.\"	$OpenBSD: strncat.3,v 1.4 2014/04/19 16:50:46 jmc Exp $
.\"
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.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
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.Dd $Mdocdate: April 19 2014 $
.Dt STRNCAT 3
.Os
.Sh NAME
.Nm strncat
.Nd concatenate a string with part of another
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fn strncat "char *dst" "const char *append" "size_t count"
.Sh DESCRIPTION
The
.Fn strncat
function appends not more than
.Fa count
characters of the string
.Fa append
to the end of the string found in the buffer
.Fa dst .
Space for the terminating
.Ql \e0
should not be included in
.Fa count .
.Pp
Bounds checking must be performed manually with great care.
If the buffer
.Fa dst
is not large enough to hold the result,
subsequent memory will be damaged.
.Sh RETURN VALUES
The
.Fn strncat
function returns the pointer
.Fa dst .
.Sh EXAMPLES
The following example shows how to use
.Fn strncat
in conjunction with
.Xr strncpy 3 :
.Bd -literal -offset indent
char buf[BUFSIZ];
char *base, *suffix;

(void)strncpy(buf, base, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = '\e0';
(void)strncat(buf, suffix, sizeof(buf) - 1 - strlen(buf));
.Ed
.Pp
The above will copy as many characters from
.Va base
to
.Va buf
as will fit.
It then appends as many characters from
.Va suffix
as will fit.
If either
.Va base
or
.Va suffix
are too large, truncation will occur without detection.
.Pp
The above example shows dangerous coding patterns, including an
inability to detect truncation.
.Fn strncat
and
.Fn strncpy
are dangerously easy to misuse.
The
.Xr strlcpy 3
and
.Xr strlcat 3
functions are safer for this kind of operation:
.Bd -literal -offset indent
if (strlcpy(buf, base, sizeof(buf)) >= sizeof(buf) ||
    strlcat(buf, suffix, sizeof(buf)) >= sizeof(buf))
        goto toolong;

.Ed
or for greatest portability,
.Bd -literal -offset indent
if (snprintf(buf, sizeof(buf), "%s%s",
    base, suffix) >= sizeof(buf))
        goto toolong;
.Ed
.Sh SEE ALSO
.Xr strlcpy 3 ,
.Xr wcscat 3 ,
.Xr wcslcpy 3
.Sh STANDARDS
The
.Fn strncat
function conforms to
.St -ansiC .
.Sh HISTORY
The
.Fn strncat
function first appeared in
.At v7 .