fopen(3) - Linux manual page (original) (raw)

fopen(3) Library Functions Manual fopen(3)

NAME top

   fopen, fdopen, freopen - stream open functions

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <stdio.h>**

   **FILE *fopen(const char *restrict** _pathname_**, const char *restrict** _mode_**);**
   **FILE *fdopen(int** _fd_**, const char ***_mode_**);**
   **FILE *freopen(const char *restrict** _pathname_**, const char *restrict** _mode_**,**
                 **FILE *restrict** _stream_**);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   **fdopen**():
       _POSIX_C_SOURCE

DESCRIPTION top

   The **fopen**() function opens the file whose name is the string
   pointed to by _pathname_ and associates a stream with it.

   The argument _mode_ points to a string beginning with one of the
   following sequences (possibly followed by additional characters,
   as described below):

   **r** Open text file for reading.  The stream is positioned at
          the beginning of the file.

   **r+** Open for reading and writing.  The stream is positioned at
          the beginning of the file.

   **w** Truncate file to zero length or create text file for
          writing.  The stream is positioned at the beginning of the
          file.

   **w+** Open for reading and writing.  The file is created if it
          does not exist, otherwise it is truncated.  The stream is
          positioned at the beginning of the file.

   **a** Open for appending (writing at end of file).  The file is
          created if it does not exist.  The stream is positioned at
          the end of the file.

   **a+** Open for reading and appending (writing at end of file).
          The file is created if it does not exist.  Output is always
          appended to the end of the file.  POSIX is silent on what
          the initial read position is when using this mode.  For
          glibc, the initial file position for reading is at the
          beginning of the file, but for Android/BSD/MacOS, the
          initial file position for reading is at the end of the
          file.

   The _mode_ string can also include the letter 'b' either as a last
   character or as a character between the characters in any of the
   two-character strings described above.  This is strictly for
   compatibility with ISO C and has no effect; the 'b' is ignored on
   all POSIX conforming systems, including Linux.  (Other systems may
   treat text files and binary files differently, and adding the 'b'
   may be a good idea if you do I/O to a binary file and expect that
   your program may be ported to non-UNIX environments.)

   See NOTES below for details of glibc extensions for _mode_.

   Any created file will have the mode **S_IRUSR** | **S_IWUSR** | **S_IRGRP** |
   **S_IWGRP** | **S_IROTH** | **S_IWOTH** (0666), as modified by the process's
   umask value (see [umask(2)](../man2/umask.2.html)).

   Reads and writes may be intermixed on read/write streams in any
   order.  Note that ANSI C requires that a file positioning function
   intervene between output and input, unless an input operation
   encounters end-of-file.  (If this condition is not met, then a
   read is allowed to return the result of writes other than the most
   recent.)  Therefore it is good practice (and indeed sometimes
   necessary under Linux) to put an [fseek(3)](../man3/fseek.3.html) or [fsetpos(3)](../man3/fsetpos.3.html) operation
   between write and read operations on such a stream.  This
   operation may be an apparent no-op (as in _fseek(..., 0L, SEEKCUR)_
   called for its synchronizing side effect).

   Opening a file in append mode (**a** as the first character of _mode_)
   causes all subsequent write operations to this stream to occur at
   end-of-file, as if preceded by the call:

       fseek(stream, 0, SEEK_END);

   The file descriptor associated with the stream is opened as if by
   a call to [open(2)](../man2/open.2.html) with the following flags:
          ┌──────────────┬───────────────────────────────┐
          │ **fopen() mode** │ **open() flags** │
          ├──────────────┼───────────────────────────────┤
          │      _r_       │ O_RDONLY                      │
          ├──────────────┼───────────────────────────────┤
          │      _w_       │ O_WRONLY | O_CREAT | O_TRUNC  │
          ├──────────────┼───────────────────────────────┤
          │      _a_       │ O_WRONLY | O_CREAT | O_APPEND │
          ├──────────────┼───────────────────────────────┤
          │      _r+_      │ O_RDWR                        │
          ├──────────────┼───────────────────────────────┤
          │      _w+_      │ O_RDWR | O_CREAT | O_TRUNC    │
          ├──────────────┼───────────────────────────────┤
          │      _a+_      │ O_RDWR | O_CREAT | O_APPEND   │
          └──────────────┴───────────────────────────────┘

fdopen() The fdopen() function associates a stream with the existing file descriptor, fd. The mode of the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor. The file position indicator of the new stream is set to that belonging to fd, and the error and end-of- file indicators are cleared. Modes "w" or "w+" do not cause truncation of the file. The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. The result of applying fdopen() to a shared memory object is undefined.

freopen() The freopen() function opens the file whose name is the string pointed to by pathname and associates the stream pointed to by stream with it. The original stream (if it exists) is closed. The mode argument is used just as in the fopen() function.

   If the _pathname_ argument is a null pointer, **freopen**() changes the
   mode of the stream to that specified in _mode_; that is, **freopen**()
   reopens the pathname that is associated with the stream.  The
   specification for this behavior was added in the C99 standard,
   which says:

          In this case, the file descriptor associated with the
          stream need not be closed if the call to **freopen**()
          succeeds.  It is implementation-defined which changes of
          mode are permitted (if any), and under what circumstances.

   The primary use of the **freopen**() function is to change the file
   associated with a standard text stream (_stderr_, _stdin_, or _stdout_).

RETURN VALUE top

   Upon successful completion **fopen**(), **fdopen**(), and **freopen**() return
   a _FILE_ pointer.  Otherwise, NULL is returned and _[errno](../man3/errno.3.html)_ is set to
   indicate the error.

ERRORS top

   **EINVAL** The _mode_ provided to **fopen**(), **fdopen**(), or **freopen**() was
          invalid.

   The **fopen**(), **fdopen**(), and **freopen**() functions may also fail and
   set _[errno](../man3/errno.3.html)_ for any of the errors specified for the routine
   [malloc(3)](../man3/malloc.3.html).

   The **fopen**() function may also fail and set _[errno](../man3/errno.3.html)_ for any of the
   errors specified for the routine [open(2)](../man2/open.2.html).

   The **fdopen**() function may also fail and set _[errno](../man3/errno.3.html)_ for any of the
   errors specified for the routine [fcntl(2)](../man2/fcntl.2.html).

   The **freopen**() function may also fail and set _[errno](../man3/errno.3.html)_ for any of the
   errors specified for the routines [open(2)](../man2/open.2.html), [fclose(3)](../man3/fclose.3.html), and
   [fflush(3)](../man3/fflush.3.html).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **fopen**(), **fdopen**(), **freopen**()         │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   **fopen**()
   **freopen**()
          C11, POSIX.1-2008.

   **fdopen**()
          POSIX.1-2008.

HISTORY top

   **fopen**()
   **freopen**()
          POSIX.1-2001, C89.

   **fdopen**()
          POSIX.1-2001.

NOTES top

glibc notes The GNU C library allows the following extensions for the string specified in mode:

   **c** (since glibc 2.3.3)
          Do not make the open operation, or subsequent read and
          write operations, thread cancelation points.  This flag is
          ignored for **fdopen**().

   **e** (since glibc 2.7)
          Open the file with the **O_CLOEXEC** flag.  See [open(2)](../man2/open.2.html) for
          more information.  This flag is ignored for **fdopen**().

   **m** (since glibc 2.3)
          Attempt to access the file using [mmap(2)](../man2/mmap.2.html), rather than I/O
          system calls ([read(2)](../man2/read.2.html), [write(2)](../man2/write.2.html)).  Currently, use of
          [mmap(2)](../man2/mmap.2.html) is attempted only for a file opened for reading.

   **x** Open the file exclusively (like the **O_EXCL** flag of
          [open(2)](../man2/open.2.html)).  If the file already exists, **fopen**() fails, and
          sets _[errno](../man3/errno.3.html)_ to **EEXIST**.  This flag is ignored for **fdopen**().

   In addition to the above characters, **fopen**() and **freopen**() support
   the following syntax in _mode_:

       **,ccs=**_string_

   The given _string_ is taken as the name of a coded character set and
   the stream is marked as wide-oriented.  Thereafter, internal
   conversion functions convert I/O to and from the character set
   _string_.  If the **,ccs=**_string_ syntax is not specified, then the
   wide-orientation of the stream is determined by the first file
   operation.  If that operation is a wide-character operation, the
   stream is marked wide-oriented, and functions to convert to the
   coded character set are loaded.

BUGS top

   When parsing for individual flag characters in _mode_ (i.e., the
   characters preceding the "ccs" specification), the glibc
   implementation of **fopen**() and **freopen**() limits the number of
   characters examined in _mode_ to 7 (or, before glibc 2.14, to 6,
   which was not enough to include possible specifications such as
   "rb+cmxe").  The current implementation of **fdopen**() parses at most
   5 characters in _mode_.

SEE ALSO top

   [open(2)](../man2/open.2.html), [fclose(3)](../man3/fclose.3.html), [fileno(3)](../man3/fileno.3.html), [fmemopen(3)](../man3/fmemopen.3.html), [fopencookie(3)](../man3/fopencookie.3.html),
   [open_memstream(3)](../man3/open%5Fmemstream.3.html)

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Linux man-pages 6.10 2024-07-23 fopen(3)

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