Provided by: manpages-posix-dev_2.16-1_all 
NAME
fopen - open a stream
SYNOPSIS
#include <stdio.h> FILE *fopen(const char *restrict filename, const char *restrict mode);
DESCRIPTION
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.
r or rb
Open file for reading.
w or wb
Truncate to zero length or create file for writing.
a or ab
Append; open or create file for writing at end-of-file.
r+ or rb+ or r+b
Open file for update (reading and writing).
w+ or wb+ or w+b
Truncate to zero length or create file for update.
a+ or ab+ or a+b
Append; open or create file for update, writing at end-of-file.
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.
RETURN VALUE
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.
ERRORS
The fopen() 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 mode are denied, or the file does not exist and write permission is
denied for the parent directory of the file to be created.
EINTR A signal was caught during fopen().
EISDIR The named file is a directory and mode requires write access.
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 filename 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 A component of filename does not name an existing file or filename is an empty string.
ENOSPC The directory or file system that would contain the new file cannot be expanded, the file does not
exist, and the file was to be created.
ENOTDIR
A component of the path prefix is not a directory.
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 mode requires write access.
The fopen() function may fail if:
EINVAL The value of the mode argument is not valid.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
EMFILE {FOPEN_MAX} streams are currently open in the calling process.
EMFILE {STREAM_MAX} streams are currently open in the calling process.
ENAMETOOLONG
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds
{PATH_MAX}.
ENOMEM Insufficient storage space is available.
ETXTBSY
The file is a pure procedure (shared text) file that is being executed and mode requires write
access.
The following sections are informative.
EXAMPLES
Opening a File
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;
...
}
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
fclose() , fdopen() , freopen() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .