popen() alternative
My question is extension of this one: popen creates an extra sh process
Motives:
1) My program need to create a child which does tail
on a file. I need to process the output line by line. That is why I am using popen
because it returns FILE *. I can easily fetch single line, do what I need to do and print it.
One problem with popen is that you do not get pid of child (tail command in my case).
2) My program should not exit before its child is done. So I need to do wait
; but without pid, I cannot do it.
How can I achieve both the goals?
A possible (kludge) solution: do execvp("tail -f file > tmpfile") and the keep readin开发者_运维百科g that tmpfile. I am not sure how good this solution is, though.
Why aren't you using pipe/fork/exec method?
pid_t pid = 0;
int pipefd[2];
FILE* output;
char line[256];
int status;
pipe(pipefd); //create a pipe
pid = fork(); //span a child process
if (pid == 0)
{
// Child. Let's redirect its standard output to our pipe and replace process with tail
close(pipefd[0]);
dup2(pipefd[1], STDOUT_FILENO);
dup2(pipefd[1], STDERR_FILENO);
execl("/usr/bin/tail", "/usr/bin/tail", "-f", "path/to/your/file", (char*) NULL);
}
//Only parent gets here. Listen to what the tail says
close(pipefd[1]);
output = fdopen(pipefd[0], "r");
while(fgets(line, sizeof(line), output)) //listen to what tail writes to its standard output
{
//if you need to kill the tail application, just kill it:
if(something_goes_wrong)
kill(pid, SIGKILL);
}
//or wait for the child process to terminate
waitpid(pid, &status, 0);
- You can use
pipe
, a function of theexec*
family andfdopen
. This is non-standard, but so ispopen
. - You don't need to
wait
. Just read the pipe up toEOF
. execvp("tail -f file > tmpfile")
won't work, redirection is a feature of the shell and you're not running the shell here. Even if it worked it would be an awful solution. Suppose you have read to the end of the file, but the child process has not ended yet. What do you do?
You can use wait
as it doesn't want a PID to wait for but simply waits for the any child process to exit. If you have created other child processes you can keep track of them, and if wait
returns an unknown PID you can assume it's from your popen
process.
I'm not sure why you need the process ID of the child. When the child exits, your pipe read will return an EOF. If you need to terminate the child, just close the pipe.
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