Why can't get data from tail -f? - c++

Question

In my program I'm redirecting output of child process to pipe and in parent getting this result and doing something (this is not important). But my program doesn't work when I'm using tail -f test.txt command and doesn't recieve any data during tail is running, and getting this data only after tail is finished (or killed).
At first I have thought that problem was that tail -f doesn't flushing and that's why no data I can recieve, but when I have tried to redirect output of tail -f to some file the data were in this file even when tail were not finished.
//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;
}
Then I'm reading from child_stderr_stream and child_stdout_stream which were passed as parameters to function the part above is from what.
For reading I'm using select() to not block program until reading from one of the streams.
Adding part of select and read
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;
}

Answer 1

This strange problem have been solved very strangely. I have just set buffers of files to 0 this way:
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");
setbuf(*child_stdout_stream, NULL);
close(errpipe[1]);
*child_stderr_stream=fdopen(errpipe[0], "rt");
setbuf(*child_stderr_stream, NULL);
*child_pid=pid;
}
This is very strange, that this helps, but in any case my program is now working well.