Provided by: manpages-dev_3.35-0.1ubuntu1_all bug

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  following  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 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.  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 character or as a
       character between the characters in any of  the  two-character  strings  described  above.
       This  is  strictly for compatibility with C89 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 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 created 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() function conforms to
       POSIX.1-1990.

NOTES

   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
              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  EEXIST.   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 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.