Concatenating strings in C, which method is more efficient?

Question

I came across these two methods to concatenate strings:

Common part:

char* first= "First";
char* second = "Second";
char* both = malloc(strlen(first) + strlen(second) + 2);

Method 1:

strcpy(both, first);
strcat(both, " ");       // or space could have been part of one of the strings
strcat(both, second);

Method 2:

sprintf(both, "%s %s", first, second);

In both cases the content of both would be "First Second".

I would like to know which one is more efficient (I have to perform several concatenation operations), or if you know a better way to do it.

Answer 1

For readability, I'd go with

char * s = malloc(snprintf(NULL, 0, "%s %s", first, second) + 1);
sprintf(s, "%s %s", first, second);

If your platform supports GNU extensions, you could also use asprintf():

char * s = NULL;
asprintf(&s, "%s %s", first, second);

If you're stuck with the MS C Runtime, you have to use _scprintf() to determine the length of the resulting string:

char * s = malloc(_scprintf("%s %s", first, second) + 1);
sprintf(s, "%s %s", first, second);

The following will most likely be the fastest solution:

size_t len1 = strlen(first);
size_t len2 = strlen(second);

char * s = malloc(len1 + len2 + 2);
memcpy(s, first, len1);
s[len1] = ' ';
memcpy(s + len1 + 1, second, len2 + 1); // includes terminating null