C Programming: malloc() inside another function

Question

I need help with malloc() inside another function.

I\'m passing a pointer and size to the fu

Answer 1

As mentioned in the other answers, we need a pointer to the pointer. But why?

We need to pass the value by a pointer in order to be able to modify the value. If you want to modify an int, you need to pass it by the int*.

In this question, the value we want to modify is a pointer int* (pointer changed from NULL to the address of the allocated memory), so we need to pass a pointer to the pointer int**.

By doing followed, pInt inside foo(int*) is a copy of the argument. When we allocate memory to the local variable, the one in the main() is intact.

void foo(int* pInt)
{
   pInt = malloc(...);
}
int main()
{
   int* pInt;
   foo(pInt);
   return 0;
}

So we need a pointer to pointer,

void foo(int** pInt)
{
   *pInt = malloc(...);
}
int main()
{
   int* pInt;
   foo(&pInt);
   return 0;
}

Answer 2

If you want your function to modify the pointer itself, you'll need to pass it as a pointer to a pointer. Here's a simplified example:

void allocate_memory(char **ptr, size_t size) {
    void *memory = malloc(size);
    if (memory == NULL) {
        // ...error handling (btw, there's no need to call free() on a null pointer. It doesn't do anything.)
    }

    *ptr = (char *)memory;
}

int main() {
   char *data;
   allocate_memory(&data, 16);
}

Answer 3

The only way I could get pointer to a pointer solution to work for a similar problem I was having for this function

    BOOL OpenBitmap2 ( LPCTSTR pszFileName, char** pszBMPFile)  

Was by assigning a temporary pointer to store the address

    char* BMPFile;
    { BMPFile = (char*)GlobalAlloc(GPTR, dwFileSize + 1);   // allocate storage using GlobalAlloc + 1 for null term string

then reassigning it

    {* pszBMPFile = BMPFile; return (0);} // Error = False

Any comment on why using "* pszBMPFile" directly with GlobalAlloc didn't work would be appreciated. I answered my own question. I forgot to carry the "*" through with pszBMPFile in the other lines of code. Good lessons from all the contributors. Many thanks.