2004-11-03 13:51:07 +00:00
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.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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2007-04-12 22:42:49 +00:00
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.\"
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2004-11-03 13:51:07 +00:00
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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2007-04-12 22:42:49 +00:00
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.\"
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2004-11-03 13:51:07 +00:00
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aeb@cwi.nl)
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.\" Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aeb@cwi.nl)
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2007-06-07 09:15:03 +00:00
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.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
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.\" Improve discussion of strncpy().
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2004-11-03 13:51:07 +00:00
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.\"
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2012-07-19 09:29:15 +00:00
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.TH STRCPY 3 2012-07-19 "GNU" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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strcpy, strncpy \- copy a string
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.SH SYNOPSIS
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.nf
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.B #include <string.h>
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.sp
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.BI "char *strcpy(char *" dest ", const char *" src );
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.sp
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.BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
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.fi
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.SH DESCRIPTION
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2007-05-12 09:06:04 +00:00
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The
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.BR strcpy ()
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function copies the string pointed to by \fIsrc\fP,
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2008-06-09 15:49:35 +00:00
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including the terminating null byte (\(aq\\0\(aq),
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2007-06-07 09:15:03 +00:00
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to the buffer pointed to by \fIdest\fP.
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2007-04-24 16:44:51 +00:00
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The strings may not overlap, and the destination string
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\fIdest\fP must be large enough to receive the copy.
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2012-07-18 14:20:55 +00:00
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.IR "Beware of buffer overruns!"
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(See BUGS.)
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.PP
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2007-05-12 09:06:04 +00:00
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The
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.BR strncpy ()
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2007-06-07 09:15:03 +00:00
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function is similar, except that at most
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2007-04-12 22:42:49 +00:00
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\fIn\fP bytes of \fIsrc\fP are copied.
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2007-06-07 09:15:03 +00:00
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.BR Warning :
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If there is no null byte
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among the first \fIn\fP bytes of \fIsrc\fP,
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2010-01-16 17:56:17 +00:00
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the string placed in \fIdest\fP will not be null-terminated.
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2004-11-03 13:51:07 +00:00
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.PP
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2007-06-07 09:15:03 +00:00
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If the length of
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2004-11-03 13:51:07 +00:00
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.I src
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is less than
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2004-11-03 13:51:07 +00:00
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.IR n ,
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2007-06-18 20:01:32 +00:00
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.BR strncpy ()
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writes additional null bytes to
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.I dest
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2012-07-18 13:52:36 +00:00
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to ensure that a total of
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.I n
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bytes are written.
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2007-06-07 09:15:03 +00:00
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.PP
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A simple implementation of
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.BR strncpy ()
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might be:
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2007-12-19 06:57:44 +00:00
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.in +4n
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2007-06-07 09:15:03 +00:00
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.nf
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2010-10-11 06:09:07 +00:00
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char *
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strncpy(char *dest, const char *src, size_t n)
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{
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size_t i;
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2010-10-11 06:09:07 +00:00
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for (i = 0; i < n && src[i] != \(aq\\0\(aq; i++)
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dest[i] = src[i];
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2010-10-11 06:09:07 +00:00
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for ( ; i < n; i++)
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dest[i] = \(aq\\0\(aq;
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return dest;
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}
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.fi
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.in
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.SH "RETURN VALUE"
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The
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.BR strcpy ()
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and
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.BR strncpy ()
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functions return a pointer to
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the destination string \fIdest\fP.
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2007-05-19 04:30:20 +00:00
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.SH "CONFORMING TO"
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SVr4, 4.3BSD, C89, C99.
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.SH NOTES
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Some programmers consider
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.BR strncpy ()
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to be inefficient and error prone.
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If the programmer knows (i.e., includes code to test!)
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that the size of \fIdest\fP is greater than
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the length of \fIsrc\fP, then
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.BR strcpy ()
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can be used.
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2012-07-19 09:29:15 +00:00
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One valid (and intended) use of
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.BR strncpy ()
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is to copy a C string to a fixed-length buffer
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while ensuring both that the buffer is not overflowed
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and that unused bytes in the target buffer are zeroed out
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(perhaps to prevent information leaks if the buffer is to be
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written to media or transmitted to another process via an
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interprocess communication technique).
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2007-06-07 09:15:03 +00:00
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If there is no terminating null byte in the first \fIn\fP
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confstr.3, strcasecmp.3, strcat.3, strcmp.3, strcpy.3, strdup.3, strftime.3, strlen.3, strnlen.3, strpbrk.3, strspn.3, strtok.3, strxfrm.3: Clarify that these functions operate on bytes, not (wide) characters
Change 'character(s)' to 'byte(s)' to make clear that these
functions operate on bytes, not wide / UTF8 characters.
(POSIX uses 'byte(s)' similarly, to make this point.)
Reported-by: Andries E. Brouwer <Andries.Brouwer@cwi.nl>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2012-05-04 04:15:02 +00:00
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bytes of \fIsrc\fP,
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2007-06-07 09:15:03 +00:00
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.BR strncpy ()
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produces an unterminated string in \fIdest\fP.
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2012-07-19 12:07:25 +00:00
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You can force termination using something like the following:
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2007-12-19 06:57:44 +00:00
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.in +4n
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2007-06-07 09:15:03 +00:00
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.nf
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2008-09-29 10:01:26 +00:00
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strncpy(buf, str, n);
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2007-06-21 22:55:04 +00:00
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if (n > 0)
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buf[n \- 1]= \(aq\\0\(aq;
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.fi
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.in
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2012-07-18 15:30:32 +00:00
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.PP
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(Of course, the above technique ignores the fact that
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information contained in
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.I src
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is lost in the copying to
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.IR dest .)
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2012-07-19 09:29:15 +00:00
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Some systems (the BSDs, Solaris, and others) provide the following function:
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size_t strlcpy(char *dest, const char *src, size_t size);
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.\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
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.\" "strlcpy and strlcat - consistent, safe, string copy and concatenation"
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.\" 1999 USENIX Annual Technical Conference
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This function is similar to
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.BR strncpy (),
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but it copies at most
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.I size\-1
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2012-08-13 22:39:21 +00:00
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bytes to
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.IR dest ,
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always adds a terminating null byte,
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and does not pad the target with (further) null bytes.
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This function fixes some of the problems of
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.BR strcpy ()
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and
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.BR strncpy (),
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but the caller must still handle the possibility of data loss if
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.I size
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is too small.
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The return value of the function is the length of
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.IR src ,
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which allows truncation to be easily detected:
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if the return value is greater than or equal to
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.IR size ,
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truncation occurred.
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If loss of data matters, the caller
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.I must
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either check the arguments before the call,
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or test the function return value.
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.BR strlcpy ()
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is not present in glibc and is not standardized by POSIX,
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.\" https://lwn.net/Articles/506530/
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but is available on Linux via the
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.IR libbsd
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library.
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2004-11-03 13:51:07 +00:00
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.SH BUGS
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2007-05-12 09:06:04 +00:00
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If the destination string of a
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.BR strcpy ()
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2008-10-28 19:57:15 +00:00
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is not large enough, then anything might happen.
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Overflowing fixed-length string buffers is a favorite cracker technique
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for taking complete control of the machine.
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Any time a program reads or copies data into a buffer,
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the program first needs to check that there's enough space.
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This may be unnecessary if you can show that overflow is impossible,
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but be careful: programs can get changed over time,
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in ways that may make the impossible possible.
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2004-11-03 13:51:07 +00:00
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.SH "SEE ALSO"
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.BR bcopy (3),
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.BR memccpy (3),
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.BR memcpy (3),
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.BR memmove (3),
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2009-12-05 05:03:25 +00:00
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.BR stpcpy (3),
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2011-09-29 04:26:17 +00:00
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.BR stpncpy (3),
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2008-06-10 08:22:21 +00:00
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.BR strdup (3),
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clock_nanosleep.2, getpriority.2, kexec_load.2, nanosleep.2, nice.2, sched_setaffinity.2, timer_create.2, timer_delete.2, timer_settime.2, utime.2, btowc.3, futimes.3, log.3, pthread_attr_setguardsize.3, pthread_kill_other_threads_np.3, pthread_setaffinity_np.3, pthread_setcancelstate.3, pthread_setschedparam.3, pthread_setschedprio.3, pthread_tryjoin_np.3, strcoll.3, strcpy.3, string.3, strxfrm.3, networks.5, aio.7, sigevent.7, udplite.7: SEE ALSO: Place entries in correct order
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-11-01 05:34:26 +00:00
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.BR string (3),
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.BR wcscpy (3),
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.BR wcsncpy (3)
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