sendto not working as expected

[gz]

I'm running rumprun on qemu (hw target) using e1000 (wm0, configured with DHCP) with the following code snippet:

// A: sched_yield();
char buf[64];
pkt_sent = 0;
while (pkt_sent < 3) {
    sprintf(buf, "pkt %zu\n", pkt_sent);
    if (sendto(fd, buf, strlen(buf)+1, 0, dst, sizeof(struct sockaddr_in)) >= 0)
        pkt_sent++;
    else {
        printf("errno %d: %s\n", errno, strerror(errno));
        exit(1);
    }
    // B: sched_yield();
}
// C: sched_yield();

On the receiver (listening on a tap device) I get the following output:

pkt 2

With the same code on Linux I see the following 3 messages on the receiver socket:

pkt 0
pkt 1
pkt 2

With uncommenting sched_yield calls I see the following outputs:

Unikernel (uncomment line A):

pkt 2

Unikernel (uncomment line B):

pkt 1
pkt 2

Unikernel (uncomment line C):

pkt 2

Unikernel (uncomment line A,B,C):

pkt 1
pkt 2

I have the following questions to help my understanding of the cooperative scheduling model:

• How do I send pkt 0 with the unikernel?
• Is there a better way to send multiple packets without inserting calls to sched_yield (e.g., will a proper poll/select loop etc. take care of this?)
• Is it a bug that sendto() does not return with an error, even though the message for pkt 0 was never sent?

[ljluestc]

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sched.h> // Include sched_yield
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <poll.h>

int main() {
    int fd;
    struct sockaddr_in dst;
    // Initialize socket and destination address (omitted for brevity)

    char buf[64];
    size_t pkt_sent = 0;
    struct pollfd pfd;
    pfd.fd = fd;
    pfd.events = POLLOUT; // We are interested in writing

    while (pkt_sent < 3) {
        sprintf(buf, "pkt %zu\n", pkt_sent);
        int ret = poll(&pfd, 1, 1000); // 1 second timeout
        if (ret > 0) {
            if (pfd.revents & POLLOUT) {
                if (sendto(fd, buf, strlen(buf)+1, 0, (struct sockaddr *)&dst, sizeof(dst)) >= 0) {
                    pkt_sent++;
                } else {
                    printf("errno %d: %s\n", errno, strerror(errno));
                    exit(1);
                }
            }
        } else if (ret == 0) {
            printf("Timeout waiting for socket to be ready\n");
        } else {
            printf("poll error: %s\n", strerror(errno));
            exit(1);
        }
    }

    return 0;
}