fopen, freopen, fdopen - open a stream

#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;
const char *type;

Fopen opens the file named by filename and associates a stream with
it. Fopen returns a pointer to be used to identify the stream in
subsequent operations.

Type is a character string having one of the following values:

"r" open for reading

"w" create for writing

"a" append: open for writing at end of file, or create for writing

In addition, each type may be followed by a "+" to have the file
opened for reading and writing. "r+" positions the stream at the
beginning of the file, "w+" creates or truncates it, and "a+"
positions it at the end. Both reads and writes may be used on
read/write streams, with the limitation that an fseek, rewind, or
reading an end-of-file must be used between a read and a write or

Freopen substitutes the named file in place of the open stream. It
returns the original value of stream. The original stream is

Freopen is typically used to attach the preopened constant names,
stdin, stdout, stderr, to specified files.

Fdopen associates a stream with a file descriptor obtained from
open, dup, creat, or pipe(2). The type of the stream must agree
with the mode of the open file.

open(2), fclose(3)

Fopen and freopen return the pointer NULL if filename cannot be
accessed, if too many files are already open, or if other resources
needed cannot be allocated.

Fdopen is not portable to systems other than UNIX.

The read/write types do not exist on all systems. Those systems
without read/write modes will probably treat the type as if the "+"
was not present. These are unreliable in any event.

In order to support the same number of open files as does the
system, fopen must allocate additional memory for data structures
using calloc after 20 files have been opened. This confuses some
programs which use their own memory allocators. An undocumented
routine, f_prealloc, may be called to force immediate allocation of
all internal memory except for buffers.