How do I properly use strcat for char array?

Question

I'm trying to concatenate another string onto arg[0].

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

int main() {

    char* arg[] = {
        "Hello",
        NULL,
        NULL,
        NULL
    };

    strcat(arg[0], " World");

}

This returns abort trap.

What am I doing wrong?

Answer 1

You are trying to rewrite a string literal with:

char* arg[] = { "Hello", ..... };    // "Hello" is a string literal, pointed to by "arg[0]".

strcat(arg[0], " World");       // Attempt to rewrite/modify a string literal.

which isn´t possible.

String literals are only available to read, but not to write. That´s what make them "literal".

---

If you wonder about why:

char* arg[] = { "Hello", ..... }; 

implies "Hello" as a string literal, You should read the answers to that question:

What is the difference between char s[] and char *s?

---

By the way, it would be even not possible (or at least get a segmentation fault at run-time) if you would do something like that:

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

int main() {

    char a[] = "Hello";  // 6 for holding "Hello" (5) + "\0" (1). 

    strcat(a, " World");   

}

because the array a needs to have 12 characters for concatenating both strings and using strcat(a, " World"); - "Hello" (5 characters) + " World" (6 characters) + \\0 (1 character) but it only has 6 characters for holding "Hello" + \\0 . There is no memory space added to the array automagically when using strcat() .

If you execute the program with these statements, you are writing beyond the bounds of the array which is Undefined Behavior and this will get you probably prompt a Segmentation Fault error.

Answer 2

One of the shortcomings of C strings is that you need to know how big they will be ahead of time.

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

int main() {
    char arg[1024] = "Hello";
    strcat(arg, " World");
    printf("%s\n", arg);
}

In this case, the size of the concatenated string must be less than 1024 characters. This is why it is safer to use something like C++, where you have std::string to prevent these types of issues.