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

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).

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(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
   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),
   open_memstream(3)

COLOPHON

   This  page  is  part of release 4.09 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/.

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