How to prevent fgets blocks when file stream has no new data

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In Linux (or any Unix-y OS), you can mark the underlying file descriptor used by popen() to be non-blocking. 

#include <fcntl.h>

FILE *proc = popen("tail -f /tmp/test.txt", "r");
int fd = fileno(proc);

int flags;
flags = fcntl(fd, F_GETFL, 0);
flags |= O_NONBLOCK;
fcntl(fd, F_SETFL, flags);

If there is no input available, fgets will return NULL with errno set to EWOULDBLOCK.

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