# strcat, strcat_s

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< c‎ | string‎ | byte

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

1) Appends a copy of the null-terminated byte string pointed to by `src` 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 resulting byte string is null-terminated.
The behavior is undefined if the destination array is not large enough for the contents of both `src` and `dest` and the terminating null character. The behavior is undefined if the strings overlap. The behavior is undefined if either `dest` or `src` is not a pointer to a null-terminated byte string.
2) Same as (1), except that it may clobber the rest of the destination array (from the last character written to `destsz`) with unspecified values 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` is zero or greater than RSIZE_MAX
  - there is no null terminator in the first `destsz` bytes of `dest`
  - truncation would occur (the available space at the end of `dest` would not fit every character, including the null terminator, of `src`)
  - 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` < strlen(dest)+strlen(src)+1 <= `destsz`; in other words, an erroneous value of `destsz` does not expose the impending buffer overflow.

  As with all bounds-checked functions, `strcat_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 null-terminated byte string to copy from
destsz | - | maximum number of characters to write, typically 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 RSIZE_MAX).

### Notes

`strcat_s` is allowed to clobber the destination array from the last character written up to `destsz` in order to improve efficiency: it may copy in multibyte blocks and then check for null bytes.

The function `strcat_s` is similar to the BSD function `strlcat`, except that

- `strlcat` truncates the source string to fit in the destination
- `strlcat` does not perform all the runtime checks that `strcat_s` does
- `strlcat` does not make failures obvious by setting the destination to a null string or calling a handler if the call fails.

Although `strcat_s` prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked strncat_s instead.

### 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);
        strcat(str, " ...");
        strcat(str, " Goodbye World!");
        puts(str);

    #ifdef __STDC_LIB_EXT1__
        set_constraint_handler_s(ignore_handler_s);
        int r = strcat_s(str, sizeof str, " ... ");
        printf("str = \"%s\", r = %d\n", str, r);
        r = strcat_s(str, sizeof str, " and this is too much");
        printf("str = \"%s\", r = %d\n", str, r);
    #endif
    }

Possible output:

    Hello World! ... Goodbye World!
    str = "Hello World! ... Goodbye World! ... ", r = 0
    str = "", r = 22

### References

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

  - 7.24.3.1 The strcat function (p: 364)

  - K.3.7.2.1 The strcat_s function (p: 617-618)

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

  - 7.21.3.1 The strcat function (p: 327)

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

  - 4.11.3.1 The strcat function

### See also

strncat strncat_s (C11) | concatenates a certain amount of characters of two strings (function)
strcpy strcpy_s (C11) | copies one string to another (function)

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