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 (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.   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 the call:

           fseek(stream, 0, SEEK_END);

       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.  This flag is ignored for fdopen().

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

       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, in glibc versions before 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

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

COLOPHON

       This  page  is  part  of  release 3.54 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/.