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man:freopen

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:
            allbox; lb lb c l.  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).
     allbox;  lbw28  lb  lb  l  l  l.  Interface Attribute Value T{ fopen(),
     fdopen(), freopen() T}   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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