signal-echo.cpp

#include <pwd.h>
#include <sys/ucontext.h>
#include <unistd.h>
#include <cerrno>
#include <csignal>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#define ERROR(fmt, ...)                                         \
    do {                                                        \
        fprintf(stderr, "%s:%d: " fmt "\n", __FILE__, __LINE__, \
                ##__VA_ARGS__);                                 \
        exit(1);                                                \
    } while (false)

void signal_handler(int _, siginfo_t* info, void* void_ctx) {
    struct passwd* passwd_data = getpwuid(info->si_uid);

    printf("Received a SIGUSR1 signal from process %d executed by %u (%s).\n",
           info->si_pid, info->si_uid, passwd_data->pw_name);

    greg_t* gregs = ((ucontext_t*)void_ctx)->uc_mcontext.gregs;
    printf("State of the context: RIP = %lld, RAX = %lld, RBX = %lld.\n",
           gregs[REG_RIP], gregs[REG_RAX], gregs[REG_RBX]);
}

int main() {
    struct sigaction action = {};
    action.sa_flags = SA_SIGINFO;
    action.sa_sigaction = signal_handler;

    if (sigaction(SIGUSR1, &action, nullptr) < 0) {
        ERROR("Failed to register signal handler: %s", strerror(errno));
    }

    pause();
}