Problem - Kevin Tafuro

a pointer to data. Instead, they return –1 on failure, and otherwise return the number
of bytes read.
When using standard C runtime functions, we recommend using read( ). If you are
reading from /dev/urandom, read( ) will successfully return unless a signal is delivered
during the call (in which case the call should be made again), the operating system
is misconfigured, or there is some other catastrophic error. Therefore, if read( )
is unsuccessful, retry when the value of errno is EINTR, and fail unconditionally otherwise.
You should also check that the return value is equal to the number of bytes you
requested to read, because some implementations may limit the amount of data you
can read at once from this device. If you get a short read, merely continue to read
until you collect enough data.
When using /dev/random, things are the same if you are performing regular blocking
reads. Of course, if not enough entropy is available, the call will hang until the
requested data is available or until a signal interrupts the call.
If you don’t like that behavior, you can make the file descriptor nonblocking, meaning
that the function will return an error and set errno to EAGAIN if there isn’t enough
data to complete the entire read. Note that if some (but not all) of the requested data
is ready, it will be returned instead of giving an error. In that case, the return value of
read( ) will be smaller than the requested amount.
Given an integer file descriptor, the following code makes the associated descriptor
nonblocking:
#include
#include
#include
void spc_make_fd_nonblocking(int fd) {
int flags;
flags = fcntl(fd, F_GETFL); /* Get flags associated with the descriptor. */
if (flags = = -1) {
perror("spc_make_fd_nonblocking failed on F_GETFL");
exit(-1);
}
flags |= O_NONBLOCK;
/* Now the flags will be the same as before, except with O_NONBLOCK set.
*/
if (fcntl(fd, F_SETFL, flags) = = -1) {
perror("spc_make_fd_nonblocking failed on F_SETFL");
exit(-1);
}
}
Here, we will demonstrate how to use /dev/random and /dev/urandom properly by
binding them to the API we developed in Recipe 11.2. We will implement spc_
entropy( ) by reading from /dev/random in nonblocking mode. We will implement
Using the Standard Unix Randomness Infrastructure | 577
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.