fopen, freopen, fdopen - open a stream
FILE ∗fopen (filename, type)
char ∗filename, ∗type;
FILE ∗freopen (filename, type, stream)
char ∗filename, ∗type;
FILE ∗fdopen (fildes, type)
The routine opens the file named by filename and associates a stream
with it. The routine returns a pointer to the FILE structure associ‐
ated with the stream.
The filename points to a character string that contains the name of the
file to be opened.
The type is a character string having one of the following values:
"r" Open for reading
"w" Truncate or create for writing
"a" Append; open for writing at end of file, or create for
"A" Append with no overwrite; open for writing at end-of-file,
or create for writing
"r+" Open for reading and writing
"w+" Truncate or create for reading and writing
"a+" Append; open or create for reading and writing at end-of-
"A+" Append with no overwrite, open or create for update at
The letter "b" can also follow r, w, or a. In some C implementations,
the "b" is needed to indicate a binary file, however, it is not needed
in ULTRIX. If "+" is used, the "b" may occur on either side, as in
"rb+" or "w+b".
The routine substitutes the named file in place of the open stream.
The original stream is closed, regardless of whether the open ulti‐
mately succeeds. The routine returns a pointer to the FILE structure
associated with stream.
The routine is typically used to attach the preopened streams associ‐
ated with stdin, stdout and stderr to other files.
The routine associates a stream with a file descriptor. File descrip‐
tors are obtained from or which open files but do not return pointers
to a FILE structure stream. Streams are necessary input for many of
the Section 3s library routines. The type of stream must agree with
the mode of the open file.
When a file is opened for update, both input and output may be done on
the resulting stream. However, output may not be directly followed by
input without an intervening or and input may not be directly followed
by output without an intervening or an input operation which encounters
When a file is opened for append with no overwrite (that is when type
is "A" or "A+"), it is impossible to overwrite information already in
the file. The routine may be used to reposition the file pointer to
any position in the file, but when output is written to the file, the
current file pointer is disregarded. All output is written at the end
of the file and causes the file pointer to be repositioned at the end
of the output. If two separate processes open the same file for
append, each process may write freely to the file without fear of
destroying output being written by the other. The output from the two
processes will be intermixed in the file in the order in which it is
The and routines return a NULL pointer on failure.
When your program is compiled using the System V environment, append
with no overwrite is specified by using the "a" or "a+" type string,
and the "A" and "A+" type strings are not allowed.
In the POSIX environment, the "a" and "a+" strings, and the "A" and
"A+" strings specify append with no overwrite.
See Alsocreat(2), dup(2), open(2), pipe(2), fclose(3s), fseek(3s).