Question

What form is correct in allocating string in C?

char *sample;

sample = malloc ( length * sizeof(char) );

or

sample = malloc ( length * sizeof(char*) );

Why does char* take 4 bytes when char takes 1 byte?

Answer 1

Assuming the goal is to store a string of length characters, the correct allocation is:

sample = malloc(length + 1);

Notes:

1. Don’t use sizeof (char), since it’s always 1 it doesn’t add any value.
2. Remember the terminator, I assumed (based on name) that length is the length in visible characters of the string, i.e. the return of strlen() will be length.
3. I know you didn’t, but it’s worth pointing out that there should be no cast of the return value from malloc(), either.

The reason char * is larger is that it’s a pointer type, and pointers are almost always larger than a single character. On many systems (such as yours, it seems) they are 32 bit, while characters are just 8 bits. The larger size is needed since the pointer needs to be able to represent any address in the machine’s memory. On 64-bit computers, pointers are often 64 bits, i.e. 8 characters.