As @Barmar said in the comments, there is no built-in property to check if the child process is about to read from the pipe. Even standard terminals allow input during process execution. So when I run the following code, I can enter text that is as not being processed by the child process, the shell tries to execute after process termination.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int a[2], b[2], child_read, child_write;
    char output[4] = {0};
    pipe(a);
    pipe(b);
    child_write = a[1];
    child_read = b[0];
    FILE *parent_write = fdopen(b[1], "w");
    FILE *parent_read = fdopen(a[0], "r");

    if (fork() == 0) {
        close(b[1]);
        close(a[0]);
        fclose(parent_read);
        fclose(parent_write);
        dup2(child_read, STDIN_FILENO);
        dup2(child_write, STDOUT_FILENO);

        sleep(3);
        printf("ok\n");

        close(child_read);
        close(child_write);
        exit(0);
    }

    close(child_read);
    close(child_write);

    if (fread(output, 1, 3, parent_read) > 0) {
        printf("%s\n", output);
    }
    
    fclose(parent_read);
    fclose(parent_write);
    close(b[1]);
    close(a[0]);
}

$ gcc -o test test.c ; ./test
ls
ok
$ ls
test  test.c