Question

I have to use the following block of code for a school assignment, STRICTLY WITHOUT ANY MODIFICATIONS.

typedef struct 
{
    char* firstName;
    char* lastName;
    int id;
    float mark;
}* pStudentRecord;

pStudentRecord* g_ppRecords;
int g_numRecords =0;

Here g_ppRecords is supposed to be an array of pointers to structs. What I am completely failing to understand is that how can the statement pStudentRecords *g_ppRecords; mean g_ppRecords to be an array because an array should be defined as

type arrayname[size];

I tried allocating memory to g_ppRecords dynamically, but that’s not helping.

g_ppRecords = (pStudentRecord*) malloc(sizeof(pStudentRecord*)*(g_numRecords+1));

Answer 1

Observe that pStudentRecord is typedef’d as a pointer to a structure. Pointers in C simply point to the start of a memory block, whether that block contains 1 element (a normal “scalar” pointer) or 10 elements (an “array” pointer). So, for example, the following

char c = 'x';
char *pc = &c;

makes pc point to a piece of memory that starts with the character 'x', while the following

char *s = "abcd";

makes s point to a piece of memory that starts with "abcd" (and followed by a null byte). The types are the same, but they might be used for different purposes.

Therefore, once allocated, I could access the elements of g_ppRecords by doing e.g. g_ppRecords[1]->firstName.

Now, to allocate this array: you want to use g_ppRecords = malloc(sizeof(pStudentRecord)*(g_numRecords+1)); (though note that sizeof(pStudentRecord*) and sizeof(pStudentRecord) are equal since both are pointer types). This makes an uninitialized array of structure pointers. For each structure pointer in the array, you’d need to give it a value by allocating a new structure. The crux of the problem is how you might allocate a single structure, i.e.

g_ppRecords[1] = malloc(/* what goes here? */);

Luckily, you can actually dereference pointers in sizeof:

g_ppRecords[1] = malloc(sizeof(*g_ppRecords[1]));

Note that sizeof is a compiler construct. Even if g_ppRecords[1] is not a valid pointer, the type is still valid, and so the compiler will compute the correct size.