FD_SET causing seg fault

Question

I am running into an issue where FD_SET is crashing my program. I am running the example code from a boost Boost ASIO sample HTTP Server. In the request handler, I put a third party api the eventually makes the following calls:

fd_set fds; 
FD_ZERO(&fds);
FD_SET(sockfd, &fds);

My program seems to be crashing on the FD_SET. Is there a situation where calling FD_SET from different threads would cause a segfault? I am not sure exactly how FD_SET works, my gut feeling would be that I am calling FD_SET on a fd that is already in use. Would this cause a problem at all?

sockfd is a class member, which is created each time a request comes in, so I should not be sharing a sockfd variable between threads.

Answer 1

At 1000 requests/second you are very likely eclipsing the FD_SETSIZE limit (typically 1024 on Linux), which results in undefined behavior:

An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET() with a value of fd that is negative or is equal to or larger than FD_SETSIZE will result in undefined behavior. Moreover, POSIX requires fd to be a valid file descriptor.

You should look at using poll, epoll, or a higher level library like Boost Asio. Any of those choices will not have the limitation of select.