strcat, strncat — concatenate two strings


#include <string.h>
char *strcat( char *restrict dest,
  const char *restrict src);
char *strncat( char *restrict dest,
  const char *restrict src,
  size_t n);


The strcat() function appends the src string to the dest string, overwriting the terminating null byte ('\0') at the end of dest, and then adds a terminating null byte. The strings may not overlap, and the dest string must have enough space for the result. If dest is not large enough, program behavior is unpredictable; buffer overruns are a favorite avenue for attacking secure programs.

The strncat() function is similar, except that

  • it will use at most n bytes from src; and

  • src does not need to be null-terminated if it contains n or more bytes.

As with strcat(), the resulting string in dest is always null-terminated.

If src contains n or more bytes, strncat() writes n+1 bytes to dest (n from src plus the terminating null byte). Therefore, the size of dest must be at least strlen(dest)+n+1.

A simple implementation of strncat() might be:

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] != '\0' ; i++)
        dest[dest_len + i] = src[i];
    dest[dest_len + i] = '\0';

    return dest;


The strcat() and strncat() functions return a pointer to the resulting string dest.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
strcat(), strncat() Thread safety MT-Safe


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


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 src to the string dest, copying at most size−strlen(dest)−1 from src, and adds a terminating null byte to the result, unless size is less than strlen(dest). This function fixes the buffer overrun problem of strcat(), but the caller must still handle the possibility of data loss if size is too small. The function returns the length of the string strlcat() tried to create; if the return value is greater than or equal to size, data loss occurred. If data loss matters, the caller must either check the arguments before the call, or test the function return value. strlcat() is not present in glibc and is not standardized by POSIX, but is available on Linux via the libbsd library.


Because strcat() and strncat() must find the null byte that terminates the string 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 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.)

Program source

#include <stdint.h>
#include <string.h>
#include <time.h>
#include <stdio.h>

main(int argc, char *argv[])
#define LIM 4000000
    char p[LIM + 1];    /* +1 for terminating null byte */
    time_t base;

    base = time(NULL);
    p[0] = '\0';

    for (int j = 0; j < LIM; j++) {
        if ((j % 10000) == 0)
            printf("%d %jd\n", j, (intmax_t) (time(NULL) − base));
        strcat(p, "a");


bcopy(3), memccpy(3), memcpy(3), strcpy(3), string(3), strncpy(3), wcscat(3), wcsncat(3)


This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright 1993 David Metcalfe (

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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 (
2007-06-15, Marc Boyer <> + mtk
    Improve discussion of strncat().