From : http://eli.thegreenplace.net/2011/01/23/how-debuggers-work-part-1

#include<stdio.h>
#include<sys/reg.h>
#include<sys/types.h>
#include<sys/ptrace.h>
#include<sys/wait.h>
#include<unistd.h>
#include<sys/user.h>
#include<sys/syscall.h>

void run_debugger(pid_t child_pid);
void run_target(const char* programname);
int main(int argc, char** argv)
{
    pid_t child_pid;

    if (argc < 2) {
        printf("Expected a program name as argument\n");
        return -1;
    }

    child_pid = fork();
    if (child_pid == 0)
        run_target(argv[1]);
    else if (child_pid > 0)
        run_debugger(child_pid);
    else {
        printf("fork");
        return -1;
    }

    return 0;
}
void run_target(const char* programname)
{
    printf("target started.#include<stdio.h>
#include<sys/reg.h>
#include<sys/types.h>
#include<sys/ptrace.h>
#include<sys/wait.h>
#include<unistd.h>
#include<sys/user.h>
#include<sys/syscall.h>

void run_debugger(pid_t child_pid);
void run_target(const char* programname);
int main(int argc, char** argv)
{
    pid_t child_pid;

    if (argc < 2) {
        printf("Expected a program name as argument\n");
        return -1;
    }

    child_pid = fork();
    if (child_pid == 0)
        run_target(argv[1]);
    else if (child_pid > 0)
        run_debugger(child_pid);
    else {
        printf("fork");
        return -1;
    }

    return 0;
}
void run_target(const char* programname)
{
    printf("target started.#include<stdio.h>
#include<sys/reg.h>
#include<sys/types.h>
#include<sys/ptrace.h>
#include<sys/wait.h>
#include<unistd.h>
#include<sys/user.h>
#include<sys/syscall.h>

void run_debugger(pid_t child_pid);
void run_target(const char* programname);
int main(int argc, char** argv)
{
    pid_t child_pid;

    if (argc < 2) {
        printf("Expected a program name as argument\n");
        return -1;
    }

    child_pid = fork();
    if (child_pid == 0)
        run_target(argv[1]);
    else if (child_pid > 0)
        run_debugger(child_pid);
    else {
        printf("fork");
        return -1;
    }

    return 0;
}
void run_target(const char* programname)
{
    printf("target started.will run '%s'\n", programname);

    /* Allow tracing of this process */
    if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
        printf("ptrace");
        return;
    }
    printf("replacing process image");
    /* Replace this process's image with the given program */
    execl(programname, programname, NULL);
}
void run_debugger(pid_t child_pid)
{
    int wait_status;
    unsigned icounter = 0;
    printf("debugger started\n");

    /* Wait for child to stop on its first instruction */
    wait(&wait_status);

    while (WIFSTOPPED(wait_status)) {
        icounter++;
        struct user_regs_struct regs;
        ptrace(PTRACE_GETREGS, child_pid, 0, &regs);
        unsigned instr = ptrace(PTRACE_PEEKTEXT, child_pid, regs.rip, 0);

        /*printf("icounter = %u.  EIP = 0x%08lld.  instr = 0x%08x\n",
                    icounter, regs.rip, instr);*/

        /* Make the child execute another instruction */
        if (ptrace(PTRACE_SINGLESTEP, child_pid, 0, 0) < 0) {
            printf("ptrace");
            return;
        }

        /* Wait for child to stop on its next instruction */
        wait(&wait_status);
    }

    printf("the child executed %u instructions\n", icounter);
}