3
在我的程序中,我将子进程的输出重定向到管道,并在父进程中获取此结果并执行某些操作(这不重要)。但是,当我使用命令时,我的程序不起作用,并且在尾部运行期间没有收到任何数据,只有尾部完成(或终止)后才能获取此数据。为什么无法从tail -f获取数据?
起初我以为问题是tail -f
不刷新,这就是为什么我没有收到数据,但是当我试图将tail -f
的输出重定向到某个文件时,即使尾部是没做完。
//the code of creating child and redirecting data (important part)
//only core is here so please don't tell me that maybe pipe() or fork() is failed
pid_t pid;
int outpipe[2]; //pipe for reading from stdout
int errpipe[2]; //pipe for reading from stderr
// Createing pipes for childs stdout and stderr streams
pipe(outpipe);
pipe(errpipe);
pid = fork();
if(pid == 0)
{
// This is the child process. Closing read end of pipes and duplicating stdout and stderr streams
close(outpipe[0]);
dup2(outpipe[1], STDOUT_FILENO);
close(errpipe[0]);
dup2(errpipe[1], STDERR_FILENO);
if(execvp(argv[0], (char * const *)argv) == -1)
{
fprintf(stderr, "Failed to execute command %s: %s", argv[0], strerror(errno));
_exit(EXIT_FAILURE);
}
_exit(EXIT_SUCCESS);
}
else if (pid != -1)
{
// This is the parent process, Closing write end of pipes and opening fds as FILE
close(outpipe[1]);
*child_stdout_stream=fdopen(outpipe[0], "rt");
close(errpipe[1]);
*child_stderr_stream=fdopen(errpipe[0], "rt");
*child_pid=pid;
}
然后,我从其中作为参数传递给函数以上的部分是由什么child_stderr_stream
和child_stdout_stream
阅读。 对于阅读,我使用select()在从其中一个流读取之前不阻止程序。
添加的选择部分和阅读
int select_and_read(FILE **files, bool *is_eof, char *chars, int *mask, int nfiles, int timeout, pid_t child_pid)
{
int max_fd_plus_1 = 0;
fd_set rfds;
struct timeval tv;
FD_ZERO(&rfds);
for(int i = 0; i < nfiles; ++i)
{
if(is_eof[i]==false)
{
FD_SET(fileno(files[i]), &rfds);
max_fd_plus_1 = (max_fd_plus_1 > fileno(files[i])) ? max_fd_plus_1 : fileno(files[i]);
}
}
++max_fd_plus_1;
tv.tv_sec = timeout/1000;
tv.tv_usec = (timeout % 1000) * 1000;
int retval = select(max_fd_plus_1, &rfds, NULL, NULL, &tv);
if(retval > 0)
{
*mask = 0;
for(int i = 0; i < nfiles; ++i)
{
if(is_eof[i]==false)
{
if(FD_ISSET(fileno(files[i]), &rfds))
{
*mask |= 1 << i;
chars[i] = fgetc(files[i]);
}
}
}
}
else
{
kill(child_pid, SIGKILL);
}
return retval;
}
问题可能在select()中。添加一些调试以查看当您连接到标准输出时FD是否准备就绪。也许你是选择()在标准输出? – FoneyOp 2011-04-15 15:35:36
是select()不告诉我可以从流中读取。它挂起,但为什么?我怎么解决这个问题? – 2011-04-15 15:37:08