Read file in non-blocking mode on Linux

Question

When opening the file /dev/urandom in nonblocking mode it is still blocking when reading. Why is the read call still blocking.

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

int main(int argc, char *argv[])
{
    int fd = open("/dev/urandom", O_NONBLOCK);
    if (fd == -1) {
        printf("Unable to open file\n");
        return 1;
    }

    int flags = fcntl(fd, F_GETFL);
    if (flags & O_NONBLOCK) {
        printf("non block is set\n");
    }

    int ret;
    char* buf = (char*)malloc(10000000);

    ret = read(fd, buf, 10000000);
    if (ret == -1) {
        printf("Error reading: %s\n", strerror(errno));
    } else {
        printf("bytes read: %d\n", ret);
    }

    return 0;
}

The output looks like this:

gcc nonblock.c -o nonblock
./nonblock 
non block is set
bytes read: 10000000

Answer 1

Opening any (device) file in nonblocking mode does not mean you never need to wait for it.

O_NONBLOCK just says return EAGAIN if there is no data available.

Obviously, the urandom driver always considers to have data available, but isn't necessarily fast to deliver it.

Answer 2

/dev/urandom is non-blocking by design:

When read, the /dev/random device will only return random bytes within the estimated number of bits of noise in the entropy pool. /dev/random should be suitable for uses that need very high quality randomness such as one-time pad or key generation. When the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered.

A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver.

If you replace it with /dev/random, your program should produce a different result.

Answer 3

In Linux, it is not possible to open regular files in non blocking mode. You have to use the AIO interface to read from /dev/urandom in non blocking mode.