unix-fork-monitor-child-progress

In this kind of situation where you only want to monitor the progress, the easiest alternative is to use shared memory. Every process updates it progress value (e.g. an integer) on a shared memory block, and the master process reads the block regularly. Basically, you don't need any locking in this scheme. Also, it is a "polling" style application because the master can read the information whenever it wants, so you do not need any event processing for handling the progress data.

If the only progress you need is "how many jobs have completed?", then a simple

while (jobs_running) {    pid = wait(&status);    for (i = 0; i < num_jobs; i++)        if (pid == jobs[i]) {            jobs_running--;            break;        }    printf("%i/%i\n", num_jobs - jobs_running, num_jobs);}

will do. For reporting progress while, well, in progress, here's dumb implementations of some of the other suggestions.

Pipes:

#include <poll.h>#include <stdio.h>#include <stdlib.h>#include <time.h>#include <unistd.h>int child(int fd) {    int i;    struct timespec ts;    for (i = 0; i < 100; i++) {        write(fd, &i, sizeof(i));        ts.tv_sec = 0;        ts.tv_nsec = rand() % 512 * 1000000;        nanosleep(&ts, NULL);    }    write(fd, &i, sizeof(i));    exit(0);}int main() {    int fds[10][2];    int i, j, total, status[10] = {0};    for (i = 0; i < 10; i++) {        pipe(fds[i]);        if (!fork())            child(fds[i][1]);    }    for (total = 0; total < 1000; sleep(1)) {        for (i = 0; i < 10; i++) {            struct pollfd pfds = {fds[i][0], POLLIN};            for (poll(&pfds, 1, 0); pfds.revents & POLLIN; poll(&pfds, 1, 0)) {                read(fds[i][0], &status[i], sizeof(status[i]));                for (total = j = 0; j < 10; j++)                    total += status[j];            }        }        printf("%i/1000\n", total);    }    return 0;}

Shared memory:

#include <semaphore.h>#include <stdio.h>#include <stdlib.h>#include <sys/mman.h>#include <time.h>#include <unistd.h>int child(int *o, sem_t *sem) {    int i;    struct timespec ts;    for (i = 0; i < 100; i++) {        sem_wait(sem);        *o = i;        sem_post(sem);        ts.tv_sec = 0;        ts.tv_nsec = rand() % 512 * 1000000;        nanosleep(&ts, NULL);    }    sem_wait(sem);    *o = i;    sem_post(sem);    exit(0);}int main() {    int i, j, size, total;    void *page;    int *status;    sem_t *sems;    size = sysconf(_SC_PAGESIZE);    size = (10 * sizeof(*status) + 10 * sizeof(*sems) + size - 1) & size;    page = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);    status = page;    sems = (void *)&status[10];    for (i = 0; i < 10; i++) {        status[i] = 0;        sem_init(&sems[i], 1, 1);        if (!fork())            child(&status[i], &sems[i]);    }    for (total = 0; total < 1000; sleep(1)) {        for (total = i = 0; i < 10; i++) {            sem_wait(&sems[i]);            total += status[i];            sem_post(&sems[i]);        }        printf("%i/1000\n", total);    }    return 0;}

Error handling etc. elided for clarity.

A few options (no idea which, if any, will suit you--a lot depends on what you are actually doing, as opped to the "uppercasing files" analogy):

• signals
• fifos / named pipes
• the STDOUT of the children or other passed handles
• message queues (if appropriate)