You can also modify actual instructions as well as data.

CodeGen3.cpp

#include <cstdio>
#include <sys/mman.h>
#include <sys/types.h>
#include <cstring>
#include <cctype>
#include <cstdlib>


int main(int argc, const char * argv[])
{

    bool Add = true;

    char a = '\0';
    char b = '\0';
    do{
    printf("Enter a number.\n");
        a = getc(stdin);
    } while(!isdigit(a));
    
    printf("OK, using %c.\n", a);
    
    do{
    printf("Enter a second number.\n");
        b = getc(stdin);
    } while(!isdigit(b));
    
    printf("OK, using %c.\n", b);
    
    char t = '\0';
    do{
        t = getc(stdin);
        printf("Press 'a' to add, 's' to subtract.\n");
    }while(t!='a' && t!='s');
    
    if(t=='s')
        Add = false;
        
    printf("Running memory test.\n");
    
    // What we want to execute.
    unsigned char nop[] = {0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0xC3};
    
    void *lPage = mmap(nullptr, 0xFF, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0);

    if((lPage==NULL)||(lPage==(void *)(~NULL))){
        printf("Memory map failed.\n");
        return EXIT_FAILURE;
    }

    memcpy(lPage, nop, sizeof(nop));

    mprotect(lPage, sizeof(nop), PROT_READ|PROT_EXEC);
    
    void (*func)(void);
    func = (void (*)(void))lPage;
    
    func();
    
    printf("Successful memory test. Performing %s:\n", Add?"addition":"subtraction");
    
    char ac[] = {a, 0};
    char bc[] = {b, 0};
                
    unsigned char adds[] = {

        0x48, 0xC7, 0xC0, (unsigned char)atoi(ac), /*put a in rax.*/
        0, 0, 0, /*align*/ 
        0x48, 0x83, (unsigned char)(Add?0xC0:0xE8), (unsigned char)atoi(bc), /*Add b to rax, storing result in rax.*/
        0xC3, /*return. Return values are in rax in x86_64.*/
    };
    
    mprotect(lPage, sizeof(adds), PROT_READ|PROT_WRITE);
    
    memcpy(lPage, adds, sizeof(adds));

    mprotect(lPage, sizeof(adds), PROT_READ|PROT_EXEC);
    
    int (*ifunc)(void);
    ifunc = (int (*)(void))lPage;
    
    int r = ifunc();
    
    printf("In program-modified machine code, %c %c %c = %i.\n", a, Add?'+':'-', b, r);
    
    return EXIT_SUCCESS;
}