Is top-level volatile or restrict significant in a function prototype?

Question

Is there any practical difference between the following prototypes?

void f(const int *p);

void f(const int *restrict p);

void f(const int *volatile p);
         


        

Answer 1

If there is nothing in the standard, then it's up to the compilers, but it seems that at least for gcc 4.9 (for x86) they are ignored. Check this small snippet that I've used to tease the compiler:

static int b;

void f(const int *p) {
  b = *p + 1;
}

int main()
{
     int a = 42;
     const int *p = &a;
     f(p);
     return a;
}

If I compile it as is, I get

f(int const*):
    pushq   %rbp
    movq    %rsp, %rbp
    movq    %rdi, -8(%rbp)
    movq    -8(%rbp), %rax
    movl    (%rax), %eax
    addl    $1, %eax
    movl    %eax, b(%rip)
    popq    %rbp
    ret
main:
    pushq   %rbp
    movq    %rsp, %rbp
    subq    $16, %rsp
    movl    $42, -12(%rbp)
    leaq    -12(%rbp), %rax
    movq    %rax, -8(%rbp)
    movq    -8(%rbp), %rax
    movq    %rax, %rdi
    call    f(int const*)
    movl    -12(%rbp), %eax
    leave
    ret

If I compile it using void f(const int *__restrict__ p) I get

f(int const*):
    pushq   %rbp
    movq    %rsp, %rbp
    movq    %rdi, -8(%rbp)
    movq    -8(%rbp), %rax
    movl    (%rax), %eax
    addl    $1, %eax
    movl    %eax, b(%rip)
    popq    %rbp
    ret
main:
    pushq   %rbp
    movq    %rsp, %rbp
    subq    $16, %rsp
    movl    $42, -12(%rbp)
    leaq    -12(%rbp), %rax
    movq    %rax, -8(%rbp)
    movq    -8(%rbp), %rax
    movq    %rax, %rdi
    call    f(int const*)
    movl    -12(%rbp), %eax
    leave
    ret

Anf finally if I compile it using void f(const int *__volatile__ p) I get

f(int const*):
    pushq   %rbp
    movq    %rsp, %rbp
    movq    %rdi, -8(%rbp)
    movq    -8(%rbp), %rax
    movl    (%rax), %eax
    addl    $1, %eax
    movl    %eax, b(%rip)
    popq    %rbp
    ret
main:
    pushq   %rbp
    movq    %rsp, %rbp
    subq    $16, %rsp
    movl    $42, -12(%rbp)
    leaq    -12(%rbp), %rax
    movq    %rax, -8(%rbp)
    movq    -8(%rbp), %rax
    movq    %rax, %rdi
    call    f(int const*)
    movl    -12(%rbp), %eax
    leave
    ret

So it seems that in practice they are ignored in C as well.