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(fi         


        

Answer 1

Here's some madness for you, I actually went and measured it. Bloody hell, imagine that. I think I got some meaningful results.

I used a dual core P4, running Windows, using mingw gcc 4.4, building with "gcc foo.c -o foo.exe -std=c99 -Wall -O2".

I tested method 1 and method 2 from the original post. Initially kept the malloc outside the benchmark loop. Method 1 was 48 times faster than method 2. Bizarrely, removing -O2 from the build command made the resulting exe 30% faster (haven't investigated why yet).

Then I added a malloc and free inside the loop. That slowed down method 1 by a factor of 4.4. Method 2 slowed down by a factor of 1.1.

So, malloc + strlen + free DO NOT dominate the profile enough to make avoiding sprintf worth while.

Here's the code I used (apart from the loops were implemented with < instead of != but that broke the HTML rendering of this post):

void a(char *first, char *second, char *both)
{
    for (int i = 0; i != 1000000 * 48; i++)
    {
        strcpy(both, first);
        strcat(both, " ");
        strcat(both, second);
    }
}

void b(char *first, char *second, char *both)
{
    for (int i = 0; i != 1000000 * 1; i++)
        sprintf(both, "%s %s", first, second);
}

int main(void)
{
    char* first= "First";
    char* second = "Second";
    char* both = (char*) malloc((strlen(first) + strlen(second) + 2) * sizeof(char));

    // Takes 3.7 sec with optimisations, 2.7 sec WITHOUT optimisations!
    a(first, second, both);

    // Takes 3.7 sec with or without optimisations
    //b(first, second, both);

    return 0;
}