C Programming: malloc() inside another function

Question

I need help with malloc() inside another function.

I'm passing a pointer and size to the function from my main() and I would like to allocate memory for that pointer dynamically using malloc() from inside that called function, but what I see is that.... the memory, which is getting allocated, is for the pointer declared within my called function and not for the pointer which is inside the main().

How should I pass a pointer to a function and allocate memory for the passed pointer from inside the called function?

---

I have written the following code and I get the output as shown below.

SOURCE:

int main() {    unsigned char *input_image;    unsigned int bmp_image_size = 262144;     if(alloc_pixels(input_image, bmp_image_size)==NULL)      printf("\nPoint2: Memory allocated: %d bytes",_msize(input_image));    else      printf("\nPoint3: Memory not allocated");         return 0; }  signed char alloc_pixels(unsigned char *ptr, unsigned int size) {     signed char status = NO_ERROR;     ptr = NULL;      ptr = (unsigned char*)malloc(size);      if(ptr== NULL)     {         status = ERROR;         free(ptr);         printf("\nERROR: Memory allocation did not complete successfully!");     }      printf("\nPoint1: Memory allocated: %d bytes",_msize(ptr));      return status; } 

PROGRAM OUTPUT:

Point1: Memory allocated ptr: 262144 bytes Point2: Memory allocated input_image: 0 bytes 

Answer 1

How should I pass a pointer to a function and allocate memory for the passed pointer from inside the called function?

Ask yourself this: if you had to write a function that had to return an int, how would you do it?

You'd either return it directly:

int foo(void) {     return 42; } 

or return it through an output parameter by adding a level of indirection (i.e., using an int* instead of int):

void foo(int* out) {     assert(out != NULL);     *out = 42; } 

So when you're returning a pointer type (T*), it's the same thing: you either return the pointer type directly:

T* foo(void) {     T* p = malloc(...);     return p; } 

or you add one level of indirection:

void foo(T** out) {     assert(out != NULL);     *out = malloc(...); }