.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
.\"     Improve discussion of strncat().
.TH STRCAT 3  2012-07-19 "GNU" "Linux Programmer's Manual"
.SH NAME
strcat, strncat \- concatenate two strings
.SH SYNOPSIS
.nf
.B #include <string.h>
.sp
.BI "char *strcat(char *" dest ", const char *" src );
.sp
.BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
.fi
.SH DESCRIPTION
The
.BR strcat ()
function appends the \fIsrc\fP string to the
\fIdest\fP string,
overwriting the terminating null byte (\(aq\\0\(aq) at the end of
\fIdest\fP, and then adds a terminating null byte.
The strings may not overlap, and the \fIdest\fP string must have
enough space for the result.
If
.I dest
is not large enough, program behavior is unpredictable;
.IR "buffer overruns are a favorite avenue for attacking secure programs" .
.PP
The
.BR strncat ()
function is similar, except that
.IP * 3
it will use at most \fIn\fP bytes from \fIsrc\fP; and
.IP *
\fIsrc\fP does not need to be null-terminated if it contains
\fIn\fP or more bytes.
.PP
As with
.BR strcat (),
the resulting string in \fIdest\fP is always null-terminated.
.PP
If \fIsrc\fP contains \fIn\fP or more bytes,
.BR strncat ()
writes \fIn+1\fP bytes to \fIdest\fP (\fIn\fP
from \fIsrc\fP plus the terminating null byte).
Therefore, the size of \fIdest\fP must be at least
\fIstrlen(dest)+n+1\fP.

A simple implementation of
.BR strncat ()
might be:
.in +4n
.nf

char*
strncat(char *dest, const char *src, size_t n)
{
    size_t dest_len = strlen(dest);
    size_t i;

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

    return dest;
}
.fi
.in
.SH "RETURN VALUE"
The
.BR strcat ()
and
.BR strncat ()
functions return a pointer to the resulting string \fIdest\fP.
.SH "CONFORMING TO"
SVr4, 4.3BSD, C89, C99.
.SH NOTES
Some systems (the BSDs, Solaris, and others) provide the following function:

    size_t strlcat(char *dest, const char *src, size_t size);

This function appends the null-terminated string
.I src
to the string
.IR dest ,
copying at most
.IR "size\-strlen(dest)\-1"
from
.IR src ,
and adds a null terminator to the result,
.I unless
.IR size
is less than
.IR strlen(dest) .
This function fixes the buffer overrun problem of
.BR strcat (),
but the caller must still handle the possibility of data loss if
.I size
is too small.
The function returns the length of the string
.BR strlcat ()
tried to create; if the return value is greater than or equal to
.IR size ,
data loss occurred.
If data loss matters, the caller
.I must
either check the arguments before the call, or test the function return value.
.BR strlcat ()
is not present in glibc and is not standardized by POSIX,
.\" https://lwn.net/Articles/506530/
but is available on Linux via the
.IR libbsd
library.
.SH "SEE ALSO"
.BR bcopy (3),
.BR memccpy (3),
.BR memcpy (3),
.BR strcpy (3),
.BR string (3),
.BR strncpy (3),
.BR wcscat (3),
.BR wcsncat (3)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.