Implement "trap and exec" with SIGSTOP

trapit.cc

@@ -1,5 +1,14 @@
+// Copyright Jiangge Zhang (License: MIT)
+
+#include <string.h>
+#include <unistd.h>
+#include <signal.h>
+
 #include <iostream>
-#include <cstring>
+
+#ifndef TRAPIT_MAX_ARGS
+#define TRAPIT_MAX_ARGS 4096
+#endif
 
 int cmd_usage(const char *prog);
 int cmd_trap(const char *prog, int argc, char **argv);
@@ -11,15 +20,15 @@ int main(int argc, char **argv) {
 
     if (_argc < 0) {
         return cmd_usage(argv[0]);
-    } else if (std::strcmp(argv[1], "exec") == 0) {
-        if (_argc > 0 && std::strcmp(_argv[0], "--") == 0) {
+    } else if (strcmp(argv[1], "exec") == 0) {
+        if (_argc > 0 && strcmp(_argv[0], "--") == 0) {
             _argc--;
             _argv++;
         }
         return cmd_trap(argv[0], _argc, _argv);
-    } else if (std::strcmp(argv[1], "wake") == 0) {
+    } else if (strcmp(argv[1], "wake") == 0) {
         return cmd_wake(argv[0], _argc, _argv);
-    } else if (std::strcmp(argv[1], "help") == 0) {
+    } else if (strcmp(argv[1], "help") == 0) {
         cmd_usage(argv[0]);
         return 0;
     } else {
@@ -38,13 +47,35 @@ int cmd_usage(const char *prog) {
 }
 
 int cmd_trap(const char *prog, int argc, char **argv) {
-    std::cout << "Work-in-Progress: exec" << std::endl;
-    while (argc > 0) {
-        std::cout << "  " << *argv << std::endl;
-        argc--;
-        argv++;
+    /*
+     * Layout: [ARG 0] [ARG 0] [ARG 1] ... [NULL]
+     */
+    const char *exec_argv[TRAPIT_MAX_ARGS];
+
+    if (argc <= 0) {
+        return cmd_usage(prog);
+    } else if (argc >= TRAPIT_MAX_ARGS - 1) {
+        std::cerr << "Up to " << TRAPIT_MAX_ARGS - 2
+            << " arguments, got " << argc << " arguments";
+        return 1;
+    } else {
+        exec_argv[0] = argv[0];
+        for (int i = 0; i < argc; i++) {
+            exec_argv[i + 1] = argv[i];
+        }
+        exec_argv[argc + 1] = NULL;
     }
-    return 0;
+
+    if (kill(getpid(), SIGSTOP) == -1) {
+        std::cerr << "Cannot pause current process: " <<
+            strerror(errno) << " (errno=" << errno << ")" << std::endl;
+        return 1;
+    }
+
+    execvp(exec_argv[0], (char *const *) exec_argv);
+    std::cerr << "Cannot execute " << exec_argv[0] << ": " <<
+        strerror(errno) << " (errno=" << errno << ")" << std::endl;
+    return 1;
 }
 
 int cmd_wake(const char *prog, int argc, char **argv) {