fopen

FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)

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  follow-
       ing sequences (possibly followed by additional characters, as described
       below):

       r      Open text file for reading.  The stream is positioned at the be-
              ginning 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 cre-
              ated 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 char-
       acter or as a character between the characters in any of the  two-char-
       acter strings described above.  This is strictly for compatibility with
       C89 and has no effect; the 'b' is ignored on all POSIX conforming  sys-
       tems,  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_IW-
       GRP  |  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() mode | open() 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 de-
       scriptor, 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 de-
       scriptor.  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  de-
       scriptor  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 im-
              plementation-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 associ-
       ated 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 in-
              valid.

       The fopen(), fdopen() and freopen() functions may also fail and set er-
       rno 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 er-
       rors specified for the routines open(2), fclose(3), and fflush(3).

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(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 speci-
       fied in mode:

       c (since glibc 2.3.3)
              Do not make the open operation, or subsequent read and write op-
              erations,  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 in-
              formation.  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  at-
              tempted 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  EEX-
              IST.  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  charac-
       ters  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 imple-
       mentation of fdopen() parses at most 5 characters in mode.

SEE ALSO
       open(2),  fclose(3),  fileno(3), fmemopen(3), fopencookie(3), open_mem-
       stream(3)

COLOPHON
       This page is part of release 5.05 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/.

GNU                               2019-05-09                          FOPEN(3)
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