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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 || _XOPEN_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.)

       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 the global variable
       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)

COLOPHON

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