#include <stdio.h>
FILE *fopen(const char *restrict filename, const char
*restrict mode);
The fopen() function shall open the file whose pathname is the string pointed to by filename, and associates a stream with it.
The mode argument points to a string. If the string is one of the following, the file shall be opened in the indicated mode. Otherwise, the behavior is undefined.
The character 'b' shall have no effect, but is allowed for ISO C standard conformance. Opening a file with read mode (r as the first character in the mode argument) shall fail if the file does not exist or cannot be read.
Opening a file with append mode (a as the first character in the mode argument) shall cause all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to fseek().
When a file is opened with update mode ( '+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call to fflush() or to a file positioning function ( fseek(), fsetpos(), or rewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.
When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream shall be cleared.
If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously exist, upon successful completion, the fopen() function 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 mode is w, wb, w+, wb+, or w+b, and the file did previously exist, upon successful completion, fopen() shall mark for update the st_ctime and st_mtime fields of the file. The fopen() function shall allocate a file descriptor as open() does.
After a successful call to the fopen() function, the orientation of the stream shall be cleared, the encoding rule shall be cleared, and the associated mbstate_t object shall be set to describe an initial conversion state.
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, fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.
The fopen() function shall fail if:
The length of the filename argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
The fopen() function may fail if:
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.
The following sections are informative.
The following example tries to open the file named file for reading. The fopen() function returns a file pointer that is used in subsequent fgets() and fclose() calls. If the program cannot open the file, it just ignores it.
#include <stdio.h>
...
FILE *fp;
...
void rgrep(const char *file)
{
...
if ((fp = fopen(file, "r")) == NULL)
return;
...
}
fclose() , fdopen() , freopen() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>