Question

I have a structure as below

typedef struct Mystruct{
char *name;
int telno;
struct Mystruct *nextp;
}data;

Now I malloc the structure

data *addnode;
addnode = malloc (sizeof(data));

Now I would add data to the char *name.

addnode->name = malloc (sizeof(MAX));

Question:Why is it required to malloc again?

I was under the assumption that malloc-ing the addnode will even allocate the memory for addnode->name but it is not so.

Answer 1

Allocating memory for Mystruct provides enough memory for a pointer to name. At this point we have no idea how many characters will be in a name so can’t possibly allocate the memory for it.

If you want to fully allocate the structure in a single allocation, you could decide on a max size for name and change the structure definition to

#define MAX_NAME (10) /* change this as required */
typedef struct Mystruct{
    char name[MAX_NAME];
    int telno;
    struct Mystruct *nextp;
}data;

Or, if you know the name when you allocate the struct, you could hide the need for two allocations from the caller by providing a constructor function

struct Mystruct* Mystruct_create(const char* name)
{
    Mystruct* ms = malloc(sizeof(*ms));
    ms->name = strdup(name);
    return ms;
}