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ComputingCStdio.h

fdopen

Stream open functions
+ View other versions (3)

Interface

#include <stdio.h>
FILE fopen (const char * restrict path, const char * restrict mode)
FILE fdopen (int fildes, const char *mode)
FILE freopen (const char *path, const char *mode, FILE *stream)

Description

The fopen function opens the file whose name is the string pointed to by path and associates a stream with it.

The argument mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):
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 writing. The file is created if it does not exist. The stream is positioned at the end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening reference:fseek or similar.
a+ Open for reading and writing. The file is created if it does not exist. The stream is positioned at the end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening reference:fseek or similar.

The mode string can also include the letter ``b'' either as a third character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with and has no effect; the ``b'' is ignored.

Any created files will have mode "S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH" (0666 as modified by the process' umask value (see umask).

Reads and writes may be intermixed on read/write streams in any order, and do not require an intermediate seek as in previous versions of stdio. This is not portable to other systems, however; ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-file.

The fdopen function associates a stream with the existing file descriptor, fildes. The mode of the stream must be compatible with the mode of the file descriptor. When the stream is closed via fclose, fildes is closed also.

The freopen function opens the file whose name is the string pointed to by path 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 path argument is NULL, freopen attempts to re-open the file associated with stream with a new mode. The new mode must be compatible with the mode that the stream was originally opened with:

  • Streams originally opened with mode "r" can only be reopened with that same mode.
  • Streams originally opened with mode "a" can be reopened with the same mode, or mode w.
  • Streams originally opened with mode "w" can be reopened with the same mode, or mode a.
  • Streams originally opened with mode "r+", "w+", or "a+" can be reopened with any mode.

The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin, or stdout).

The code below attempts to open the text file "fred.txt" in the current directory and read each character in it until the EOF symbol is encountered. The fopen function has been used to open the file.

Example:

Example - Stream open functions
Workings
#include <stdio.h>
int main()
{
  FILE *in;
  if (in = fopen("fred.txt", "rt"))
  {
    for (char c; !feof(in); fscanf(in, "%c", &c));
    fclose(in);
  }
  return 0;
}

Return Values

Upon successful completion fopen, fdopen and freopen return a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to indicate the error.

Errors

EINVAL The mode argument 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 reference:malloc.

The fopen function may also fail and set errno for any of the errors specified for the routine open.

The fdopen function may also fail and set errno for any of the errors specified for the routine fcntl.

The freopen function may also fail and set errno for any of the errors specified for the routines open, fclose and fflush.