CPU - Raspberry Pi Hardware Reference - page 48

The reason behind this is likely that it was thought that the traditional Unix errno
approach would be phased out in the near future (at the time POSIX threads were being
standardized). The original use of errno was as follows:

extern int errno;

However, this approach didn't work for threaded programs. Imagine two threads
concurrently opening files with open(2) , which sets the errno value upon failure. Both
threads cannot share the same int value for errno .
Rather than change a vast body of code already using errno in this manner, other
approaches were implemented to provide each thread with its own private copy of errno .
This is one reason that programs today using errno must include the header file errno.h .
The header file takes care of defining the thread specific reference to errno .
Because the pthread standard was developing before the errno solution generally
emerged, the pthread library returns the error code directly when there is an error and
returns zero when the call is a success. If Unix were to be rewritten from scratch today, all
system calls would probably work this way.

pthread_create(3)
The function pthread_create(3) is used to create a new thread of execution. The
function call looks more daunting than it really is:

int pthread_create(
pthread_t

thread,
const pthread_attr_t

∗

attr,

∗

void ( start_routine)(void ),

∗ ∗ ∗

void arg

∗

);

The call to pthread_create(3) creates a new stack, sets up registers, and performs
other housekeeping. Let's describe the arguments:

thread : This first argument is simply a pointer to a pthread_t
variable to receive the created thread's ID value. The ID value
allows you to query and control the created thread. If the call
succeeds, the thread ID is returned to the calling program.

attr : This is a pointer to a pthread_attr_t attribute object
that supplies various options and parameters. If you can accept
the defaults, simply supply zero or NULL .
start_routine : As shown in the following code, this is simply
the name of a start routine that accepts a void pointer and
returns a void pointer.