fopen, freopen, fdopen

Purpose

     Opens a stream.

Library

     Standard I/O Library (libc.a)

Syntax

     #include <stdio.h>

     FILE *fopen (path, type)                  FILE *fdopen (fildes, type)
     char *path, *type;                        int fildes;
                                               char *type;
     FILE *freopen (path, type, stream)
     char *path, *type;
     FILE *stream;
     Description

     The fopen  subroutine opens  the file  named by  the path
     parameter and associates a stream with it.  fopen returns
     a pointer to the FILE structure of this stream.

     The path parameter points to a character string that con-
     tains the name of the file to be opened.

     The type parameter points to  a character string that has
     one of the following values:

     ""r""   Open the file for reading.
     ""w""   Truncate or create a new file for writing.
     ""a""   Append  (open  for writing  at  end  of file,  or
             create for writing).
     ""r+""  Open for update (reading and writing).
     ""w+""  Truncate or create for update.
     ""a+""  Append  (open  or create  for  update  at end  of
             file).

     The  freopen subroutine  substitutes  the  named file  in
     place of the open stream.   The original stream is closed
     whether  or not  the  open succeeds.   freopen returns  a
     pointer  to the  FILE structure  associated with  stream.
     The freopen  subroutine is  typically used to  attach the
     pre-opened  streams associated  with  stdin, stdout,  and
     stderr to other files.

     The  fdopen subroutine  associates a  stream with  a file
     descriptor  obtained from  an open,  dup, creat,  or pipe
     system call.   These system calls  open files but  do not
     return pointers to FILE structures.  Many of the standard
     I/O library  subroutines require pointers to  FILE struc-

     tures.  Note that the type of stream specified must agree
     with the mode of the open file.

     When you  open a  file for update,  you can  perform both
     input  and output  operations  on  the resulting  stream.
     However, an output operation  cannot be directly followed
     by  an input  operation without  an intervening  fseek or
     rewind.  Also, an input operation cannot be directly fol-
     lowed  by  an  output operation  without  an  intervening
     fseek, rewind, or an  input operation that encounters the
     end of the file.

     When you  open a file for  append (that is, when  type is
     ""a"" or ""a+""), it  is impossible to overwrite informa-
     tion already in  the file.  You can use  fseek to reposi-
     tion the  file pointer to  any position in the  file, but
     when  output is  written to  the file,  the current  file
     pointer is ignored.  All output  is written at the end of
     the file and  causes the file pointer  to be repositioned
     to the end of the output.

     If two separate processes open  the same file for append,
     each  process  can  write  freely  to  the  file  without
     destroying the  output being  written by the  other.  The
     output from the two processes  is intermixed in the order
     in which  it is written  to the  file.  Note that  if the
     data is buffered,  then it is not  actually written until
     it is flushed.

     If the fopen or freopen  subroutine fails, a NULL pointer
     is returned.

     Related Information

     In this book:  "creat," "open," "fclose, fflush," "fseek,
     rewind,  ftell,"  "setbuf,  setvbuf," and  "standard  i/o
     library."