#include <stdio.h>FILE *fopen(const char *filename, const char *mode);
The fopen() function opens the file whose pathname is the string pointed to by filename, and associates a stream with it.
The argument mode points to a string beginning with one of the following sequences:
Open file for reading.
Truncate to zero length or create file for writing.
Append; open or create file for writing at end-of-file.
Open file for update (reading and writing).
Truncate to zero length or create file for update.
Append; open or create file for update, writing at end-of-file.
The character b has no effect, but is allowed for ISO C standard conformance (see standards(5)). Opening a file with read mode (r as the first character in the mode argument) fails 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) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to fseek(3C). If two separate processes open the same file for append, each process may write freely to the file without fear of destroying output being written by the other. The output from the two processes will be intermixed in the file in the order in which it is written.
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, output must not be directly followed by input without an intervening call to fflush(3C) or to a file positioning function ( fseek(3C), fsetpos(3C) or rewind(3C)), and input must not be 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 are cleared.
If mode is w, a, w+ or a+ and the file did not previously exist, upon successful completion, fopen() function will 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 or w+ and the file did previously exist, upon successful completion, fopen() will mark for update the st_ctime and st_mtime fields of the file. The fopen() function will allocate a file descriptor as open(2) does.
The largest value that can be represented correctly in an object of type off_t will be established as the offset maximum in the open file description.
Upon successful completion, fopen() returns a pointer to the object controlling the stream. Otherwise, a null pointer is returned and errno is set to indicate the error.
The fopen() function may fail and not set errno if there are no free stdio streams.
The fopen() function will fail if:
Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created.
A signal was caught during the execution of fopen().
The named file is a directory and mode requires write access.
Too many symbolic links were encountered in resolving path.
There are OPEN_MAX file descriptors currently open in the calling process.
The length of the filename exceeds PATH_MAX or a pathname component is longer than NAME_MAX.
The maximum allowable number of files is currently open in the system.
A component of filename does not name an existing file or filename is an empty string.
The directory or file system that would contain the new file cannot be expanded, the file does not exist, and it was to be created.
A component of the path prefix is not a directory.
The named file is a character special or block special file, and the device associated with this special file does not exist.
The current value of the file position cannot be represented correctly in an object of type fpos_t.
The named file resides on a read-only file system and mode requires write access.
The fopen() function may fail if:
The value of the mode argument is not valid.
The number of streams currently open in the calling process is either FOPEN_MAX or STREAM_MAX.
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds PATH_MAX.
Insufficient storage space is available.
The file is a pure procedure (shared text) file that is being executed and mode requires write access.
The number of streams that a process can have open at one time is STREAM_MAX. If defined, it has the same value as FOPEN_MAX.
The fopen() function has a transitional interface for 64-bit file offsets. See lf64(5).
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|