Question

I have an exercise where I need to write a wrapper function for strcat. After it prints the string length (for debugging) it seg faults and I am not quite sure why. Any help would be greatly appreciated.

EDIT: I didn’t specify that the wrapper function is supposed to guarantee that it never goes outside of the bounds of memory allocated for destination. This is why I allocated only enough memory for “string” in destination and reallocating more in the Strcat() wrapper.

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    char* Strcat(char* destination, const char* source);

    int main(void)
    {
        (void)printf("This strcat cannot fail!\n");
        char* destination = (char*)malloc(sizeof(char)*7);
        destination = "string";
        (void)printf("%s\n", destination);
        Strcat(destination, " concatination");
        (void)printf("%s\n", destination);
        (void)printf("It works!\n");

        free(destination);

        return 0;
    }

    char* Strcat(char* destination, const char* source)
    {
        (void)printf("%d\n", (strlen(destination)+strlen(source))*sizeof(char));
        if((sizeof((strlen(destination)+strlen(source)))+1) > sizeof(destination))
            destination = (char*)realloc(destination, sizeof((strlen(destination)+strlen(source)))+1);
        return strcat(destination, source);
    }

Answer 1

This line:

destination = "string";

overwrites the pointer returned from malloc(3) so you lose that memory forever. You probably meant to copy that string, so use strcpy(3) or something.