Why I am getting wrong output for source string when I am implementing strcat()?

Question

Here I tried implementing strcat function in C. Problem is that when I am printing the source string it is showing me "hyam" not "shyam"

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

char *fun(char *target, const char *source) {
    printf("%s\n", source);
    int x = strlen(target);
    while (*source != '\0') {
        *(target + x++) = *source++;
    }
    *(target + x) = '\0';
    return target;
}

int main() {
    char target[] = "ram";
    char source[] = "shyam";
    fun(target, source);
    printf("%s\n", target);
    printf("%s", source);
    return 0;
}

Here in last line of output hyam is shown but it should be shyam.

shyam
ramshyam
hyam

Answer 1

Your target array is too small - it needs at least nine elements (the length of both strings plus a terminating zero).

Writing outside target has undefined behaviour, but in practice, it looks like your arrays happen to be laid out end to end, like this:

 |r|a|m|\0|s|h|y|a|m|\0|
 ^        ^
 |        | 
target   source

and then you concatenate, going past the end of target into source:

 |r|a|m|s|h|y|a|m|\0|\0|
 ^       ^
 |       | 
target  source

which makes it look like an 's' has disappeared.

(Note that this is undefined, so anything can happen. You can't rely on this behaviour, unless the documentation for your compiler says that it's fine and should do this. Which it most likely won't.)