How to send 2D char pointer to a function for memory allocation?

Question

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


void createDynamicArrayForChar(int dimension, char **ptr)
{
    ptr = (char**)malloc(dimension*sizeof(char*));

    for (int i = 0; i < dimension; i++)
    {
        ptr[i] = (char*)malloc(20 * sizeof(char));
        ptr[i] = "value";
    }
}

int main()
{
    char **ptrArray;
    createDynamicArrayForChar(5, ptrArray);
    printf("%s", ptrArray[3]);


    getchar(); getchar();
    return 0;
}

It gives some errors when I try to compile this codes. How can I solve this problem? How to send 2D char pointer to a function in C?

Please [see why not to cast](http://stackoverflow.com/q/605845/2173917) the return value of `malloc()` and family in `C`. — Sourav Ghosh, Aug 21 '15 at 06:58
`some errors`..it will be nice if you can share those with us. :) — Sourav Ghosh, Aug 21 '15 at 06:58
@Bathsheba Good morning Sir !! I did not quite understand your comment. What is it actually? :) — Sourav Ghosh, Aug 21 '15 at 07:04
@Bathsheba Sir, ( _I'm not very good at english, but as per my understanding_ ) I'll take this as a compliment. :) — Sourav Ghosh, Aug 21 '15 at 07:13

score 3 · Accepted Answer · edited May 23 '17 at 12:29

Firstly, as per the present code, I see two issues.

You're passing ptrArray to the function and trying to allocate memory inside the function. Please be aware, C uses pass by value for function argument passing, so, if you want to allocate memory to ptrArray and expect that to be refeclted back to the caller, without returning, you'll be needing to pass a pointer to that `ptrArray.
in the code
```
ptr[i] = (char*)malloc(20 * sizeof(char));
ptr[i] = "value";
```
You're leaking memory. Once you've allocated memory using malloc(), you should use strcpy() to copy the data into the allocated memory.

That said, some advice:

Please see why not to cast the return value of malloc() and family in C.
sizeof(char) is guaranteed to be 1 in C. Using that as a multiplier is not required.
Always check the success of malloc() before using the returned pointer.

Thanks Sourav, can you edit my codes and show us? Because I didn't understand your first statement completely. I'm trying to learn more English. — Hakan, Aug 21 '15 at 07:17
It gives "error C4700: uninitialized local variable 'ptrArray' used". — Hakan, Aug 21 '15 at 07:42

score 2 · Answer 2 · answered Aug 21 '15 at 07:05

You probably need this (no error checking and not debugged code):

void createDynamicArrayForChar(int dimension, char ***ptr)
{
    *ptr = (char**)malloc(dimension*sizeof(char*));

    for (int i = 0; i < dimension; i++)
    {
        (*ptr)[i] = (char*)malloc(20 * sizeof(char));
        strcpy((*ptr)[i],"value");
    }
}

or

char **createDynamicArrayForChar(int dimension)
{
    char **ptr = (char**)malloc(dimension*sizeof(char*));

    for (int i = 0; i < dimension; i++)
    {
        ptr[i] = (char*)malloc(20 * sizeof(char));
        strcpy(ptr[i],"value");
    }

    return ptr;
}

int main()
{
    char **ptrArray;
    ptrArray = createDynamicArrayForChar(5);
    ...

How to send 2D char pointer to a function for memory allocation?

2 Answers2