CODEWITHSUNDEEP
cmemory-managementdynamic-memory-allocation
Roxor9999
Roxor9999

Reputation: 115

C - Memory size pointer of char

I am trying to write a function to assign text input to a char**. Where each word is stored in a char*. When I was doing this I was getting errors that I think indicated that I didn't allocate enough memory so I wrote a little test program.

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

int main(){
    char **input=NULL;
    char **help=NULL;
    for(int i=1;i<300;i++){
        help=realloc(input,i*(sizeof(char*)));
        if (!help)
            return 0;
        input=help;
        input[i]="test";
    }
    return 1;
}

Now this works just fine, but when I assign per character as follows.

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

int main(){
    char **input=NULL;
    char **help=NULL; 
    char *help2=NULL;
    for(int i=1;i<300;i++){
        help=realloc(input,i*(sizeof(char*)));
        if (!help)
            return 0;
        input=help;
        for (int j=1;j<5;j++){
            help2=realloc(input[i-1],j);
            if (!help2)
                return 0;
            input[i-1]=help2;
            input[i-1][j-1]='t';    
        } 
    }
    return 1;
}

I get a segmentation fault on line 14. I think this is because the input does not have enough space. But since the size of a character is one and I used sizeof(char*) I don't see how it is not enough. How much memory would I need to allocate and why did it work in my first example? Thanks!

Upvotes: 2

Views: 96

Answers (1)

Schwern
Schwern

Reputation: 164639

clang finds the problem right away.

test(72707,0x7fffe32443c0) malloc: *** error for object 0x7fffe0051090: pointer being realloc'd was not allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap: 6

The problem is here.

help2=realloc(input[i-1],j);

While input has been allocated, none of its elements have been initialized. realloc will act like malloc if the pointer is null, but if its asked to reallocate the uninitialized garbage pointer in input[i-1] it will freak.

Instead you must properly initialize input before trying to realloc its elements.

A memory checking tool such as Valgrind will help find these sorts of mistakes.

Upvotes: 1

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