The Mechanism: Ptrace

ptrace(2) is  how a debugger can control a process

long ptrace(enum __ptrace_request request, pid_t pid,
            void *addr, void *data);
PTRACE_TRACEME
PTRACE_ATTACH
PTRACE_KILL
PTRACE_DETACH

TRACE_PEEKTEXT, PTRACE_PEEKDATA
PTRACE_PEEKUSER
PTRACE_POKETEXT, PTRACE_POKEDATA
PTRACE_POKEUSER
PTRACE_GETREGS, PTRACE_GETFPREGS
PTRACE_SETREGS, PTRACE_SETFPREGS
PTRACE_CONT
PTRACE_SYSCALL, PTRACE_SINGLESTEP

Example: ptracetest