即使stdout和stderr重定向，Unix程序如何在屏幕上显示输出？ [英] How can a Unix program display output on screen even when stdout and stderr are redirected?

问题描述

我在我的Ubuntu计算机上运行一个程序（实际上是valgrind），并且将stdout和stderr重定向到不同的文件。我惊讶地看到一条短信出现在屏幕上 - 怎么可能？

I was running a program (valgrind, actually) on my Ubuntu machine, and had redirected both stdout and stderr to different files. I was surprised to see a short message appear on the screen -- how is that possible? How could I do that myself in a C++ program?

编辑：这是我使用的命令和输出：

Here's the command I used, and the output:

$ valgrind ./myprogram > val.out 2> val.err
*** stack smashing detected ***: ./myprogram terminated

编辑2：使用它一点，事实证明myprogram，而不是valgrind，导致消息被打印，如下面的回答，看起来像gcc堆栈捣毁检测代码正在打印到/ dev / tty

Playing with it a little more, it turns out that myprogram, not valgrind, is causing the message to be printed, and as answered below, it looks like gcc stack smashing detection code is printing to /dev/tty

推荐答案

它不是由valgrind写的，而是glibc和./myprogram使用glibc：

It is not written by valgrind but rather glibc and your ./myprogram is using glibc:

#define _PATH_TTY   "/dev/tty"

/* Open a descriptor for /dev/tty unless the user explicitly
   requests errors on standard error.  */
const char *on_2 = __libc_secure_getenv ("LIBC_FATAL_STDERR_");
if (on_2 == NULL || *on_2 == '\0')
  fd = open_not_cancel_2 (_PATH_TTY, O_RDWR | O_NOCTTY | O_NDELAY);

if (fd == -1)
  fd = STDERR_FILENO;

...
written = WRITEV_FOR_FATAL (fd, iov, nlist, total);

以下是glibc的一些相关部分：

Below are some relevant parts of glibc:

void
__attribute__ ((noreturn))
__stack_chk_fail (void)
{
  __fortify_fail ("stack smashing detected");
}

void
__attribute__ ((noreturn))
__fortify_fail (msg)
     const char *msg;
{
  /* The loop is added only to keep gcc happy.  */
  while (1)
    __libc_message (2, "*** %s ***: %s terminated\n",
            msg, __libc_argv[0] ?: "<unknown>");
}


/* Abort with an error message.  */
void
__libc_message (int do_abort, const char *fmt, ...)
{
  va_list ap;
  int fd = -1;

  va_start (ap, fmt);

#ifdef FATAL_PREPARE
  FATAL_PREPARE;
#endif

  /* Open a descriptor for /dev/tty unless the user explicitly
     requests errors on standard error.  */
  const char *on_2 = __libc_secure_getenv ("LIBC_FATAL_STDERR_");
  if (on_2 == NULL || *on_2 == '\0')
    fd = open_not_cancel_2 (_PATH_TTY, O_RDWR | O_NOCTTY | O_NDELAY);

  if (fd == -1)
    fd = STDERR_FILENO;

  ...
  written = WRITEV_FOR_FATAL (fd, iov, nlist, total);

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