[英]How can a Unix program display output on screen even when stdout and stderr are redirected?
我在我的Ubuntu机器上运行了一个程序(实际上是valgrind),并将stdout和stderr重定向到不同的文件。 我很惊讶地看到屏幕上出现一条短信 - 这怎么可能? 我怎么能在C ++程序中自己做到这一点?
编辑:这是我使用的命令,输出:
$ valgrind ./myprogram > val.out 2> val.err
*** stack smashing detected ***: ./myprogram terminated
EDIT2:玩了一下,事实证明myprogram,而不是valgrind,正在打印消息,并且如下面的回答,它看起来像gcc堆栈粉碎检测代码正在打印到/ dev / tty
它不是由valgrind写的,而是glibc,你的./myprogram使用的是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:
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);
下面是一些示例代码,它完全符合要求(感谢早期的答案指向我正确的方向)。 两者都是用g ++编译的,即使重定向stdout和stderr,它也会在屏幕上打印一条消息。
对于Linux(Ubuntu 14):
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
int main( int, char *[]) {
printf("This goes to stdout\n");
fprintf(stderr, "This goes to stderr\n");
int ttyfd = open("/dev/tty", O_RDWR);
const char *msg = "This goes to screen\n";
write(ttyfd, msg, strlen(msg));
}
对于Windows 7,使用MinGW:
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <conio.h>
void writeConsole( const char *s) {
while( *s) {
putch(*(s++));
}
}
int main( int, char *[]) {
printf("This goes to stdout\n");
fprintf(stderr, "This goes to stderr\n");
writeConsole( "This goes to screen\n");
}
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