$ ./a.out w5 s1 w5 s1 w 50000 | ./a.out r | ./a.out r1 check r &  ps -ef  | grep a.out
[3] 8408
ale       8406  8066  0 19:33 pts/6    00:00:00 ./a.out w5 s1 w5 s1 w 50000
ale       8407  8066  0 19:33 pts/6    00:00:00 ./a.out r
ale       8408  8066  0 19:33 pts/6    00:00:00 ./a.out r1 check r
ale       8410  8066  0 19:33 pts/6    00:00:00 grep a.out
$ 2025-03-03T19:33:50.616080+01:00 pcale dummy pipe[8406]: exiting
2025-03-03T19:33:50.616455+01:00 pcale dummy pipe[8407]: exiting
2025-03-03T19:33:50.616691+01:00 pcale dummy pipe[8408]: kill 8406: No such process
2025-03-03T19:33:50.616843+01:00 pcale dummy pipe[8408]: exiting

That is, by the time the third instance starts reading, the first one has already finished, despite the time it spent sleeping and the huge amount it writes. This means that at some point all of those 50,000 rows are stored together.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <syslog.h>
#include <errno.h>
#include <limits.h>

static void do_open(void)
{
    openlog("dummy pipe", LOG_PID, LOG_USER);
}

int main(int argc, char *argv[])
{
    do_open();
    pid_t me = 1;
    int wrote = 0, n;
    for (int i = 1; i < argc; ++i)
    {
        if (argv[i][0] == 'w' && (n = atoi(&argv[i][1])) > 0)
        {
            if (wrote == 0)
            {
                me = getpid();
                setsid();
                wrote = 1;
            }
            if (n --> 0)
                printf("Pid: %d\n", me);
            while (n --> 0)
                puts("Lorem ipsum lorem ipsum lorem ipsum lorem ipsum");
        }
        else if (argv[i][0] == 's' && (n = atoi(&argv[i][1])) > 0)
            sleep(n);
        else if (argv[i][0] == 'r')
        {
            if (argv[i][1] == 0)
                n = INT_MAX;
            else
                n = atoi(&argv[i][1]);

            char buf[1024];
            while (n --> 0)
            {
                char *p = fgets(buf, sizeof buf, stdin);
                if (p == NULL)
                    break;
                if (buf[0] == 'P')
                    sscanf(buf, "Pid: %d\n", &me);
                if (!isatty(fileno(stdout)))
                    printf("%s", buf);
            }
        }
        else if (strcmp(argv[i], "check") == 0)
        {
            if (me != 1)
            {
                n = kill(me, 0);
                syslog(LOG_NOTICE, "kill %d: %s\n", me,
                    n? strerror(errno): "still running");
                closelog();
                do_open();
            }
        }
    }
    syslog(LOG_NOTICE, "exiting\n");
    closelog();
    return 0;
}

$ ./a.out w5 s1 w5 s1 w50 | ./a.out r | ./a.out r1 check r &  ps -ef  | grep a.out
[3] 10908
ale      10906  8066  0 10:24 pts/6    00:00:00 ./a.out w5 s1 w5 s1 w50
ale      10907  8066  0 10:24 pts/6    00:00:00 ./a.out r
ale      10908  8066  0 10:24 pts/6    00:00:00 ./a.out r1 check r
ale      10910  8066  0 10:24 pts/6    00:00:00 grep a.out
$ 2025-03-04T10:24:56.628434+01:00 pcale dummy pipe[10906]: exiting (read 0 bytes)
2025-03-04T10:24:56.628925+01:00 pcale dummy pipe[10907]: exiting (read 2769 bytes)
2025-03-04T10:24:56.629188+01:00 pcale dummy pipe[10908]: kill 10906: No such process
2025-03-04T10:24:56.629377+01:00 pcale dummy pipe[10908]: exiting (read 2769 bytes)

That is, by the time the third instance starts reading, the first one has already finished, despite the time it spent sleeping. This means that at some point all of those 2,769 bytes rows are stored together.

Increasing the amount of data, as Scott noted, concurrency enters.

$ ./a.out w5 s1 w5 s1 w150 | ./a.out r | ./a.out r1 check r &  ps -ef  | grep a.out
[3] 11254
ale      11252  8066  0 10:31 pts/6    00:00:00 ./a.out w5 s1 w5 s1 w150
ale      11253  8066  0 10:31 pts/6    00:00:00 ./a.out r
ale      11254  8066  0 10:31 pts/6    00:00:00 ./a.out r1 check r
ale      11256  8066  0 10:31 pts/6    00:00:00 grep a.out
$ 2025-03-04T10:31:13.002766+01:00 pcale dummy pipe[11252]: exiting (read 0 bytes)
2025-03-04T10:31:13.003068+01:00 pcale dummy pipe[11254]: kill 11252: still running
2025-03-04T10:31:13.003235+01:00 pcale dummy pipe[11253]: exiting (read 7569 bytes)
2025-03-04T10:31:13.003502+01:00 pcale dummy pipe[11254]: exiting (read 7569 bytes)

I tried to determine by dichotomic search the value that triggers the buffering strategy, but couldn't. It changes from time to time.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <syslog.h>
#include <errno.h>
#include <limits.h>

static void do_open(void)
{
    openlog("dummy pipe", LOG_PID, LOG_USER);
}

int main(int argc, char *argv[])
{
    do_open();
    pid_t me = 1;
    int wrote = 0, n;
    long total_read = 0;
    for (int i = 1; i < argc; ++i)
    {
        if (argv[i][0] == 'w' && (n = atoi(&argv[i][1])) > 0)
        {
            if (wrote == 0)
            {
                me = getpid();
                setsid();
                wrote = 1;
            }
            if (n --> 0)
                printf("Pid: %d\n", me);
            while (n --> 0)
                puts("Lorem ipsum lorem ipsum lorem ipsum lorem ipsum");
        }
        else if (argv[i][0] == 's' && (n = atoi(&argv[i][1])) > 0)
            sleep(n);
        else if (argv[i][0] == 'r')
        {
            if (argv[i][1] == 0)
                n = INT_MAX;
            else
                n = atoi(&argv[i][1]);

            char buf[1024];
            while (n --> 0)
            {
                char *p = fgets(buf, sizeof buf, stdin);
                if (p == NULL)
                    break;
                if (buf[0] == 'P')
                    sscanf(buf, "Pid: %d\n", &me);
                total_read += strlen(buf);
                if (!isatty(fileno(stdout)))
                    printf("%s", buf);
            }
        }
        else if (strcmp(argv[i], "check") == 0)
        {
            if (me != 1)
            {
                n = kill(me, 0);
                syslog(LOG_NOTICE, "kill %d: %s\n", me,
                    n? strerror(errno): "still running");
                closelog();
                do_open();
            }
        }
    }
    syslog(LOG_NOTICE, "exiting (read %'ld bytes)\n", total_read);
    closelog();
    return 0;
}