Segmentation fault when trying to concatenate an element of a 2d array

Question

I wanted to use strcat() to concatenate an element of an array of strings. I tried:

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

int main() {
    char **str = malloc(sizeof(char *) * 3);

    for (int i = 0; i < 3; i++) {
        str[i] = malloc(sizeof(char) * 8);
    }

    str[0] = "foo";
    str[1] = "bar";

    strcat(str[0], "H");

    for (int i = 0; i < 3; i++) {
        printf("%s\n", str[i]);
    }

    free(str);

    return 0;
}

and I get the error:

Segmentation fault (core dumped)

What should I do to get it right?

Answer 1

For starters the program has memory leaks. At first memory was allocated and its addresses were stored in pointers str[i]

for (int i = 0; i < 3; i++) {
    str[i] = malloc(sizeof(char) * 8);
}

and then the pointers str[0] and str[1] were reassigned by addresses of string literals.

str[0] = "foo";
str[1] = "bar";

As a result you may not change a string literal by this statement

strcat(str[0], "H");

because this invokes undefined behavior

You have to write

strcpy( str[0], "foo" );
strcpy( str[1], "bar" );

And this loop

for (int i = 0; i < 3; i++) {
    printf("%s\n", str[i]);
}

also invokes undefined behavior because the element str[2] was not initialized by a string.

Either you need to change the condition or the loop like i < 2 or to initialize the element str[2].

And you need to free all the allocated memory like

for (int i = 0; i < 3; i++) {
    free( str[i] );
}

free( str );