strcat, strcat_s

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 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

Because strcat needs to seek to the end of dest on each call, it is inefficient to concatenate many strings into one using strcat.

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

#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