从C中的不兼容指针类型[默认启用]警告返回

when I'm calling this function

当我调用这个函数时

 4 char *file_reader(FILE *fp, char *file_name, const char *mode) {
  5     char *buffer;
  6     fp = fopen(file_name, mode);
  7 
  8     if (fp != NULL) {
  9         fseek(fp, 0L, SEEK_END);
 10         long s = ftell(fp);
 11         rewind(fp);
 12         buffer = malloc(s);
 13         if (buffer != NULL) {
 14             fread(buffer, s, 1, fp);
 15             fclose(fp);
 16             fp = NULL;
 17         }
 18         if (fp != NULL)
 19             fclose(fp);
 20     } else {
 21         printf("No such file.\n");
 22         return "ERR";
 23     }
 24     return buffer;
 25 }
 4