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FREOPEN(3P)		   POSIX Programmer's Manual		   FREOPEN(3P)

       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

       freopen — open a stream

       #include <stdio.h>

       FILE *freopen(const char *restrict pathname, const char *restrict mode,
	   FILE *restrict stream);

       The functionality described on this reference page is aligned with  the
       ISO C  standard.	 Any  conflict between the requirements described here
       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2008
       defers to the ISO C standard.

       The  freopen() function shall first attempt to flush the stream associ‐
       ated with stream as if by a call to fflush(stream).  Failure  to	 flush
       the  stream  successfully  shall	 be ignored. If pathname is not a null
       pointer, freopen() shall close  any  file  descriptor  associated  with
       stream.	 Failure  to  close  the file descriptor successfully shall be
       ignored.	 The error and end-of-file indicators for the stream shall  be

       The freopen() function shall open the file whose pathname is the string
       pointed to by pathname and associate the stream pointed	to  by	stream
       with it. The mode argument shall be used just as in fopen().

       The  original  stream  shall be closed regardless of whether the subse‐
       quent open succeeds.

       If pathname is a null pointer, the freopen() function shall attempt  to
       change the mode of the stream to that specified by mode, as if the name
       of the file currently associated with the stream had been used. 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.

       After  a	 successful call to the freopen() function, the orientation of
       the stream shall be cleared, the encoding rule shall  be	 cleared,  and
       the  associated	mbstate_t  object  shall be set to describe an initial
       conversion state.

       If pathname is not a null pointer, or if pathname is a null pointer and
       the  specified  mode change necessitates the file descriptor associated
       with the stream to be closed and reopened, the file descriptor  associ‐
       ated  with the reopened stream shall be allocated and opened as if by a
       call to open() with the following flags:

		   │ freopen() Mode  │	     open() Flags	 │
		   │r or rb	     │ O_RDONLY			 │
		   │w or wb	     │ O_WRONLY|O_CREAT|O_TRUNC	 │
		   │a or ab	     │ O_WRONLY|O_CREAT|O_APPEND │
		   │r+ or rb+ or r+b │ O_RDWR			 │
		   │w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC	 │
		   │a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND	 │
       Upon successful completion, freopen() shall return the value of stream.
       Otherwise,  a null pointer shall be returned, and errno shall be set to
       indicate the error.

       The freopen() function shall fail if:

       EACCES Search permission is denied on a component of the	 path  prefix,
	      or  the  file  exists  and the permissions specified by mode are
	      denied, or the file does	not  exist  and	 write	permission  is
	      denied for the parent directory of the file to be created.

       EBADF  The  file	 descriptor  underlying the stream is not a valid file
	      descriptor when pathname is a null pointer.

       EINTR  A signal was caught during freopen().

       EISDIR The named file is a directory and mode requires write access.

       ELOOP  A loop exists in symbolic links encountered during resolution of
	      the path argument.

       EMFILE All  file	 descriptors  available	 to  the process are currently

	      The  length  of  a  component  of	 a  pathname  is  longer  than

       ENFILE The  maximum  allowable number of files is currently open in the

       ENOENT The mode string begins with 'r' and a component of pathname does
	      not  name an existing file, or mode begins with 'w' or 'a' and a
	      component of the path prefix of pathname does not name an exist‐
	      ing file, or pathname is an empty string.

	      The  pathname argument contains at least one non-<slash> charac‐
	      ter and ends with one or more trailing  <slash>  characters.  If
	      pathname	names  an  existing  file, an [ENOENT] error shall not

       ENOSPC The directory or file system that would  contain	the  new  file
	      cannot  be  expanded,  the file does not exist, and it was to be

	      A component of the path prefix names an existing	file  that  is
	      neither  a  directory nor a symbolic link to a directory, or the
	      pathname argument contains at least  one	non-<slash>  character
	      and  ends	 with  one or more trailing <slash> characters and the
	      last pathname component names an existing file that is neither a
	      directory nor a symbolic link to a directory.

       ENXIO  The named file is a character special or block special file, and
	      the device associated with this special file does not exist.

	      The named file is a regular file and the size of the file cannot
	      be represented correctly in an object of type off_t.

       EROFS  The  named  file	resides	 on  a	read-only file system and mode
	      requires write access.

       The freopen() function may fail if:

       EBADF  The mode with which the file descriptor  underlying  the	stream
	      was  opened does not support the requested mode when pathname is
	      a null pointer.

       EINVAL The value of the mode argument is not valid.

       ELOOP  More than {SYMLOOP_MAX} symbolic links were  encountered	during
	      resolution of the path argument.

	      The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
	      tion of a symbolic link produced an intermediate result  with  a
	      length that exceeds {PATH_MAX}.

       ENOMEM Insufficient storage space is available.

       ENXIO  A	 request  was made of a nonexistent device, or the request was
	      outside the capabilities of the device.

	      The file is a pure procedure (shared text) file  that  is	 being
	      executed and mode requires write access.

       The following sections are informative.

   Directing Standard Output to a File
       The  following  example	logs  all  standard output to the /tmp/logfile

	   #include <stdio.h>
	   FILE *fp;
	   fp = freopen ("/tmp/logfile", "a+", stdout);

       The freopen() function is  typically  used  to  attach  the  pre-opened
       streams associated with stdin, stdout, and stderr to other files.

       Since  implementations  are  not	 required  to  support any stream mode
       changes when the pathname argument is NULL, portable applications  can‐
       not  rely on the use of freopen() to change the stream mode, and use of
       this feature is discouraged. The feature was originally	added  to  the
       ISO C  standard	in  order  to  facilitate changing stdin and stdout to
       binary mode. Since a 'b' character in the mode has no effect  on	 POSIX
       systems,	 this use of the feature is unnecessary in POSIX applications.
       However, even though the 'b' is ignored,	 a  successful	call  to  fre‐
       open(NULL,  "wb", stdout) does have an effect. In particular, for regu‐
       lar files it truncates the file and sets	 the  file-position  indicator
       for  the	 stream	 to  the  start of the file. It is possible that these
       side-effects are an unintended consequence of the way  the  feature  is
       specified  in  the  ISO/IEC 9899:1999 standard, but unless or until the
       ISO C standard is changed, applications which  successfully  call  fre‐
       open(NULL,  "wb",  stdout) will behave in unexpected ways on conforming
       systems in situations such as:

	   { appl file1; appl file2; } > file3

       which will result in file3 containing only the output from  the	second
       invocation of appl.



       Section 2.5, Standard I/O Streams, fclose(), fdopen(), fflush(), fmemo‐
       pen(), fopen(), mbsinit(), open(), open_memstream()

       The Base Definitions volume of POSIX.1‐2008, <stdio.h>

       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal and Electronics Engineers,  Inc  and	 The  Open  Group.   (This  is
       POSIX.1-2008  with  the	2013  Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained	online
       at .

       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker‐ .

IEEE/The Open Group		     2013			   FREOPEN(3P)

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