path: root/man3/strcpy.3

.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aeb@cwi.nl)
.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
.\"     Improve discussion of strncpy().
.\"
.TH STRCPY 3  2021-03-22 "Linux man-pages (unreleased)"
.SH NAME
strcpy, strncpy \- copy a string
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.PP
.BI "char *strcpy(char *restrict " dest ", const char *" src );
.BI "char *strncpy(char *restrict " dest ", const char *restrict " src \
", size_t " n );
.fi
.SH DESCRIPTION
The
.BR strcpy ()
function copies the string pointed to by
.IR src ,
including the terminating null byte (\(aq\e0\(aq),
to the buffer pointed to by
.IR dest .
The strings may not overlap, and the destination string
.I dest
must be large enough to receive the copy.
.I Beware of buffer overruns!
(See BUGS.)
.PP
The
.BR strncpy ()
function is similar, except that at most
.I n
bytes of
.I src
are copied.
.BR Warning :
If there is no null byte
among the first
.I n
bytes of
.IR src ,
the string placed in
.I dest
will not be null-terminated.
.PP
If the length of
.I src
is less than
.IR n ,
.BR strncpy ()
writes additional null bytes to
.I dest
to ensure that a total of
.I n
bytes are written.
.PP
A simple implementation of
.BR strncpy ()
might be:
.PP
.in +4n
.EX
char *
strncpy(char *dest, const char *src, size_t n)
{
    size_t i;

    for (i = 0; i < n && src[i] != \(aq\e0\(aq; i++)
        dest[i] = src[i];
    for ( ; i < n; i++)
        dest[i] = \(aq\e0\(aq;

    return dest;
}
.EE
.in
.SH RETURN VALUE
The
.BR strcpy ()
and
.BR strncpy ()
functions return a pointer to
the destination string
.IR dest .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR strcpy (),
.BR strncpy ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
.SH NOTES
Some programmers consider
.BR strncpy ()
to be inefficient and error prone.
If the programmer knows (i.e., includes code to test!)
that the size of
.I dest
is greater than
the length of
.IR src ,
then
.BR strcpy ()
can be used.
.PP
One valid (and intended) use of
.BR strncpy ()
is to copy a C string to a fixed-length buffer
while ensuring both that the buffer is not overflowed
and that unused bytes in the destination buffer are zeroed out
(perhaps to prevent information leaks if the buffer is to be
written to media or transmitted to another process via an
interprocess communication technique).
.PP
If there is no terminating null byte in the first
.I n
bytes of
.IR src ,
.BR strncpy ()
produces an unterminated string in
.IR dest .
If
.I buf
has length
.IR buflen ,
you can force termination using something like the following:
.PP
.in +4n
.EX
if (buflen > 0) {
    strncpy(buf, str, buflen \- 1);
    buf[buflen \- 1]= \(aq\e0\(aq;
}
.EE
.in
.PP
(Of course, the above technique ignores the fact that, if
.I src
contains more than
.I "buflen\ \-\ 1"
bytes, information is lost in the copying to
.IR dest .)
.\"
.SS strlcpy()
Some systems (the BSDs, Solaris, and others) provide the following function:
.PP
.in +4n
.EX
size_t strlcpy(char *dest, const char *src, size_t size);
.EE
.in
.PP
.\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
.\"     "strlcpy and strlcat - consistent, safe, string copy and concatenation"
.\"     1999 USENIX Annual Technical Conference
This function is similar to
.BR strncpy (),
but it copies at most
.I size\-1
bytes to
.IR dest ,
always adds a terminating null byte,
and does not pad the destination with (further) null bytes.
This function fixes some of the problems of
.BR strcpy ()
and
.BR strncpy (),
but the caller must still handle the possibility of data loss if
.I size
is too small.
The return value of the function is the length of
.IR src ,
which allows truncation to be easily detected:
if the return value is greater than or equal to
.IR size ,
truncation occurred.
If loss of data matters, the caller
.I must
either check the arguments before the call,
or test the function return value.
.BR strlcpy ()
is not present in glibc and is not standardized by POSIX,
.\" https://lwn.net/Articles/506530/
but is available on Linux via the
.I libbsd
library.
.SH BUGS
If the destination string of a
.BR strcpy ()
is not large enough, then anything might happen.
Overflowing fixed-length string buffers is a favorite cracker technique
for taking complete control of the machine.
Any time a program reads or copies data into a buffer,
the program first needs to check that there's enough space.
This may be unnecessary if you can show that overflow is impossible,
but be careful: programs can get changed over time,
in ways that may make the impossible possible.
.SH SEE ALSO
.BR bcopy (3),
.BR memccpy (3),
.BR memcpy (3),
.BR memmove (3),
.BR stpcpy (3),
.BR stpncpy (3),
.BR strdup (3),
.BR string (3),
.BR wcscpy (3),
.BR wcsncpy (3)