Example of ptrace for FreeBSD

This is an example of using ptrace in FreeBSD. It's based on the example code of the Perl module FreeBSD::i386::Ptrace, translated into C. This example runs "ls". Example output:
# 00001 202 return: 0
# 00002 477 return: 2807A000
# 00003 073 return: 0
... etc.
The meanings of the system calls can be found by looking in /usr/include/sys/syscall.h.
/* This is a translation of the Perl routine. */

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <machine/reg.h>
#include <signal.h>

int getcall (int pid)
{
    int status;

    struct reg registers;
    status = ptrace (PT_GETREGS, pid, (caddr_t) & registers, 0);
    if (status != 0) {
        return -1;
    }
    return registers.r_eax;
}

void run ()
{
    int status;
    status = ptrace (PT_TRACE_ME, 0, 0, 0);
    if (status != 0) {
        exit (EXIT_FAILURE);
    }
    execl ("/bin/ls", "ls", NULL);
}

int main (int argc, char ** argv)
{
    int pid;

    pid = fork ();
    if (pid == -1) {
        fprintf (stderr, "Fork failed: %s\n", strerror (errno));
        exit (EXIT_FAILURE);
    }
    else if (pid == 0) {
        run ();
    }
    else {
        int count = 0;
        if (wait (0) == -1) {
            fprintf (stderr, "Wait failed.\n");
            exit (EXIT_FAILURE);
        }
        while (ptrace (PT_TO_SCE, pid, (caddr_t) 1, 0) == 0) {
            int call;
            int retval;
            if (wait (0) == -1) {
                break;
	    }
            call = getcall (pid);
            if (call == -1) {
                break;
	    }
            ptrace (PT_TO_SCX, pid, (caddr_t) 1, 0);
            if (wait (0) == -1) {
                break;
	    }
            retval = getcall (pid);
            count++;
            printf ("# %05d %03d return: %X\n", count, call, retval);
        }
        printf ("%d system calls issued.\n", count);
    }
    return 0;
}

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