195 lines
5.0 KiB
Groff
195 lines
5.0 KiB
Groff
.\" $NetBSD: fgets.3,v 1.19 2003/12/09 21:40:54 grant Exp $
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.\"
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.\" Copyright (c) 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Chris Torek and the American National Standards Committee X3,
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.\" on Information Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fgets.3 8.1 (Berkeley) 6/4/93
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.\"
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.Dd June 4, 1993
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.Dt FGETS 3
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.Os
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.Sh NAME
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.Nm fgets ,
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.Nm gets
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.Nd get a line from a stream
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdio.h
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.Ft char *
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.Fn fgets "char * restrict str" "int size" "FILE * restrict stream"
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.Ft char *
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.Fn gets "char *str"
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.Sh DESCRIPTION
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The
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.Fn fgets
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function
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reads at most one less than the number of characters specified by
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.Fa size
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from the given
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.Fa stream
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and stores them in the string
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.Fa str .
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Reading stops when a newline character is found,
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at end-of-file or error.
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The newline, if any, is retained, and a
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.Ql \e0
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character is appended to end the string.
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.Pp
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The
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.Fn gets
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function
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is equivalent to
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.Fn fgets
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with an infinite
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.Fa size
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and a
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.Fa stream
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of
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.Em stdin ,
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except that the newline character (if any) is not stored in the string.
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It is the caller's responsibility to ensure that the input line,
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if any, is sufficiently short to fit in the string.
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn fgets
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and
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.Fn gets
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return
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a pointer to the string.
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If end-of-file or an error occurs before any characters are read,
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they return
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.Dv NULL .
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The
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.Fn fgets
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and
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.Fn gets
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functions
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do not distinguish between end-of-file and error, and callers must use
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.Xr feof 3
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and
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.Xr ferror 3
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to determine which occurred.
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.Sh ERRORS
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.Bl -tag -width Er
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.It Bq Er EBADF
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The given
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.Fa stream
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is not a readable stream.
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.El
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.Pp
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The function
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.Fn fgets
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may also fail and set
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.Va errno
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for any of the errors specified for the routines
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.Xr fflush 3 ,
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.Xr fstat 2 ,
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.Xr read 2 ,
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or
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.Xr malloc 3 .
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.Pp
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The function
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.Fn gets
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may also fail and set
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.Va errno
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for any of the errors specified for the routine
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.Xr getchar 3 .
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.Sh SEE ALSO
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.Xr feof 3 ,
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.Xr ferror 3 ,
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.Xr fgetln 3
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.Sh STANDARDS
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The functions
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.Fn fgets
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and
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.Fn gets
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conform to
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.St -ansiC .
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.Sh CAVEATS
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The following bit of code illustrates a case where the programmer assumes a
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string is too long if it does not contain a newline:
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.Bd -literal
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char buf[1024], *p;
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while (fgets(buf, sizeof(buf), fp) != NULL) {
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if ((p = strchr(buf, '\en')) == NULL) {
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fprintf(stderr, "input line too long.\en");
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exit(1);
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}
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*p = '\e0';
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printf("%s\en", buf);
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}
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.Ed
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.Pp
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While the error would be true if a line > 1023 characters were read, it would
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be false in two other cases:
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.Bl -enum -offset indent
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.It
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If the last line in a file does not contain a newline, the string returned by
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.Fn fgets
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will not contain a newline either.
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Thus
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.Fn strchr
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will return
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.Dv NULL
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and the program will terminate, even if the line was valid.
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.It
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All C string functions, including
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.Fn strchr ,
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correctly assume the end of the string is represented by a null
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.Pq Sq \e0
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character.
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If the first character of a line returned by
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.Fn fgets
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were null,
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.Fn strchr
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would immediately return without considering the rest of the returned text
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which may indeed include a newline.
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.El
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.Pp
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Consider using
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.Xr fgetln 3
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instead when dealing with untrusted input.
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.Sh SECURITY CONSIDERATIONS
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Since it is usually impossible to ensure that the next input line
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is less than some arbitrary length, and because overflowing the
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input buffer is almost invariably a security violation, programs
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should
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.Em NEVER
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use
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.Fn gets .
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The
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.Fn gets
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function
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exists purely to conform to
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.St -ansiC .
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