Child process not continuing execution when ptrace'ing - c

Question

I made the following simple example to read memory from a child process using ptrace.
I want to see the value at a specific address, 0x601050, every second during the execution of a small matrix multiplication program. I use PTRACE_PEEKDATA followed by PTRACE_CONT and sleep for 1 second, in an infinite loop, to do so.
However, the matrix multiplication program never proceeds--it should print to stdout in the first instruction, but it never seems to execute. I understood that ptrace(PTRACE_CONT,pid) would signal the child to resume execution and that sleep(1) would allow it to execute for a second (until the next ptrace call), but that is not the case.
#include <string.h>
#include <errno.h>
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <sys/reg.h>
int read_mem(long *out, pid_t pid, long addr, size_t sz)
{
long tmp;
size_t copied = 0;
while(copied < sz)
{
tmp = ptrace(PTRACE_PEEKDATA, pid, addr+copied);
if(errno)
{
fprintf(stderr,"ptrace: error : %s\n",strerror(errno));
return copied;
}
memcpy(out,&tmp,sizeof(long));
copied += sizeof(long);
out++;
printf("ptrace: copied %d bytes\n",copied);
}
return copied;
}
int main()
{
pid_t child;
long result;
struct user_regs_struct regs;
int status;
long addr = 0x601050;
size_t sz = sizeof(double);
long *buf = (long*)malloc(sz);
child = fork();
if(child == 0)
{
ptrace(PTRACE_TRACEME);
execl("./matmul", "matmul", NULL);
}
else
{
ptrace(PTRACE_GETREGS, child, &regs);
printf("ptrace: regs.rip : 0x%lx\n", regs.rip);
while(1)
{
read_mem(buf, child, addr, sz);
printf("ptrace: read(0x%lx) : %f\n", addr, (double)(*buf));
ptrace(PTRACE_CONT, child);
sleep(1);
}
}
return 0;
}

Answer 1

You don't seem to set a PTRACE_O_TRACEEXEC option. Failing to do so results in SIGTRAP being sent to tracee upon a call to exec; if it is not prepared, the default action is a termination with a core dump.