how to execute self-generated x86 machine code on Linux

x86-codegen-at-runtime.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/mman.h>

int main(int argc, char *argv[]) {
  unsigned int pagesize;
  pagesize = sysconf(_SC_PAGE_SIZE);
  if (pagesize == -1) { perror("sysconf"); }

  size_t bufsize = 4 * pagesize;
  uint8_t *buf = aligned_alloc(pagesize, bufsize);
  if (buf == 0) { perror("memalign"); }

  // write x86 opcodes directly into memory. this is almost always a
  // very bad idea.
  memcpy(buf,
         "\xB8\x00\x11\x22\x33" // literal => eax
         "\xC3" // => retn (return from near procedure)
         , 6);

  // then write-protect and make executable
  if (mprotect(buf, bufsize, PROT_EXEC|PROT_READ ) < 0) {
    fprintf(stderr, "mprotect failed: %s\n", strerror(errno));
    return 1;
  }

  int (*ptr)(int) = (int (*)(int)) buf;
  printf("returns: 0x%X\n", ptr(42));
  return 0;
}

$ gcc x86-codegen-at-runtime.c -o x86-codegen-at-runtime && ./x86-codegen-at-runtime
returns: 0x33221100