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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FOPEN(3P) POSIX Programmer's Manual FOPEN(3P)
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.
fopen — open a stream
#include <stdio.h>
FILE *fopen(const char *restrict pathname, const char *restrict mode);
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 │
└──────────────────┴───────────────────────────┘
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:
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.
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;
...
}
None.
None.
None.
Section 2.5, Standard I/O Streams, creat(3p), fclose(3p),
fdopen(3p), fmemopen(3p), freopen(3p), open_memstream(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)
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 .
IEEE/The Open Group 2017 FOPEN(3P)
Pages that refer to this page: stdio.h(0p), close(3p), fclose(3p), fdopen(3p), feof(3p), ferror(3p), fgetpos(3p), fgets(3p), fgetwc(3p), fgetws(3p), fileno(3p), fmemopen(3p), fputc(3p), fputs(3p), fputwc(3p), fputws(3p), fread(3p), freopen(3p), fseek(3p), fsetpos(3p), ftell(3p), fwrite(3p), lockf(3p), open_memstream(3p), puts(3p), setbuf(3p), setvbuf(3p), stdin(3p), tempnam(3p), tmpfile(3p), tmpnam(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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