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

NAME
       fopen, fdopen, freopen - stream open functions

SYNOPSIS
       #include <stdio.h>

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

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

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

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

       fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

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

       The argument mode points to a string beginning with one of the  follow-
       ing sequences (Additional characters may follow these sequences.):

       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  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
              always 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 files will have 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)).

       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 by an

           fseek(stream,0,SEEK_END);

       call.

       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  cre-
       ated  by  fdopen()  is  closed.   The  result of applying fdopen() to a
       shared memory object is undefined.

       The freopen() function opens the file whose name is the string  pointed
       to by path 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.  The primary use of the freopen()
       function is to change the file associated 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
              invalid.

       The fopen(), fdopen() and freopen() functions may  also  fail  and  set
       errno 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
       errors specified for the routines open(2), fclose(3) and fflush(3).

CONFORMING TO
       The fopen() and freopen() functions conform to C89.  The fdopen() func-
       tion conforms to POSIX.1-1990.

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
              operations, thread cancellation points.

       e (since glibc 2.7)
              Open  the  file  with  the O_CLOEXEC flag.  See open(2) for more
              information.

       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 only
              attempted 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().

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

COLOPHON
       This page is part of release 3.25 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU                               2009-02-23                          FOPEN(3)
 

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