FOPEN(3) BSD Programmer's Manual FOPEN(3)NAME
fopen, fdopen, freopen - stream open functions
SYNOPSIS
#include <stdio.h>
FILE *
fopen(const char *path, const char *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 be-
ginning 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 appending. The file is created if it does not exist.
The stream is positioned at the end of the file.
``a+'' Open for reading and appending. The file is created if it does
not exist. The stream is positioned at the end of the file.
The mode string can also include the letter ``b'' either as a third char-
acter or as a character between the characters in any of the two-charac-
ter strings described above. This is strictly for compatibility with AN-
SI C X3.159-1989 (``ANSI C '') 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(2)).
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 descrip-
tor, fildes. The mode of the stream must be compatible with the mode of
the file descriptor.
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. The primary use of the freopen() function is
to change the file associated with a standard text stream (stderr, stdin,
or stdout).
Note that on any stream opened for appending, all writes are forced to
the end of the file. While this behavior differs from traditional BSD
systems, it is required by several standards. Appending is done with
O_APPEND mode whenever possible, as this is more efficient and does not
suffer from race conditions when separate processes are writing to the
same file (e.g., when appending to a log file). For fdopen() streams,
however, the underlying file flags are left unchanged; if the file de-
scriptor does not already have O_APPEND set, appending writes are done
with individual seeks.
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 provided to fopen(), fdopen(), or freopen() was in-
valid.
The fopen(), fdopen() and freopen() functions may also fail and set errno
for any of the errors specified for the routine malloc(3).
The fopen() function may also fail and set errno for any of the errors
specified for the routine open(2).
The fdopen() function may also fail and set errno for any of the errors
specified for the routine fcntl(2).
The freopen() function may also fail and set errno for any of the errors
specified for the routines open(2), fclose(3) and fflush(3).
SEE ALSOopen(2), fclose(3), fseek(3), funopen(3)STANDARDS
The fopen() and freopen() functions conform to ANSI C X3.159-1989 (``ANSI
C ''). The fdopen() function conforms to IEEE Std1003.1-1988 (``POSIX'').
BUGS
There should be mode strings corresponding to O_RDWR | O_CREAT and
O_WRONLY | O_CREAT (note that ``w'' and ``w+'' include the effect of
O_TRUNC, while ``r'' and ``r+'' lack the effect of O_CREAT).
BSDI BSD/OS June 4, 1993 2
