poll() function 试图检测来自 jq 的标准输出

Question

I am trying to write a function in C that checks the presence of stdin using poll()

#include <stdio.h>
#include <sys/poll.h>

\\other code

void check_stdin(){
    struct pollfd fds;
    int ret; fds.fd = 0; fds.events = POLLIN;
    ret = poll(&fds, 1, 10);
    printf("Return value: %d\n", ret);
    if(ret != 1){
        printf("stdin could not be read\n");
    }
}

Here fds.fd=0 refers to file descriptor for STDIN. fds.events = POLLIN refers to the event that there is data to read. I am using a timeout of 10 milliseconds. When I run

echo "{\"key\": 1}" | jq .key | ./test_stdin

where test_stdin is the object file for the C program, I get the output

Return value: 0
stdin could not be read

The value of ret should be 1 if there was data found to be read in the STDIN. Is the STDOUT from jq not considered as STDIN for ./test_stdin here?

Answer 1

You have a race condition in your shell pipeline.

ret = poll(&fds, 1, 10);

You're telling poll() to wait for 10 milliseconds before timing out. jq wasn't producing any output in that short time when you tested it (Didn't for me either). If you use a longer timeout, say 500 milliseconds, you'll likely see

Return value: 1

as the output instead.

Commands in a pipeline are all run concurrently, and what order they execute in is dependent on your OS's scheduler. So the C program at the end might actually be ending before jq even starts to execute. If programs intended for use in a pipeline use blocking reads, they'll never notice, but with that very short timeout, you're seeing the effects.