CODEWITHSUNDEEP
cstringmemory-management
AntikM
AntikM

Reputation: 651

C - Appending to string - Possible memory errors

I am trying to a create a function that keeps on appending a string to a char variable. However, some times it works and other times it doesn't. I am wondering where the bug is?

char *final_output = NULL;
void add_string(const char *);

int main(void) {
    add_string("Hello world\n");
    add_string("This is my new function!\n");

    /* Let's print */
    while (final_output && *final_output) {
        printf("%c", *final_output);
        *final_output++;
    }
}

void add_string(const char *text) {
    if (final_output == NULL) {
        final_output = malloc(strlen(text) + 1);
    }
    else {
        final_output = (char *) realloc(final_output, strlen(final_output) + strlen(text) + 2);
    }

    strncat(final_output, text, strlen(text));
}

Upvotes: 1

Views: 62

Answers (1)

Vlad from Moscow
Vlad from Moscow

Reputation: 311058

The problem is in function add_string. You do not append the allocated or copied array with the terminating zero after statements

final_output = malloc(strlen(text) + 1);

and

strncat(final_output, text, strlen(text));

Rewrite the function the following way

void add_string( const char *s ) 
{
    if ( final_output == NULL ) 
    {
        final_output = malloc( strlen( s ) + 1 );
        final_output[0] = '\0';
    }
    else 
    {
        final_output = realloc( final_output, strlen( final_output ) + strlen( s ) + 1 );
    }

    strcat( final_output, s );
}

Upvotes: 2

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