Multithreaded Programming Guide

Providing for Static Local Variables

Example 9-2 shows a problem similar to the errno problem, but involving static storage instead of global storage. The function gethostbyname(3N) is called with the computer name as its argument. The return value is a pointer to a structure containing the required information for contacting the computer through network communications.

Example 9-2 The gethostbyname() Problem

struct hostent *gethostbyname(char *name) {
    static struct hostent result;
        /* Lookup name in hosts database */
        /* Put answer in result */

Returning a pointer to a local variable is generally not a good idea, although it works in this case because the variable is static. However, when two threads call this variable at once with different computer names, the use of static storage conflicts.

Thread-specific data could be used as a replacement for static storage, as in the errno problem, but this involves dynamic allocation of storage and adds to the expense of the call.

A better way to handle this kind of problem is to make the caller of gethostbyname() supply the storage for the result of the call. This is done by having the caller supply an additional argument, an output argument, to the routine. This requires a new interface to gethostbyname().

This technique is used in threads to fix many of these problems. In most cases, the name of the new interface is the old name with "_r" appended, as in gethostbyname_r(3N).