C++ getline()从c中的外部函数传递指针时输出null

我试图从文件中读取行，当我在Batch_MODE函数中直接使用getline时，它工作得很好，但当我在一个单独的函数中重构代码时，它给了我一个错误.

int main(int argc, char *argv[]) {
  if (argc > 2)
    return EXIT_FAILURE;

  batch_mode(argv[1]);

  return EXIT_SUCCESS;
}

Batch_MODE函数

void batch_mode(char *path) {
  char *line = NULL;
  ssize_t error = 0;
  char *cmd = NULL;
  FILE *file = fopen(path, "r");

  if (file == NULL) {
    printf("Coudn't read file\n");
    return;
  }

  while (error != -1) {
    read_line(line, file, cmd);
    error = -1;

    int thread = fork();

    if (thread == 0) {
      char *myargs[] = {NULL};

      execv(cmd, myargs);
    } else {
      thread = wait(NULL);
    }
  }

  if (file != NULL)
    fclose(file);
  free(line);
}

lines are read correctly in this function when printf but output null in the Batch_MODE函数

ssize_t read_line(char *line, FILE *file, char *cmd) {
  size_t len = 0;
  ssize_t read = 0;

  read = getline(&line, &len, file);

  printf("%s\n", line);
  if (read == -1) {
    printf("Coudn't read line\n");
    return -1;
  }

  char *copy = line;
  cmd = strsep(&copy, " ");
  printf("%s\n", cmd);

  return 0;
}

当try 此代码时，它输出无法读取文件，因为它正在多次执行

void batch_mode(char *path) {
  char *line = NULL;
  ssize_t error = 0;
  char *cmd = NULL;
  FILE *file = fopen(path, "r");

  if (file == NULL) {
    printf("Coudn't read file\n");
    return;
  }

  while ((error = read_line(line, file, cmd)) != -1) {

    int thread = fork();

    if (thread == 0) {
      char *myargs[] = {NULL};

      execv(cmd, myargs);
    } else {
      thread = wait(NULL);
    }
  }

  if (file != NULL)
    fclose(file);
  free(line);
}