FOPEN(3) SysV FOPEN(3)
NAME
fopen, freopen, fdopen - Opens a stream
SYNOPSIS
#include <stdio.h>
FILE *fopen (filename, type)
const char *filename, *type;
FILE *freopen (filename, type, stream)
const char *filename, *type;
FILE *stream;
FILE *fdopen (fildes, type)
int fildes;
char *type;
DESCRIPTION
The fopen function opens the file named by the path argument and
associates a stream with it, returning a pointer to the FILE structure of
this stream.
path Points to a character string that contains the name of the file to
be opened. If the final component of the path argument specifies a
symbolic link, the link is traversed and pathname resolution
continues.
type Points to a character string that has one of the following values:
r Open file for reading.
w Create a new file for writing, or open and truncate to zero
length. (The file is not truncated under the fdopen function.)
a Append (open file for writing at the end of the file, or create
for writing).
rb Open file for reading.
wb Create a file for writing, or open and truncate to zero length.
ab Append (open file for update, writing at the end of the file,
or create for writing).
r+ Open for update (reading and writing).
w+ Truncate or create for update. (The file is not truncated
under the fdopen function.)
a+ Append (open file for update, writing at End-of-File, or create
for writing).
r+b or rb+
Open file for update (reading and writing).
w+b or wb+
Create file for update, or open and truncate to zero length.
a+b or ab+
Append (open a file for update, writing at the end of the file,
or create for writing).
The presence or absence of the character "b" in any of the above
list of mode argument values has no effect.
stream
Specifies the input stream.
filedes
Specifies a valid open file descriptor.
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
fflush(3S function call or a file positioning operation (fseek(3S),
fsetpos(3S), or rewind(3S) function). Also, an input operation cannot be
directly followed by an output operation without an intervening flush or
file positioning operation, unless the input operation encounters the end
of the file.
When you open a file for append (that is, when the type argument is a or
a+), it is impossible to overwrite information already in the file. You
can use the fseek(3S) function to reposition 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
the file pointer is repositioned to the end of the output.
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 are cleared.
The fopen function allocates a file descriptor as open does. If the type
argument is w, a, w+, or a+ and the file did not previously exist, upon
successful completion the fopen function marks the st_atime, st_ctime and
st_mtime fields of the file and the st_ctime and st_mtime fields of the
parent directory for update. If the type argument is w or w+ and the
file did previously exist, upon successful completion the fopen function
marks the st_ctime and st_mtime fields of the file for update.
The freopen function substitutes the named file in place of the open
stream. The original stream is closed regardless of whether the open
function succeeds with the named file. The freopen function returns a
pointer to the FILE structure associated with the stream argument. The
freopen function is typically used to attach the preopened streams
associated with stdin, stdout, and stderr to other files.
The fdopen function associates a stream with a file descriptor obtained
from an open(2), dup(2), creat(2), or pipe(2) function. These functions
open files but do not return pointers to FILE structures. Many of the
standard I/O package functions require pointers to FILE structures. Note
that the type of stream specified must agree with the mode of the open
file.
DIAGNOSTICS
If the fopen, fdopen, or freopen function fails, a null pointer is
returned and errno is set to indicate the error.
ERRORS
fopen and freopen fail if one of the following are true:
[EACCES] Search permission is denied on a component of the path
prefix, or the file exists and the permissions specified
by the type argument are denied, or the file does not
exist and write permission is denied for the parent
directory of the file to be created.
[EINTR] The fopen function was interrupted by a signal which was
caught.
[EISDIR] The named file is a directory and type requires write
access.
[EMFILE] OPEN_MAX file descriptors are currently open in the
calling process.
[ENAMETOOLONG]
The length of the path string exceeds PATH_MAX or a
pathname component is longer than NAME_MAX.
[ENFILE] Too many files are currently open in the system.
[ENOENT] The named file does not exist or the path argument points
to an empty string.
[ENOSPC] The directory or file system that would contain the new
file cannot be expanded.
[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.
[EROFS] The named file resides on a read only file system and type
requires write access.
No errors are defined for fdopen
SEE ALSO
open(2), fclose(3), fseek(3), setbuf(3)