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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
fopen — open a stream
SYNOPSIS
#include <stdio.h>
FILE *fopen(const char *restrict pathname, const char *restrict mode);
DESCRIPTION
The functionality described on this reference page is aligned with the ISO C standard. Any
conflict between the requirements described here and the ISO C standard is unintentional.
This volume of POSIX.1‐2008 defers to the ISO C standard.
The fopen() function shall open the file whose pathname is the string pointed to by
pathname, 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, fopen() shall mark for update the last data access,
last data modification, and last file status change timestamps of the file and the last
file status change and last data modification timestamps of the parent directory.
If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously
exist, the fopen() function shall create a file as if it called the creat() function with
a value appropriate for the path argument interpreted from pathname and a value of S_IRUSR
| S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.
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 last data modification and last file status
change timestamps of the file.
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 file descriptor associated with the opened stream shall be allocated and opened as if
by a call to open() with the following flags:
┌─────────────────┬───────────────────────────┐
│ fopen() Mode │ open() Flags │
├─────────────────┼───────────────────────────┤
│r or rb │ O_RDONLY │
│w or wb │ O_WRONLY|O_CREAT|O_TRUNC │
│a or ab │ O_WRONLY|O_CREAT|O_APPEND │
│r+ or rb+ or r+b │ O_RDWR │
│w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC │
│a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND │
└─────────────────┴───────────────────────────┘
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 All file descriptors available to the process are currently open.
EMFILE {STREAM_MAX} streams are currently open in the calling process.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic
link produced an intermediate result with a length that exceeds {PATH_MAX}.
ENFILE The maximum allowable number of files is currently open in the system.
ENOENT The mode string begins with 'r' and a component of pathname does not name an
existing file, or mode begins with 'w' or 'a' and a component of the path prefix of
pathname does not name an existing file, or pathname is an empty string.
ENOENT or ENOTDIR
The pathname argument contains at least one non-<slash> character and ends with one
or more trailing <slash> characters. If pathname names an existing file, an
[ENOENT] error shall not occur.
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 names an existing file that is neither a directory
nor a symbolic link to a directory, or the pathname argument contains at least one
non-<slash> character and ends with one or more trailing <slash> characters and the
last pathname component names an existing file that is neither a directory nor a
symbolic link to 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.
ENAMETOOLONG
The length of a component of a pathname is longer than {NAME_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
Section 2.5, Standard I/O Streams, creat(), fclose(), fdopen(), fmemopen(), freopen(),
open_memstream()
The Base Definitions volume of POSIX.1‐2008, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .