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Subjectshort read from /dev/urandom
The /dev/urandom device is advertised as always returning the requested 
number of bytes. Yet, it fails to do this under some situations.
Compile this

int main (void)
alarm (100);
char buf[32];
int fd = open ("/dev/urandom", 0);
while (1)
if (read (fd, buf, sizeof buf) != sizeof (buf))
abort ();


gcc -o r r.c -g -O2 -pg

Note the -pg at the end to enable profiling. Running this code fails
for me after less than a second.

The relevant code in the kernel is this

while (nbytes) {
* Check if we need to break out or reschedule....
if ((flags & EXTRACT_ENTROPY_USER) && need_resched()) {
if (signal_pending(current)) {
if (ret == 0)

Here the loop is left prematurely if a signal is pending.

One solution is to redefine the /dev/urandom interface. The problem is
that this will cause program to fail. I know since I found this while
debugging such a program.

➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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