Question

In one function I have written:

char *ab; 

ab=malloc(10);

Then in another function I want to know the size of memory pointed by the ab pointer.
Is there any way that I can know that ab is pointing to 10 chars of memory?

Answer 1

It’s a deep secret that only free() knows for sure. It’s likely in your system, but in a totally implementation dependent manner.

A bit awkward, but if you want to keep everything together:

typedef struct
{   // size of data followed by data (C only trick! NOT for C++)
    int        dimension;   // number of data elements
    int        data[1];     // variable number of data elements
} malloc_int_t;

malloc_int_t  *ab;

int  dimension = 10;
ab = malloc( sizeof(*ab) + (dimension-1)*sizeof(int) );
ab->dimension = dimension;

ab->data[n]  // data access 

I’ve changed the data type to int to make the code a more generic template.