Checking NULL Pointer In C Doesn't Work

First of all,

a = malloc(sizeof(user) * user_counts); 

has a problem - you want to allocate user_counts instances of pointers to user, not instances of user, so that line should be

a = malloc(sizeof(user *) * user_counts); 

However, there's an easier way around this - the sizeof operator can take expressions as arguments as well as type names. So you can rewrite that line as

a = malloc( sizeof *a * user_counts ); 

The expression *a has type user *, so sizeof *a is equivalent to sizeof (user *). This makes your life a bit simpler, in that you don't have to puzzle out the type that a points to - make the compiler do the hard work.

You should always check the result of a malloc call.

a = malloc( sizeof *a * users_count ); if ( a ) { // do stuff with a } 

Second of all, don't use *(a+i) to index into a - use a[i] instead. Makes things a bit easier to read. So,

 *(a+i) = malloc(sizeof(user)); (*(a+i))->id = 1; (*(a+i))->name = malloc(sizeof(char) * 25); 

becomes

 a[i] = malloc(sizeof *a[i]); if ( a[i] ) { a[i]->id = 1; a[i]->name = malloc(sizeof *a[i]->name * 25); } else { /* deal with memory allocation failure */ } 

Now, to your actual problem. Your code is crashing for one of the following reasons:

• the initial malloc of a failed, so you're crashing on the first line that uses a[i];
• the malloc of one of your a[i] failed, so you're crashing on a[i]->id = 1;
• you've successfully allocated memory for all users_count elements of a - no a[i] is NULL, so you loop past the last element of your array and try to dereference the object immediately following it, which is most likely not a valid pointer.

In addition to adding the checks after each malloc call, you should probably loop based on users_count:

for ( i = 0; i < users_count; i++ ) printf("ID: %d \t %s\n", a[i]->id, a[i]->name); 

Or, you need to allocate one extra element for a and set it to NULL:

a = malloc( sizeof *a * (users_count + 1) ); if ( a ) { a[users_count] = NULL; for ( i = 0; i < users_count; i++ ) ... } 

Note that calloc initializes all allocated memory to 0, so you can use that and not worry about explicitly setting an element to NULL:

a = calloc( users_count + 1, sizeof *a ); if ( a ) { for ( i = 0; i < users_count; i++ ) ... }