Introduction of POSIX Timer and Thread

Abstract

POSIX time is a system for describing a point in time, also known as Unit time. It is widely used in operation systems and file formats. The C POSIX library is a specification of a C standard library for POSIX systems, which includes the time types and functions definied in header file. Recently, one of my friends shared the conception of POSIX timer with me. But, I seldom used and was not focus on the POSIX-TIMERS API before. So that I roughly survey this topic and try to sort the relative information out in this post. Here we go.

POSIX Timers C API

Summary of C standard library for POSIX systems:

Even if these functions seem not be implemented in Mac OS X, we can still take a look at the simple example of timer posted in stackover flow and further compile it and understand how the timer works. The porpose of this program is to count down 10 seconds , and hopefully only print the “Hello, World!” with the new thread to console.

Example of Simple Timer using pthreads

timer.h
typedef struct Timer {
    void (*fn)(void);
    bool (*timer_delegate)(pthread_t, unsigned int, unsigned int);
    unsigned int seconds;
} Timer;

void* timer_run(void *t);

bool should_kill_thread(pthread_t self_id, unsigned int start_time, unsigned int utime_new); 
 
void hello_world();

timer.c#include "posix_timer.h"

void* timer_run(void *t) {
    unsigned int start_time = time(NULL);
    while(1) {
        Timer tmr = *((Timer *) t);
        bool should_kill_thread = tmr.timer_delegate(pthread_self(), start_time, time(NULL));
        if (should_kill_thread) 
            pthread_cancel(pthread_self());
        tmr.fn();
        sleep(tmr.seconds);
    }
}

bool should_kill_thread(pthread_t self_id, unsigned int start_time, unsigned int utime_new) {
    printf("Thread ID: %u\n",(unsigned) self_id);
    printf("the start time was %d and the new time is %d \n", start_time, utime_new);
    // Count down 10 seconds
    if (utime_new - start_time >= 9) {
        return true;
    }
    return false;
}

void hello_world() {
    printf("%s\n", "Hello, World!");
}

main.c#include "posix_timer.h"

int main(int argc, char const *argv[])
{
    pthread_t t1;
    Timer args;
    args.fn = &hello_world; // function pointer
    args.timer_delegate = should_kill_thread; // function pointer
    args.seconds = 1; // call every 1 second
    // New thread starts execution by invoking timer_run()
    int id = pthread_create(&t1, NULL, timer_run, &args);
    if (id) {
       printf("ERROR; return code from pthread_create() is %d\n", id);
       exit(EXIT_FAILURE);
    }
    pthread_join(t1, NULL); // blocks main thread
    printf("%s\n", "DONE"); // never reached until t1 is killed
    return 0;
}

The corresponding output$ ./exe/timer
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083090 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083091 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083092 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083093 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083094 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083095 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083096 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083097 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083098 
Hello, World!
Thread ID: 47472640
the start time was 1609083090 and the new time is 1609083099 
Hello, World!
DONE

Note:

• pthread_create function starts a new thread in the calling process. The new thread starts execution by invoking start_routine(). arg is passed as the sole argument of start_routine().

  int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);
  

• pthread_t type is definied as described in <sys/types.h>
• A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executing by another thread.

=========== To be continued…. ==========

Reference

[1] POSIX Timers (POSIX-TIMERS)

[2] Create C timer in macOS

[3] pthread_create function

[4] The Open Group: pthread.h

Feel free to leave the comments below or email to me. Any pieces of advice are always welcome. :)