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man: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
            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 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.  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  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_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() 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
     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  cre-
     ated  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 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
            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 speci-
     fied 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  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 4.16 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 2017-09-15 FOPEN(3)

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