fopen, freopen, fdopen - Open a stream
const char *path,
const char *mode ); FILE *fdopen(
const char *mode ); FILE *freopen(
const char *path,
const char *mode,
FILE *stream );
Standard C Library (libc)
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
fopen(), freopen(): ISO C, XSH4.0, XSH4.2, XSH5.0
fdopen(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
Points to a character string that contains the name of the file to be
opened. If the final component of the path parameter specifies a sym‐
bolic link, the link is traversed and pathname resolution continues.
Points to a character string that controls whether the file is opened
for reading (r), writing (w), or appending (a) and whether the file is
opened for updating (+). The mode can also include a b parameter, but
this is ignored (see DESCRIPTION). Specifies the input stream. Speci‐
fies a valid open file descriptor.
The fopen() function opens the file named by the path parameter and as‐
sociates a stream with it, returning a pointer to the FILE structure of
The mode parameter controls the access allowed to the stream. The
parameter can have one of the following values. In this list of values,
the b character indicates a binary file. UNIX systems do not distin‐
guish between binary and text files in the context of opening a stream,
and the b character is therefore ignored. However, the b character is
included in X/Open standards for alignment with ISO C syntax that can
apply to other kinds of operating systems. Opens the file for reading.
Creates a new file for writing, or opens and truncates a file to zero
length. (The file is not truncated under the fdopen() function.)
Appends (opens a file for writing at the end of the file, or creates a
file for writing). Opens a file for update (reading and writing).
Truncates to zero length or creates a file for update. (The file is not
truncated under the fdopen() function.) Appends (opens a file for
update, writing at the end of the file, or creates a file for writing).
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 call
to the fflush() function or a file-positioning operation (fseek(),
fsetpos(), or rewind() 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 mode parameter is a
or a+), it is impossible to overwrite information already in the file.
You can use the fseek() function to reposition the file pointer to any
position in the file, but when output is written to the file, the cur‐
rent 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.
[Tru64 UNIX] 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 inter‐
mixed in the order in which it is written to the file. Note that if the
data is buffered, it is not actually written until it is flushed.
When opened, a stream is fully buffered if and only if it can be deter‐
mined that it does not refer to an interactive device. The error and
End-of-File indicators for the stream are cleared.
If the mode parameter is w, a, w+, or a+ and the file did not previ‐
ously 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 mode parame‐
ter is w or w+ and the file did previously exist, upon successful com‐
pletion 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 parameter.
The freopen() function is typically used to attach the preopened
streams associated with stdin, stdout, and stderr to other files. The
function uses the mode argument in the same way as described for
The fdopen() function associates a stream with a file descriptor
obtained from an open(), dup(), creat(), or pipe() 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 mode of the stream specified must agree with
the mode of the open file. The meaning of mode values is the same as
described for fopen(), except that for fdopen(), the values beginning
with w do not cause truncation of the file.
The fdopen() function sets the file position indicator associated with
the new stream to the position indicated by the file offset associated
with the file descriptor. The function also clears the error and End-
of-File indicators for the stream.
[ISO C] The current version of the ISO C standard specifies that
streams have an orientation: unbounded immediately after the stream is
opened, byte oriented if operated on by a byte-oriented I/O function,
and wide-character oriented if operated on by a wide-character I/O
function. Furthermore, the standard defines the fwide() function for
explicitly querying stream orientation and, if preceded by a call to
freopen(), for changing stream orientation without actually operating
on the stream (see fwide(3)). The reasons for these additions to the
standard are as follows. Both I/O functions that are byte oriented and
those that are wide-character oriented can refer to the same FILE
object. For wide-character functions, the FILE object is assumed to
include an mb_state object to keep track of conversion-state informa‐
tion that currently applies to the stream. Wide-character functions
are designed with the assumption that they always begin execution with
the stream positioned at the boundary between two multibyte characters.
The conversion-state information in the mb_state object is needed for
correct positioning when an application executes in a locale that sup‐
ports shift-state encoding. While wide-character functions can main‐
tain consistency between the conversion state and the stream, byte-ori‐
ented functions cannot. Therefore, a wide-character function cannot be
assured of correct boundary positioning in an open stream if that same
stream is also operated on by a byte-oriented function. The additions
to the ISO C standard allow application developers a way to make sure
that a stream opened by another function has the correct orientation
(either unbounded or wide-character) before its associated FILE object
is referred to by a wide-character function.
Currently, Tru64 UNIX systems do not provide locales that support
shift-state encoding. Therefore, the ISO C rules and definitions
related to shift-state encoding and stream orientation have no practi‐
cal use when applications run on Tru64 UNIX systems. However, these
rules and definitions are included for your convenience in using Tru64
UNIX to develop applications that run on multiple platforms, some of
which may support locales with shift-state encoding.
On successful completion, the fopen(), fdopen(), and freopen() func‐
tions return a pointer to the FILE object controlling the stream.
If these functions fail, they return a null pointer and set errno to
indicate the error.
The fopen(), fdopen(), and freopen() functions set errno to the speci‐
fied values for the following conditions:
Search permission is denied on a component of the pathname prefix; or
the file exists and the permissions specified by the mode parameter are
denied; or the file does not exist and write permission is denied for
the parent directory of the file to be created (fopen() or freopen()).
The filedes parameter is not a valid file descriptor (fdopen() only).
The function (fopen() or freopen()) was interrupted by a signal that
was caught. The mode parameter is not a valid mode. The named file is
a directory and mode requires write access (fopen() or freopen()). Too
many links were encountered in translating path. OPEN_MAX file
descriptors are currently open in the calling process (freopen()).
[Tru64 UNIX] Either the OPEN_MAX value or the per-process soft
descriptor limit is checked.
The FOPEN_MAX or STREAM_MAX streams are currently open in the
calling process (fopen() or fdopen()). The length of the path
string exceeds PATH_MAX or a pathname component is longer than
NAME_MAX (fopen() or freopen()). Too many files are currently
open in the system (fopen() or freopen()). The named file does
not exist or the path parameter points to an empty string
(fopen() or freopen()). There is insufficient space to allocate
a buffer. The directory or file system that would contain the
new file cannot be expanded (fopen() or freopen()). A component
of the pathname prefix is not a directory (fopen() or fre‐
open()). The named file is a character-special or block-special
file and the device associated with this special file does not
exist (fopen() or freopen()). [Tru64 UNIX] The named file is a
socket bound to the file system (a UNIX domain socket) and can‐
not be opened. The named file resides on a read-only file sys‐
tem and mode requires write access (fopen() or freopen()). The
file is being executed and the mode requires write access
(fopen() or freopen()).
Functions: open(2), fclose(3), fseek(3), fwide(3), setbuf(3), setlo‐
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