Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)
#include <signal.h>
#include <stdio.h>
volatile struct two_words { int a, b; } memory;
void
handler(int signum)
{
printf ("%d,%d\n", memory.a, memory.b);
alarm (1);
}
int
main (void)
{
static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
signal (SIGALRM, handler);
memory = zeros;
alarm (1);
while (1)
{
memory = zeros;
memory = ones;
}
}
This program fills memory with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents. (Calling printf in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)
Clearly, this program can print a pair of zeros or a pair of ones. But
that's not all it can do! On most machines, it takes several
instructions to store a new value in memory, and the value is
stored one word at a time. If the signal is delivered in between these
instructions, the handler might find that memory.a is zero and
memory.b is one (or vice versa).
On some machines it may be possible to store a new value in
memory with just one instruction that cannot be interrupted. On
these machines, the handler will always print two zeros or two ones.