NAME

       fopen - open a stream

SYNOPSIS

       #include <stdio.h>

       FILE *fopen(const char *restrict filename, const char *restrict mode);

DESCRIPTION

       The  fopen()  function  shall  open  the  file  whose  pathname is the string pointed to by filename, and
       associates a stream with it.

       The mode argument points to a string. If the string is one of the following, the file shall be opened  in
       the indicated mode. Otherwise, the behavior is undefined.

       r or rb
              Open file for reading.

       w or wb
              Truncate to zero length or create file for writing.

       a or ab
              Append; open or create file for writing at end-of-file.

       r+ or rb+ or r+b
              Open file for update (reading and writing).

       w+ or wb+ or w+b
              Truncate to zero length or create file for update.

       a+ or ab+ or a+b
              Append; open or create file for update, writing at end-of-file.

       The  character  'b'  shall have no effect, but is allowed for ISO C standard conformance.  Opening a file
       with read mode (r as the first character in the mode argument) shall fail if the file does not  exist  or
       cannot be read.

       Opening  a  file  with  append  mode  (a  as  the  first  character in the mode argument) shall cause all
       subsequent writes to the file to be forced to the then current  end-of-file,  regardless  of  intervening
       calls to fseek().

       When a file is opened with update mode ( '+' as the second or third character in the mode argument), both
       input  and  output  may be performed on the associated stream. However, the application shall ensure that
       output is not directly followed by input without an intervening call to fflush() or to a file positioning
       function ( fseek(), fsetpos(), or rewind()), and input is not directly  followed  by  output  without  an
       intervening call to a file positioning function, unless the input operation encounters end-of-file.

       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 for the stream shall be cleared.

       If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the  file  did  not  previously  exist,  upon
       successful  completion,  the  fopen() function shall mark for update the st_atime, st_ctime, and st_mtime
       fields of the file and the st_ctime and st_mtime fields of the parent directory.

       If mode is w, wb, w+, wb+, or w+b, and the file did previously exist, upon successful completion, fopen()
       shall mark for update the st_ctime and st_mtime fields of the file. The fopen() function shall allocate a
       file descriptor as open() does.

       After a successful call to the fopen() 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.

       The largest value that can be represented correctly in an object of type off_t shall  be  established  as
       the offset maximum in the open file description.

RETURN VALUE

       Upon  successful  completion,  fopen()  shall  return  a  pointer  to  the object controlling the stream.
       Otherwise, a null pointer shall be returned,    and errno shall be set to indicate the error.

ERRORS

       The fopen() 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.

       EINTR  A signal was caught during fopen().

       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 {OPEN_MAX} file descriptors are currently open in the calling process.

       ENAMETOOLONG

              The length of the filename argument exceeds {PATH_MAX} or a  pathname  component  is  longer  than
              {NAME_MAX}.

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

       ENOENT A component of filename does not name an existing file or filename is an empty string.

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

       ENOTDIR
              A component of the path prefix is not 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.

       EOVERFLOW
              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 fopen() function may fail if:

       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.

       EMFILE {FOPEN_MAX} streams are currently open in the calling process.

       EMFILE {STREAM_MAX} streams are currently open in the calling process.

       ENAMETOOLONG

              Pathname  resolution  of  a  symbolic  link  produced  an intermediate result whose length exceeds
              {PATH_MAX}.

       ENOMEM Insufficient storage space is available.

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

       The following sections are informative.

EXAMPLES

   Opening a File
       The  following example tries to open the file named file for reading. The fopen() function returns a file
       pointer that is used in subsequent fgets() and fclose() calls.  If the program cannot open the  file,  it
       just ignores it.

              #include <stdio.h>
              ...
              FILE *fp;
              ...
              void rgrep(const char *file)
              {
              ...
                  if ((fp = fopen(file, "r")) == NULL)
                      return;
              ...
              }

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       fclose() , fdopen() , freopen() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc and The Open Group. 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 http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                              FOPEN(P)