Example of executing bytes from an array

test.asm

; Compile with nasm -f elf64 test.asm
BITS 64
SECTION .text
global _start
_start:
    sub rsp, 8    ; Align the stack pointer for a function call
    mov rax, 0x14 ; Move syscall number 14 to rax (sys_getpid)
    int 0x80      ; Call system call
    push rax      ; push rax to the stack
    mov rbx, rax  ; move rax to rbx
    mov rax, 1    ; move syscall number 1 to rax (sys_exit)
    int 0x80      ; call system call
    ret           ; return

test.c

#if 0
0000000000000000 <_start>:
   0:   48 83 ec 08             sub    $0x8,%rsp
   4:   b8 14 00 00 00          mov    $0x14,%eax
   9:   cd 80                   int    $0x80
   b:   50                      push   %rax
   c:   48 89 c3                mov    %rax,%rbx
   f:   b8 01 00 00 00          mov    $0x1,%eax
  14:   cd 80                   int    $0x80
  16:   c3                      retq
#endif

char code[] =
{
    0x48, 0x83, 0xec, 0x08,       // sub rsp, 0x8
    0xb8, 0x14, 0x00, 0x00, 0x00, // mov rax, $0x14
    0xcd, 0x80,                   // int 0x80
    0x48, 0x89, 0xc3,             // mov rbx, rax
    0xb8, 0x01, 0x00, 0x00, 0x00, // mov rax, 0x1
    0xcd, 0x80,                   // int 0x80
    0xc3                          // ret
};

char code2[] = 
{
    0x48, 0x83, 0xec, 0x08,       // sub rsp, 0x8
    0xb8, 0x14, 0x00, 0x00, 0x00, // mov rax, 0x14
    0xcd, 0x80,                   // int 0x80
    0x50,                         // push rax
    0xc3                          // ret
};

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

#ifdef __cplusplus
// C++ does not like casting pointers to function pointers
// so this union works around that.
#define CAST(f, s) function_cast<f>(s)
template<class F> F function_cast(void *buf)
{
    union
    {
        void *buf;
        F func;
    } cast;
    cast.buf = buf;
    return cast.func;
}
#else
// C doesn't give 2 shits about pointers. Foot-shooting it is~
# define CAST(f, s) (f)s
#endif

int main()
{
    printf("Our corrent PID: %d\n", getpid());
    // Make a mmapped buffer so we can copy and execute the code
    void *buf =  mmap(0, sizeof(code2), PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANON, -1, 0);
    memcpy(buf, code2, sizeof(code2));

    int (*funct)(void) = CAST(int (*)(void), buf);

    printf("PID returned to us via func: %d\n", funct());
}

Hooray for foot-shooting!