Code generation on x86_64

adder_nowx.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h> // mmap, mprotect, munmap
#include <unistd.h>
typeof(int (*)(int)) adder(int y)
{
    void *mem = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0);
    if (mem == MAP_FAILED) {
        int e = errno;
        fprintf(stderr, "mmap: %s (%d)\n", strerror(e), e);
        abort();
    }
    unsigned char *instr = mem;
    *instr++ = 0x81; *instr++ = 0xc7; // add edi, ...
    memcpy(instr, &y, 4); instr += 4; // <imm32>
    *instr++ = 0x89; *instr++ = 0xf8; // mov eax, edi
    *instr++ = 0xc3; // ret
    __builtin___clear_cache(mem, (void *)instr);
    int ret = mprotect(mem, getpagesize(), PROT_READ | PROT_EXEC);
    if (ret != 0) {
        int e = errno;
        fprintf(stderr, "mprotect: %s (%d)\n", strerror(e), e);
        abort();
    }
    return (int (*)(int))mem;
}
int main(void)
{
    int (*f)(int) = adder(3);
    int (*g)(int) = adder(-7);
    int (*h)(int) = adder(42);
    printf("f(0) = %d\n", f(0));
    printf("f(-5) = %d\n", f(-5));
    printf("f(2) = %d\n", f(2));
    printf("g(0) = %d\n", g(0));
    printf("g(-5) = %d\n", g(-5));
    printf("g(2) = %d\n", g(2));
    printf("h(0) = %d\n", h(0));
    printf("h(-5) = %d\n", h(-5));
    printf("h(2) = %d\n", h(2));
}

adder_wx.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h> // mmap, mprotect, munmap
#include <unistd.h>
typeof(int (*)(int)) adder(int y)
{
    void *mem = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANON | MAP_PRIVATE, -1, 0);
    if (mem == MAP_FAILED) {
        int e = errno;
        fprintf(stderr, "mmap: %s (%d)\n", strerror(e), e);
        abort();
    }
    unsigned char *instr = mem;
    *instr++ = 0x81; *instr++ = 0xc7; // add edi, ...
    memcpy(instr, &y, 4); instr += 4; // <imm32>
    *instr++ = 0x89; *instr++ = 0xf8; // mov eax, edi
    *instr++ = 0xc3; // ret
    __builtin___clear_cache(mem, (void *)instr);
    return (int (*)(int))mem;
}
int main(void)
{
    int (*f)(int) = adder(3);
    int (*g)(int) = adder(-7);
    int (*h)(int) = adder(42);
    printf("f(0) = %d\n", f(0));
    printf("f(-5) = %d\n", f(-5));
    printf("f(2) = %d\n", f(2));
    printf("g(0) = %d\n", g(0));
    printf("g(-5) = %d\n", g(-5));
    printf("g(2) = %d\n", g(2));
    printf("h(0) = %d\n", h(0));
    printf("h(-5) = %d\n", h(-5));
    printf("h(2) = %d\n", h(2));
}