init

main.c

@@ -0,0 +1,125 @@
+#define _DEFAULT_SOURCE
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define exit_with(f)                                                           \
+  exit_code = f;                                                               \
+  goto cleanup;
+
+#define throw_on(f, e, c)                                                      \
+  if (f == e) {                                                                \
+    exit_with(c);                                                              \
+  }
+
+#define free_if_used(f)                                                        \
+  if (f != NULL) {                                                             \
+    free(f);                                                                   \
+  }
+
+int main(int argc, char *argv[]) {
+
+  int exit_code = 0;
+  if (argc < 2) {
+    exit(EINVAL);
+  }
+
+  // open file
+  char *script_path = argv[1];
+  FILE *script = fopen(script_path, "r");
+  if (script == NULL) {
+    exit_with(errno);
+  }
+
+  // init variables
+  char found_path [PATH_MAX+1];
+  int nargc = 0;
+  char *args [128];
+
+  // read magic bytes
+  size_t read = 0;
+  char *line = NULL;
+  read = getline(&line, &read, script);
+  if (read == -1) {
+    exit_with(errno);
+  }
+
+  // close file
+  throw_on(fclose(script), EOF, EIO);
+
+  if (strncmp(line, "#?", 2) != 0) {
+    exit_with(ENOEXEC);
+  }
+
+  // read argv[0]
+  char *token = strtok(&line[2], " ");
+  throw_on(token, NULL, EINVAL);
+
+  args[nargc] = malloc(sizeof(char) * strlen(token));
+  // avoid copying the magic bytes and newline
+  char* p = mempcpy(args[nargc], token, strlen(token) - 1);
+  *p = '\0';
+  token = strtok(NULL, " ");
+  ++nargc;
+
+  // collect rest of args
+  while (token != NULL && nargc < 127) {
+    args[nargc] = malloc(sizeof(char) * (strlen(token) + 1));
+    strcpy(args[nargc++], token);
+    token = strtok(NULL, " ");
+  }
+  args[nargc] = script_path;
+
+
+  // search for path env var
+  char *path_env = getenv("PATH");
+  if (path_env == NULL) {
+    exit(EINVAL);
+  }
+
+  // split it by colon
+  token = strtok(path_env, ":");
+  // search each member of PATH for executable with name
+  struct dirent *entity;
+  DIR *dir;
+  char rpath [PATH_MAX+1];
+  while (token != NULL) {
+    char* __attribute__((unused)) _ = realpath(token, rpath);
+    dir = opendir(token);
+    if (dir != NULL) {
+      while ((entity = readdir(dir)) != NULL) {
+        if (entity->d_type == DT_REG && strcmp(args[0], entity->d_name) == 0) {
+          throw_on(closedir(dir), -1, EIO);
+          int len = strlen(token) + strlen(args[0]) + 2;
+          snprintf(found_path, len, "%s/%s", token, args[0]);
+          goto end_loop;
+        }
+      }
+      throw_on(closedir(dir), -1, EIO);
+    }
+    token = strtok(NULL, ":");
+  }
+
+end_loop:
+  if (strlen(found_path) == 0) {
+    exit_with(ENOENT);
+  }
+
+cleanup:
+  free_if_used(line);
+
+  if (exit_code == 0) {
+    if (execv(found_path, args) == -1) {
+      exit(errno);
+    }
+  }
+  for (int n = 0; n < nargc; ++n) {
+    free_if_used(args[nargc]);
+  }
+
+  return exit_code;
+}