strcpy.3: Add some text to emphasize possibility of buffer runs with strcpy()

[thirdparty/man-pages.git] / man3 / strcpy.3

.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" 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  2012-07-18 "GNU" "Linux Programmer's Manual"
.SH NAME
strcpy, strncpy \- copy a string
.SH SYNOPSIS
.nf
.B #include <string.h>
.sp
.BI "char *strcpy(char *" dest ", const char *" src );
.sp
.BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
.fi
.SH DESCRIPTION
The
.BR strcpy ()
function copies the string pointed to by \fIsrc\fP,
including the terminating null byte (\(aq\\0\(aq),
to the buffer pointed to by \fIdest\fP.
The strings may not overlap, and the destination string
\fIdest\fP must be large enough to receive the copy.
.IR "Beware of buffer overruns!"
(See BUGS.)
.PP
The
.BR strncpy ()
function is similar, except that at most
\fIn\fP bytes of \fIsrc\fP are copied.
.BR Warning :
If there is no null byte
among the first \fIn\fP bytes of \fIsrc\fP,
the string placed in \fIdest\fP 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:
.in +4n
.nf

char *
strncpy(char *dest, const char *src, size_t n)
{
    size_t i;

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

    return dest;
}
.fi
.in
.SH "RETURN VALUE"
The
.BR strcpy ()
and
.BR strncpy ()
functions return a pointer to
the destination string \fIdest\fP.
.SH "CONFORMING TO"
SVr4, 4.3BSD, C89, C99.
.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 \fIdest\fP is greater than
the length of \fIsrc\fP, then
.BR strcpy ()
can be used.

If there is no terminating null byte in the first \fIn\fP
bytes of \fIsrc\fP,
.BR strncpy ()
produces an unterminated string in \fIdest\fP.
Programmers often prevent this mistake by forcing termination
as follows:
.in +4n
.nf

strncpy(buf, str, n);
if (n > 0)
    buf[n \- 1]= \(aq\\0\(aq;
.fi
.in
.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)