.\" Copyright 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 18:11:47 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  2017-09-15 "GNU" "Linux Programmer's Manual"

.SH NAME

strcat, strncat \- concatenate two strings

.SH SYNOPSIS

.nf

.B #include <string.h>

.PP

.BI "char *strcat(char *" dest ", const char *" src );

.PP

.BI "char *strncat(char *" dest ", const char *" src ", size_t " n );

.fi

.SH DESCRIPTION

The

.BR strcat ()

function appends the

.I src

string to the

.I dest

string,

overwriting the terminating null byte (\(aq\\0\(aq) at the end of

.IR dest ,

and then adds a terminating null byte.

The strings may not overlap, and the

.I dest

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

.I n

bytes from

.IR src ;

and

.IP *

.I src

does not need to be null-terminated if it contains

.I n

or more bytes.

.PP

As with

.BR strcat (),

the resulting string in

.I dest

is always null-terminated.

.PP

If

.IR src

contains

.I n

or more bytes,

.BR strncat ()

writes

.I n+1

bytes to

.I dest

.RI ( n

from

.I src

plus the terminating null byte).

Therefore, the size of

.I dest

must be at least

.IR "strlen(dest)+n+1" .

.PP

A simple implementation of

.BR strncat ()

might be:

.PP

.in +4n

.EX

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;

}

.EE

.in

.SH RETURN VALUE

The

.BR strcat ()

and

.BR strncat ()

functions return a pointer to the resulting string

.IR dest .

.SH ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw19 lb lb

l l l.

Interface	Attribute	Value

T{

.BR strcat (),

.BR strncat ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

.SH NOTES

Some systems (the BSDs, Solaris, and others) provide the following function:

.PP

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

.PP

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 terminating null byte 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 EXAMPLE

Because

.BR strcat ()

and

.BR strncat ()

must find the null byte that terminates the string

.I dest

using a search that starts at the beginning of the string,

the execution time of these functions

scales according to the length of the string

.IR dest .

This can be demonstrated by running the program below.

(If the goal is to concatenate many strings to one target,

then manually copying the bytes from each source string

while maintaining a pointer to the end of the target string

will provide better performance.)

.\"

.SS Program source

\&

.EX

#include <string.h>

#include <time.h>

#include <stdio.h>

int

main(int argc, char *argv[])

{

#define LIM 4000000

    int j;

    char p[LIM];

    time_t base;

    base = time(NULL);

    p[0] = \(aq\\0\(aq;

    for (j = 0; j < LIM; j++) {

        if ((j % 10000) == 0)

            printf("%d %ld\\n", j, (long) (time(NULL) \- base));

        strcat(p, "a");

    }

}

.EE

.\"

.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 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.