In the late 1980s and early 1990s, several different threading APIs existed. In 1995, POSIX.1c standardized the POSIX threads API, and this standard was later incorporated into SUSv3.
The Pthreads API defines a number of data types, some of which are listed in Table 29-1. We describe most of these data types in the following pages.
In the traditional UNIX API, errno is a global integer variable. However, this doesn’t suffice for threaded programs. If a thread made a function call that returned an error in a global errno variable, then this would confuse other threads that might also be making function calls and checking errno. In other words, race conditions would result. Therefore, in threaded programs, each thread has its own errno value. On Linux, a thread-specific errno is achieved in a similar manner to most other UNIX implementations: errno is defined as a macro that expands into a function call returning a modifiable lvalue that is distinct for each thread. (Since the lvalue is modifiable, it is still possible to write assignment statements of the form errno = value in threaded programs.)
Because each reference to errno in a threaded program carries the overhead of a function call, our example programs don’t directly assign the return value of a Pthreads function to errno. Instead, we use an intermediate variable and employ our errExitEN() diagnostic function (Common Functions and Header Files), like so:
pthread_t *thread;
int s;
s = pthread_create(&thread, NULL, func, &arg);
if (s != 0)
errExitEN(s, "pthread_create");