$ man fopenをすると出てくる内容です．
下記を読むと，fopen()関数を使うなら，その前に"#include <stdio.h>"と書きなさいと指示があることがわかります．
FOPEN(3) Linux 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 fd, const char *mode);
FILE *freopen(const char *path, const char *mode, FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
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 follow-
ing sequences (Additional characters may follow these sequences.):
r Open text file for reading. The stream is positioned at the
beginning 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 (writing at end of file). The file is cre-
ated if it does not exist. The stream is positioned at the end
of the file.
a+ Open for reading and appending (writing at end of file). The
file is created if it does not exist. The initial file position
for reading is at the beginning of the file, but output is
always appended to the end of the file.
The mode string can also include the letter 'b' either as a last char-
acter or as a character between the characters in any of the two-char-
acter strings described above. This is strictly for compatibility with
C89 and has no effect; the 'b' is ignored on all POSIX conforming sys-
tems, including Linux. (Other systems may treat text files and binary
files differently, and adding the 'b' may be a good idea if you do I/O
to a binary file and expect that your program may be ported to non-Unix
environments.)
See NOTES below for details of glibc extensions for mode.
Any created files will have mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP
| S_IROTH | S_IWOTH (0666), as modified by the process's umask value
(see umask(2)).
Reads and writes may be intermixed on read/write streams in any order.
Note that ANSI C requires that a file positioning function intervene
between output and input, unless an input operation encounters end-of-
file. (If this condition is not met, then a read is allowed to return
the result of writes other than the most recent.) Therefore it is good
practice (and indeed sometimes necessary under Linux) to put an
fseek(3) or fgetpos(3) operation between write and read operations on
such a stream. This operation may be an apparent no-op (as in
fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect.
Opening a file in append mode (a as the first character of mode) causes
all subsequent write operations to this stream to occur at end-of-file,
as if preceded by an
fseek(stream,0,SEEK_END);
call.
The fdopen() function associates a stream with the existing file
descriptor, fd. The mode of the stream (one of the values "r", "r+",
"w", "w+", "a", "a+") must be compatible with the mode of the file
descriptor. The file position indicator of the new stream is set to
that belonging to fd, and the error and end-of-file indicators are
cleared. Modes "w" or "w+" do not cause truncation of the file. The
file descriptor is not dup'ed, and will be closed when the stream cre-
ated by fdopen() is closed. The result of applying fdopen() to a
shared memory object is undefined.
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).
RETURN VALUE
Upon successful completion fopen(), fdopen() and freopen() return a
FILE pointer. Otherwise, NULL is returned and errno is set to indicate
the error.
ERRORS
EINVAL The mode provided to fopen(), fdopen(), or freopen() was
invalid.
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).
CONFORMING TO
The fopen() and freopen() functions conform to C89. The fdopen() func-
tion conforms to POSIX.1-1990.
NOTES
Glibc Notes
The GNU C library allows the following extensions for the string speci-
fied in mode:
c (since glibc 2.3.3)
Do not make the open operation, or subsequent read and write
operations, thread cancellation points.
e (since glibc 2.7)
Open the file with the O_CLOEXEC flag. See open(2) for more
information.
m (since glibc 2.3)
Attempt to access the file using mmap(2), rather than I/O system
calls (read(2), write(2)). Currently, use of mmap(2) is only
attempted for a file opened for reading.
x Open the file exclusively (like the O_EXCL flag of open(2)). If
the file already exists, fopen() fails, and sets errno to EEX-
IST. This flag is ignored for fdopen().
SEE ALSO
open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3)
COLOPHON
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
GNU 2009-02-23 FOPEN(3)