fopen(3)



FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)

NAME
       fopen, fdopen, freopen - stream open functions

SYNOPSIS
       #include <stdio.h>

       FILE *fopen(const char *pathname, const char *mode);

       FILE *fdopen(int fd, const char *mode);

       FILE *freopen(const char *pathname, const char *mode, FILE *stream);

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

       fdopen(): _POSIX_C_SOURCE

DESCRIPTION
       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  follow-
       ing sequences (possibly followed by additional characters, as described
       below):

       r      Open text file for reading.  The stream is positioned at the be-
              ginning 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 cre-
              ated 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.  The initial file position
              for  reading  is at the beginning of the file, but output is al-
              ways appended to the end of the file.

       The mode string can also include the letter 'b' either as a last  char-
       acter  or as a character between the characters in any of the two-char-
       acter strings described above.  This is strictly for compatibility with
       C89  and has no effect; the 'b' is ignored on all POSIX conforming sys-
       tems, 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_IW-
       GRP | S_IROTH | S_IWOTH (0666), as  modified  by  the  process's  umask
       value (see umask(2)).

       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)  or  fgetpos(3) operation between write and read operations on
       such a stream.   This  operation  may  be  an  apparent  no-op  (as  in
       fseek(..., 0L, SEEK_CUR) 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 the call:

           fseek(stream, 0, SEEK_END);

       The  file  descriptor  associated  with the stream is opened as if by a
       call to open(2) 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  de-
       scriptor,  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  de-
       scriptor.  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 de-
       scriptor 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 im-
              plementation-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 associ-
       ated with a standard text stream (stderr, stdin, or stdout).

RETURN VALUE
       Upon successful completion fopen(), fdopen()  and  freopen()  return  a
       FILE pointer.  Otherwise, NULL is returned and errno is set to indicate
       the error.

ERRORS
       EINVAL The mode provided to fopen(), fdopen(),  or  freopen()  was  in-
              valid.

       The fopen(), fdopen() and freopen() functions may also fail and set er-
       rno for any of the errors specified for the routine malloc(3).

       The fopen() function may also fail and set errno for any of the  errors
       specified for the routine open(2).

       The fdopen() function may also fail and set errno for any of the errors
       specified for the routine fcntl(2).

       The freopen() function may also fail and set errno for any of  the  er-
       rors specified for the routines open(2), fclose(3), and fflush(3).

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +-----------------------------+---------------+---------+
       |Interface                    | Attribute     | Value   |
       +-----------------------------+---------------+---------+
       |fopen(), fdopen(), freopen() | Thread safety | MT-Safe |
       +-----------------------------+---------------+---------+
CONFORMING TO
       fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       fdopen(): POSIX.1-2001, POSIX.1-2008.

NOTES
   Glibc notes
       The GNU C library allows the following extensions for the string speci-
       fied in mode:

       c (since glibc 2.3.3)
              Do not make the open operation, or subsequent read and write op-
              erations, thread cancellation points.  This flag is ignored  for
              fdopen().

       e (since glibc 2.7)
              Open the file with the O_CLOEXEC flag.  See open(2) for more in-
              formation.  This flag is ignored for fdopen().

       m (since glibc 2.3)
              Attempt to access the file using mmap(2), rather than I/O system
              calls  (read(2),  write(2)).   Currently,  use of mmap(2) is at-
              tempted only for a file opened for reading.

       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If
              the  file  already exists, fopen() fails, and sets errno to EEX-
              IST.  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
       When  parsing for individual flag characters in mode (i.e., the charac-
       ters preceding the "ccs" specification), the  glibc  implementation  of
       fopen()  and freopen() limits the number of characters examined in mode
       to 7 (or, in glibc versions before 2.14, to 6, which was not enough  to
       include possible specifications such as "rb+cmxe").  The current imple-
       mentation of fdopen() parses at most 5 characters in mode.

SEE ALSO
       open(2), fclose(3), fileno(3), fmemopen(3),  fopencookie(3),  open_mem-
       stream(3)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

GNU                               2017-09-15                          FOPEN(3)

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