Question

I’ve got a small program that does a large amount of processing. The progress of which you can get a print of by hitting the enter key.

The way I’ve implemented this is by having the processing done in the main thread whilst I have a pthread constantly looping on getchar() to wait for the enter key.

The problem is when I have finished with the processing. When this happens the main thread finishes, but still waits for enter to be pressed because getchar() is blocking.

How do I “cancel” getchar()?

Answer 1

The most portable solution I can think of is:

• Use pipe() to construct two FDs, one a reader and the other a writer. Give the reader to your read() loop; give the writer to whoever needs to terminate the reader.
• Use select() from the read thread to wait for readability of both stdin and the reader pipe.
• If stdin becomes readable, read a character, process it, and then restart the loop.
• If the reader pipe becomes readable, close it and terminate the loop.

Now, all you should have to do is close the other end of the pipe and this will wake up the reader thread out of its select() and it should then terminate.

The traditional approach involves using signals, however this pipe-based solution allows you to check for input on stdin as well as check if you should terminate using the same polling mechanism.

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Note that mixing getchar() and select() will not work, since getchar() will effectively use fread() under the hood, and the buffering performed by fread() can cause select() to block even though there is data available. Use read() instead. Here is an example program I used to test this approach.

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/select.h>

void * entry_point(void * p) {
    int readpipe = *(int *)p;
    fd_set rfds;

    char c;

    for (;;) {
        FD_ZERO(&rfds);
        FD_SET(STDIN_FILENO, &rfds);
        FD_SET(readpipe, &rfds);

        while (select(readpipe + 1, &rfds, NULL, NULL, NULL) == 0);

        if (FD_ISSET(readpipe, &rfds)) {
            close(readpipe);
            break;
        }

        if (FD_ISSET(STDIN_FILENO, &rfds)) {
            if (read(STDIN_FILENO, &c, sizeof(c)) > 0) {
                printf("Read: %d\n", c);
            }
        }
    }

    printf("Thread terminating\n");

    pthread_exit(NULL);
}

int main() {
    pthread_t thread;
    int r;
    int pipes[2];

    pipe(pipes);

    if (r = pthread_create(&thread, NULL, entry_point, &pipes[0])) {
        printf("Error: %d\n", r);
        return 1;
    }

    sleep(5);

    printf("Closing pipe and joining thread.\n");

    close(pipes[1]);
    pthread_join(thread, NULL);

    pthread_exit(NULL);
}

Example run:

$ time ./test
1
Read: 49
Read: 10
2
Read: 50
Read: 10
3
Read: 51
Read: 10
4
Read: 52
Read: 10
5
Read: 53
Read: 10
Closing pipe and joining thread.
Thread terminating

real    0m5.004s
user    0m0.004s
sys     0m0.000s