Secure Programming Cookbook for C and C++

3.13. Preventing File Descriptor Overflows When Using select( )

Problem

Your program uses the select( ) system call to determine when sockets are ready for writing, have data waiting to be read, or have an exceptional condition (e.g., out-of-band data has arrived). Using select( ) requires the use of the fd_set data type, which typically entails the use of the FD_*( ) family of macros. In most implementations, FD_SET( ) and FD_CLR( ), in particular, are susceptible to an array overrun.

Solution

Do not use the FD_*( ) family of macros. Instead, use the macros that are provided in this recipe. The FD_SET( ) and FD_CLR( ) macros will modify an fd_set object without performing any bounds checking. The macros we provide will do proper bounds checking.

Discussion

The select( ) system call is normally used to multiplex sockets. In a single-threaded environment, select( ) allows you to build sets of socket descriptors for which you wish to wait for data to become available or that you wish to have available to write data to. The fd_set data type is used to hold a list of the socket descriptors, and several standard macros are used to manipulate objects of this type.

Normally, fd_set is defined as a structure with a single member that is a statically allocated array of long integers. Because socket descriptors are always numbered starting with 0 and ending with the highest allowable descriptor, the array of integers in an fd_set is actually treated as a bitmask with a one-to-one correspondence ...