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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.1‐2017 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 without the trailing <slash> characters would name 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.1‐2017, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 .