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How to make it so the pointer == null test passes?


allUsers + 3 means simply the forth struct customer_account over from the address allocated to allUsers. It cannot possibly be NULL under any circumstances. It is the same as

&allUsers[3]

Its content can be zeroed out, but the pointer itself is a positive offset from a valid pointer, and the standard requires it to be non-NULL, because it is one byte past the end of the dynamically allocated array.

To see what is going on, do this experiment:

for (int i = 0 ; i != 4 ; i++) {
    printf("%p
", (void*)(allUsers + i));
}

This will print four numbers - the first is allUsers, the second is allUsers plus the size of one struct customer_account, the third is allUsers plus the size of two struct customer_account, and the last one is what your program prints. As you can see, these addresses are a result of simple calculations, and have nothing to do with the content of the actual array.


Categories : C

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