C select is overwriting timeout value [duplicate] - sockets

Question

This question already has answers here:
Is timeout changed after a call to select in c?
(5 answers)
Closed 25 days ago.
In a very simple C program, using select() to check any new read data on a socket, when I use the optional timeout parameter, it is being overwritten by select(). It looks like it resets it to values of seconds and microseconds it actually waited, so when data is coming sooner than the timeout, it will have much smaller values, leading to smaller and smaller timeouts unless timeout is reset, when select() is called in a loop.
I could not find any information on this behavior in select() description. I am using Linux Ubuntu 18.04 in my testing. It looks like I have to reset the timeout value every time before calling select() to keep the same timeout?
The code snippet is this:
void *main_udp_loop(void *arg)
{
struct UDP_CTX *ctx = (UDP_CTX*)arg;
fd_set readfds = {};
struct sockaddr peer_addr = { 0 };
int peer_addr_len = sizeof(peer_addr);
while (1)
{
struct timeval timeout;
timeout.tv_sec = 0;
timeout.tv_usec = 850000; // wait 0.85 second.
FD_ZERO(&readfds);
FD_SET(ctx->udp_socketfd, &readfds);
int activity = select( ctx->udp_socketfd + 1 , &readfds , NULL , NULL , &timeout);
if ((activity < 0) && (errno != EINTR))
{
printf("Select error: Exiting main thread\n");
return NULL;
}
if (timeout.tv_usec != 850000)
{
printf ("Timeout changed: %ld %ld\n", (long)timeout.tv_sec, (long)timeout.tv_usec);
}
if (activity == 0)
{
printf ("No activity from select: %ld \n", (long)time(0));
continue;
}
...
}

Answer 1

This is documented behavior in the Linux select() man page:
On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1 permits either behavior.) This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple select()s in a loop without reinitializing it. Consider timeout to be undefined after select() returns.
So, yes, you have to reset the timeout value every time you call select().