fopen(3S)            STANDARD I/O FUNCTIONS             fopen(3S)



NAME
     fopen, freopen, fdopen - open a stream

SYNOPSIS
     #include <stdio.h>

     FILE *fopen (const char *filename, const char *type);

     FILE *freopen (const char *filename, const char *type, FILE
         *stream);

     FILE *fdopen (int fildes, const char *type);

DESCRIPTION
     fopen opens the file named  by  filename  and  associates  a
     stream  with it.  fopen returns a pointer to the FILE struc-
     ture associated with the stream.

     filename points to a character string that contains the name
     of the file to be opened.

     type is a character string beginning with one of the follow-
     ing sequences:

       "r" or "rb"
                open for reading

       "w" or "wb"
                truncate to zero length or create for writing

       "a" or "ab"
                append; open for  writing  at  end  of  file,  or
                create for writing

       "r+", "r+b" or "rb+"
                open for update (reading and writing)

       "w+", "w+b" or "wb+"
                truncate or create for update

       "a+", "a+b" or "ab+"
                append; open or create for update at end-of-file

     The ``b'' is ignored in the above types. The ``b'' exists to
     distinguish binary files from text files.  However, there is
     no distinction between these types of files on a  UNIX  sys-
     tem.

     freopen substitutes the named file  in  place  of  the  open
     stream.   A  flush is first attempted, and then the original
     stream is closed, regardless of whether the open  ultimately
     succeeds.   Failure to flush or close stream successfully is



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fopen(3S)            STANDARD I/O FUNCTIONS             fopen(3S)



     ignored.  freopen returns a pointer to  the  FILE  structure
     associated with stream.

     freopen is typically used to attach  the  preopened  streams
     associated  with  stdin,  stdout, and stderr to other files.
     stderr is by default unbuffered, but the use of freopen will
     cause it to become buffered or line-buffered.

     fdopen associates a stream with  a  file  descriptor.   File
     descriptors  are  obtained  from  open, dup, creat, or pipe,
     which open files but do not return pointers to a FILE struc-
     ture  stream.  Streams are necessary input for almost all of
     the Section 3S library routines.  The type  of  stream  must
     agree  with  the  mode  of the open file.  The file position
     indicator associated with stream  is  set  to  the  position
     indicated by the file offset associated with fildes.

     When a file is opened for update, both input and output  may
     be done on the resulting stream.  However, output may not be
     directly followed by input without  an  intervening  fflush,
     fseek,  fsetpos,  or  rewind,  and input may not be directly
     followed by output without an intervening fseek, fsetpos, or
     rewind, or an input operation that encounters end-of-file.

     When a file is opened for append (i.e., when  type  is  "a",
     "ab",  "a+", or "ab+"), it is impossible to overwrite infor-
     mation already in the file.  fseek may be used to reposition
     the  file pointer to any position in the file, but when out-
     put is written to the file,  the  current  file  pointer  is
     disregarded.   All  output is written at the end of the file
     and causes the file pointer to be repositioned at the end of
     the  output.   If  two separate processes open the same file
     for append, each  process  may  write  freely  to  the  file
     without  fear  of  destroying  output  being  written by the
     other.  The output from the two processes will be intermixed
     in the file in the order in which it is written.

     When opened, a stream is fully buffered if and  only  if  it
     can  be  determined  not  to refer to an interactive device.
     The error and end-of-file indicators  are  cleared  for  the
     stream.

SEE ALSO
     close(2),  creat(2),  dup(2),  open(2),  pipe(2),  write(2),
     fclose(3S), fseek(3S), setbuf(3S), stdio(3S).

DIAGNOSTICS
     The functions fopen and freopen return  a  null  pointer  if
     path  cannot  be  accessed, or if type is invalid, or if the
     file cannot be opened.





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fopen(3S)            STANDARD I/O FUNCTIONS             fopen(3S)



     The function fdopen returns a null pointer if fildes is  not
     an  open  file  descriptor, or if type is invalid, or if the
     file cannot be opened.

     The functions fopen or fdopen may fail and not set errno  if
     there are no free stdio streams.

     File descriptors used by fdopen must be less than 255.















































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