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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.1‐2008 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 │
└─────────────────┴───────────────────────────┘
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 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.
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.1‐2008, stdio.h(0p)
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 .
IEEE/The Open Group 2013 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)