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C type casting doubts when type are converted many times here and there

What is this type casting doing?

ph = (struct Proghdr *)((uint8_t *) ELFHDR + ELFHDR->e_phoff);

Here, ELFHDR is already defined as #define ELFHDR ((struct Elf *) 0x10000).

I want to ask: At the end, the value in ph will be (struct Proghdr *) then what is the need for the (uint8_t *)?

I will be grateful if someone solves my problem related to type-casting.

Solution

  • Remember the equivalence p[i] == *( p + i ).

    Given pointer p and integer i, p + i advances pointer p by i times the size of *p.

    For example, you see p advanced by 8 times the time size of a uint32_t in the following:

    uint32_t a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
uint32_t *p = a;
p = p + 8;
printf( "%" PRIu32 "\n", *p );  // 8

    So if you wanted to advance the pointer by i bytes, you'd need a pointer to a byte (char).

    uint32_t a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
uint32_t *p = a;
p = (uint32_t *)( (char *)p + 8 );
printf( "%" PRIu32 "\n", *p );  // 2

    uint8_t is used as an equivalent of char in the snippet you posted.

    Demo on Compile Explorer