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NAME

       fopen, fdopen, freopen - stream open functions

SYNOPSIS

       #include <stdio.h>

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

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

       FILE *freopen(const char *pathname, 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 pathname 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.  Output is always appended to the end of the file.  POSIX is silent
              on  what the initial read position is when using this mode.  For glibc, the initial
              file  position  for  reading  is  at  the  beginning   of   the   file,   but   for
              Android/BSD/MacOS, the initial file position for reading is at 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 file will have the 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 file descriptor associated with the stream is opened as if by a call to  open(2)  with
       the following flags:

              ┌─────────────┬───────────────────────────────┐
              │fopen() modeopen() flags                  │
              ├─────────────┼───────────────────────────────┤
              │     r       │ O_RDONLY                      │
              ├─────────────┼───────────────────────────────┤
              │     w       │ O_WRONLY | O_CREAT | O_TRUNC  │
              ├─────────────┼───────────────────────────────┤
              │     a       │ O_WRONLY | O_CREAT | O_APPEND │
              ├─────────────┼───────────────────────────────┤
              │     r+      │ O_RDWR                        │
              ├─────────────┼───────────────────────────────┤
              │     w+      │ O_RDWR | O_CREAT | O_TRUNC    │
              ├─────────────┼───────────────────────────────┤
              │     a+      │ O_RDWR | O_CREAT | O_APPEND   │
              └─────────────┴───────────────────────────────┘
   fdopen()
       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.

   freopen()
       The  freopen() function opens the file whose name is the string pointed to by pathname 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.

       If  the  pathname  argument is a null pointer, freopen() changes the mode of the stream to
       that specified in mode; that is, freopen() reopens the pathname that  is  associated  with
       the  stream.   The  specification  for  this behavior was added in the C99 standard, which
       says:

              In this case, the file descriptor associated with the stream need not be closed  if
              the call to freopen() succeeds.  It is implementation-defined which changes of mode
              are permitted (if any), and under what circumstances.

       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).

       ┌─────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├─────────────────────────────┼───────────────┼─────────┤
       │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

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.