C++写入configfs/sysfs失败，bash命令可行，求非system()解决方案

问题描述 #

调试手机USB Gadget，要实现手机向PC提供ISO镜像服务时遇到异常：用C++代码写入/sys/kernel/config/usb_gadget/g1/UDC时始终报Device or resource busy错误，但命令行用echo执行完全正常，求不使用system()的解决办法。

（小更新：已通过freopen成功写入sysfs路径）

代码片段 #

void sysfs_write(char *path, char *content)
{
    std::cout << "Write: " << content << " -> " << path << std::endl;
    int fd = open(path, O_WRONLY);
    if (fd == -1)
    {
        perror("Failed");
        return;
    }
    int n = write(fd, content, strlen(content));
    if (n == -1)
    {
        perror("Failed");
    }
    close(fd);
}


sysfs_write("/sys/kernel/config/usb_gadget/g1/UDC", "");
sysfs_write("/sys/kernel/config/usb_gadget/g1/UDC" , "a600000.dwc3");
// 两次调用都报Device or resource busy错误

Stat信息 #

• /sys/kernel/config/usb_gadget/g1/UDC的stat输出：

File: /sys/kernel/config/usb_gadget/g1/UDC
  Size: 4096        Blocks: 0          IO Block: 4096   regular file
Device: 0,17    Inode: 249229      Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-02-01 01:17:16.229999999 +0200
Modify: 2023-02-01 01:28:46.424746782 +0200
Change: 2023-02-01 01:28:46.424746782 +0200
 Birth: -

• /sys/kernel/config/usb_gadget/g1的stat输出：

File: /sys/kernel/config/usb_gadget/g1
  Size: 0           Blocks: 0          IO Block: 4096   directory
Device: 0,17    Inode: 18684       Links: 4
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-02-01 01:18:00.646458114 +0200
Modify: 2023-02-01 01:17:23.679999997 +0200
Change: 2023-02-01 01:17:23.679999997 +0200
 Birth: -

解决办法 #

• 添加换行符到写入内容：命令行echo默认会在输出末尾添加换行符，而你的代码里直接写字符串没有加。sysfs的很多节点要求写入的内容以换行结尾，否则会判定为无效操作，进而返回资源忙错误。修改代码，在写入的字符串末尾加上\n：

  sysfs_write("/sys/kernel/config/usb_gadget/g1/UDC", "\n");
  sysfs_write("/sys/kernel/config/usb_gadget/g1/UDC" , "a600000.dwc3\n");
  

• 确保写入完整内容：检查write的返回值，确认是否写入了全部字节。改成循环写入，保证内容完全写入：

  void sysfs_write(char *path, char *content)
  {
      std::cout << "Write: " << content << " -> " << path << std::endl;
      int fd = open(path, O_WRONLY);
      if (fd == -1)
      {
          perror("Failed");
          return;
      }
      size_t len = strlen(content);
      ssize_t written = 0;
      while (written < len) {
          ssize_t ret = write(fd, content + written, len - written);
          if (ret == -1) {
              perror("Failed");
              break;
          }
          written += ret;
      }
      close(fd);
  }
  

• 调整文件打开模式：尝试添加O_TRUNC标志打开文件，确保文件被截断后再写入：

  int fd = open(path, O_WRONLY | O_TRUNC);
  

内容的提问来源于stack exchange，提问作者Alexandru Nitan