# strncat, strncat_s

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< c‎ | string‎ | byte

Defined in header `<string.h>` |  |
| (1) |
char *strncat( char *dest, const char *src, size_t count ); |  | (until C99)
char *strncat( char *restrict dest, const char *restrict src, size_t count ); |  | (since C99)
errno_t strncat_s(char *restrict dest, rsize_t destsz, const char *restrict src, rsize_t count); | (2) | (since C11)

1) Appends at most `count` characters from the character array pointed to by `src`, stopping if the null character is found, to the end of the null-terminated byte string pointed to by `dest`. The character src[0] replaces the null terminator at the end of `dest`. The terminating null character is always appended in the end (so the maximum number of bytes the function may write is count+1).
The behavior is undefined if the destination array does not have enough space for the contents of both `dest` and the first `count` characters of `src`, plus the terminating null character. The behavior is undefined if the source and destination objects overlap. The behavior is undefined if either `dest` is not a pointer to a null-terminated byte string or `src` is not a pointer to a character array,
2) Same as (1), except that this function may clobber the remainder of the destination array (from the last byte written to `destsz`) and that the following errors are detected at runtime and call the currently installed constraint handler function:

  - `src` or `dest` is a null pointer
  - `destsz` or `count` is zero or greater than RSIZE_MAX
  - there is no null character in the first `destsz` bytes of `dest`
  - truncation would occur: `count` or the length of `src`, whichever is less, exceeds the space available between the null terminator of `dest` and `destsz`.
  - overlap would occur between the source and the destination strings

The behavior is undefined if the size of the character array pointed to by `dest` < strnlen(dest,destsz)+strnlen(src,count)+1 < `destsz`; in other words, an erroneous value of `destsz` does not expose the impending buffer overflow. The behavior is undefined if the size of the character array pointed to by `src` < strnlen(src,count) < `destsz`; in other words, an erroneous value of `count` does not expose the impending buffer overflow.

  As all bounds-checked functions, `strncat_s` is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including `string.h`.

### Parameters

dest | - | pointer to the null-terminated byte string to append to
src | - | pointer to the character array to copy from
count | - | maximum number of characters to copy
destsz | - | the size of the destination buffer

### Return value

1) returns a copy of `dest`
2) returns zero on success, returns non-zero on error. Also, on error, writes zero to dest[0] (unless `dest` is a null pointer or `destsz` is zero or greater than RMAX_SIZE).

### Notes

Although truncation to fit the destination buffer is a security risk and therefore a runtime constraints violation for `strncat_s`, it is possible to get the truncating behavior by specifying `count` equal to the size of the destination array minus one: it will copy the first `count` bytes and append the null terminator as always: strncat_s(dst, sizeof dst, src, (sizeof dst)-strnlen_s(dst, sizeof dst)-1);

### Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #include <string.h>
    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
        char str[50] = "Hello ";
        char str2[50] = "World!";
        strcat(str, str2);
        strncat(str, " Goodbye World!", 3);
        puts(str);

    #ifdef __STDC_LIB_EXT1__
        set_constraint_handler_s(ignore_handler_s);
        char s1[100] = "good";
        char s5[1000] = "bye";
        int r1 = strncat_s(s1, 100, s5, 1000); // r1 is 0, s1 holds "goodbye\0"
        printf("s1 = %s, r1 = %d\n", s1, r1);
        char s2[6] = "hello";
        int r2 = strncat_s(s2, 6, "", 1); // r2 is 0, s2 holds "hello\0"
        printf("s2 = %s, r2 = %d\n", s2, r2);
        char s3[6] = "hello";
        int r3 = strncat_s(s3, 6, "X", 2); // r3 is non-zero, s3 holds "\0"
        printf("s3 = %s, r3 = %d\n", s3, r3);
        // the strncat_s truncation idiom:
        char s4[7] = "abc";
        int r4 = strncat_s(s4, 7, "defghijklmn", 3); // r is 0, s4 holds "abcdef\0"
        printf("s4 = %s, r4 = %d\n", s4, r4);
    #endif
    }

Possible output:

    Hello World! Go
    s1 = goodbye, r1 = 0
    s2 = hello, r2 = 0
    s3 = , r3 = 22
    s4 = abcdef, r4 = 0

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.24.3.2 The strncat function (p: 364-365)

  - K.3.7.2.2 The strncat_s function (p: 618-620)

- C99 standard (ISO/IEC 9899:1999):

  - 7.21.3.2 The strncat function (p: 327-328)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.11.3.2 The strncat function

### See also

strcat strcat_s (C11) | concatenates two strings (function)
strcpy strcpy_s (C11) | copies one string to another (function)
C++ documentation for strncat

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