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What happens when you redirect both stdin and stdout?
A common problem when people create a process and redirect both stdin and stdout to pipes is that they fail to keep the pipes flowing. Once a pipe clogs, the disturbance propagates backward until everything clogs up. Here is a common error, in pseudocode:
How to combine stderr and stdout in Bash?
To combine stdout and stderr you would redirect the latter to the former using 2>&1. This redirects stderr (file descriptor 2) to stdout (file descriptor 1), e.g.: $ { echo “stdout”; echo “stderr” 1>&2; } | grep -v std stderr $ stdout goes to stdout, stderr goes to stderr. grep only sees stdout, hence stderr prints to the terminal.
How is a process connected to a stdin?
We write some data into one pipe (connected to a process’s stdin) and then read from another pipe (connected to a process’s stdout). For example, the program might take some input, do some transformation on it, and print the result to stdout. Consider: And now we’re deadlocked.
How does grep see both stdout and stderr?
stdout goes to stdout, stderr goes to stderr. grep only sees stdout, hence stderr prints to the terminal. On the other hand: After writing to both stdout and stderr, 2>&1 redirects stderr back to stdout and grep sees both strings on stdin, thus filters out both. You can read more about redirection here.
How to close stdout and stdin file descriptors?
To do this I close the file descriptor associated with the stdout or stdin, so that when I open the file it opens under the descriptor that I just closed. This works, but the problem is that once this is done, I need to restore the stdout and stdin to what they should really be.
What happens if you close stdout and stderr in C?
Specifically: If you use a C library function that uses stdout or stderr (which are FILE* pointers (see their definition) then writing to these whilst FILE* is closed is undefined behaviour. This will likely crash your program in unexpected ways, not always at the point of the bug either.
How to clone a stdin or stdout file?
What I can do for stdout is open (“/dev/tty”,O_WRONLY); But I’m not sure why this works, and more importantly I don’t know of an equivalent statement for stdin. You should use dup () and dup2 () to clone a file descriptor.
Can a program read from stdout while writing to stdin?
To avoid this problem, your program needs to keep reading from stdout while it’s writing to stdin, so that neither will block the other. The easiest way to do this is to perform the two operations on separate threads.
How to perform two operations on separate threads?
The easiest way to do this is to perform the two operations on separate threads. Next time, another common problem with pipes. Exercise : A customer reported that this function would sometimes hang waiting for the process to exit. Discuss.