minix/lib/libc/stdio/fgets.3

.\"	$NetBSD: fgets.3,v 1.22 2010/05/13 18:38:24 jruoho Exp $
.\"
.\" Copyright (c) 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)fgets.3	8.1 (Berkeley) 6/4/93
.\"
.Dd May 13, 2010
.Dt FGETS 3
.Os
.Sh NAME
.Nm fgets ,
.Nm gets
.Nd get a line from a stream
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft char *
.Fn fgets "char * restrict str" "int size" "FILE * restrict stream"
.Ft char *
.Fn gets "char *str"
.Sh DESCRIPTION
The
.Fn fgets
function
reads at most one less than the number of characters specified by
.Fa size
from the given
.Fa stream
and stores them in the string
.Fa str .
Reading stops when a newline character is found,
at end-of-file or error.
The newline, if any, is retained, and a
.Ql \e0
character is appended to end the string.
.Pp
The
.Fn gets
function
is equivalent to
.Fn fgets
with an infinite
.Fa size
and a
.Fa stream
of
.Em stdin ,
except that the newline character (if any) is not stored in the string.
It is the caller's responsibility to ensure that the input line,
if any, is sufficiently short to fit in the string.
.Sh RETURN VALUES
Upon successful completion,
.Fn fgets
and
.Fn gets
return
a pointer to the string.
If end-of-file or an error occurs before any characters are read,
they return
.Dv NULL .
The
.Fn fgets
and
.Fn gets
functions
do not distinguish between end-of-file and error, and callers must use
.Xr feof 3
and
.Xr ferror 3
to determine which occurred.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The given
.Fa stream
is not a readable stream.
.El
.Pp
The function
.Fn fgets
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr fflush 3 ,
.Xr fstat 2 ,
.Xr read 2 ,
or
.Xr malloc 3 .
.Pp
The function
.Fn gets
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr getchar 3 .
.Sh SEE ALSO
.Xr feof 3 ,
.Xr ferror 3 ,
.Xr fgetln 3
.Sh STANDARDS
The functions
.Fn fgets
and
.Fn gets
conform to
.St -ansiC
and
.St -p1003.1-2001 .
The
.St -p1003.1-2008
revision marked
.Fn gets
as obsolescent.
.Sh CAVEATS
The following bit of code illustrates a case where the programmer assumes a
string is too long if it does not contain a newline:
.Bd -literal
	char buf[1024], *p;

	while (fgets(buf, sizeof(buf), fp) != NULL) {
		if ((p = strchr(buf, '\en')) == NULL) {
			fprintf(stderr, "input line too long.\en");
			exit(1);
		}
		*p = '\e0';
		printf("%s\en", buf);
	}
.Ed
.Pp
While the error would be true if a line longer than 1023 characters
were read, it would be false in two other cases:
.Bl -enum -offset indent
.It
If the last line in a file does not contain a newline, the string returned by
.Fn fgets
will not contain a newline either.
Thus
.Fn strchr
will return
.Dv NULL
and the program will terminate, even if the line was valid.
.It
All C string functions, including
.Fn strchr ,
correctly assume the end of the string is represented by a null
.Pq Sq \e0
character.
If the first character of a line returned by
.Fn fgets
were null,
.Fn strchr
would immediately return without considering the rest of the returned text
which may indeed include a newline.
.El
.Pp
Consider using
.Xr fgetln 3
instead when dealing with untrusted input.
.Sh SECURITY CONSIDERATIONS
Since it is usually impossible to ensure that the next input line
is less than some arbitrary length, and because overflowing the
input buffer is almost invariably a security violation, programs
should
.Em NEVER
use
.Fn gets .
The
.Fn gets
function
exists purely to conform to
.St -ansiC .