c++ multi threading priority implementation failed - c++

Question

=)
i am a new user here, and I am new to c++, so it is a bit hard for me to work on it...
so i am asking you guys some questions! =)
i am doing a work for school, that asks me to implement threading priority into this:
#include <pthread.h>
#include <stdio.h>
#include <sched.h>
int sched_yield(void);
// Parameters to print_function.
struct char_print_parms{
char character; // char to print
int count; // times to print
};
void* char_print (void* parameters){
int i;
struct char_print_parms* p;
p = (struct char_print_parms*) parameters;
for (i = 0; i < p->count; ++i){
fputc (p->character, stderr);
sched_yield();
}
return NULL;
}
int main (){
pthread_t thread1_id,thread2_id;
struct char_print_parms thread1_args,thread2_args;
// Create a new thread to print 200 x's.
thread1_args.character = 'x';
thread1_args.count = 200;
pthread_create (&thread1_id, NULL, &char_print, &thread1_args);
// Create a new thread to print 200 o's.
thread2_args.character = 'o';
thread2_args.count = 200;
pthread_create (&thread2_id, NULL,
&char_print, &thread2_args);
// main waits for the threads to complete
pthread_join(thread1_id, NULL);
pthread_join(thread2_id, NULL);
return 0;
}
This gives is "oxoxoxo..." etc.
The objective is to get more "o", until it finishes.
What I did was:
#include <pthread.h>
#include <stdio.h>
#include <sched.h>
int sched_yield(void);
// Parameters to print_function.
struct char_print_parms{
char character; // char to print
int count; // times to print
};
void* char_print (void* parameters){
int i;
struct char_print_parms* p;
p = (struct char_print_parms*) parameters;
for (i = 0; i < p->count; ++i){
fputc (p->character, stderr);
sched_yield();
}
return NULL;
}
int main (){
pthread_t thread1_id,thread2_id;
struct char_print_parms thread1_args,thread2_args;
//new code lines
struct sched_param param;
pthread_attr_t pta;
pthread_attr_init(&pta);
pthread_attr_getschedparam(&pta, &param);
//end of new code lines
// Create a new thread to print 200 x's.
thread1_args.character = 'x';
thread1_args.count = 200;
//more new code lines
param.sched_priority = 0;
pthread_attr_setschedparam(&pta, &param);
pthread_setschedparam(thread1_id, SCHED_OTHER, &param);
//end of more new code lines
pthread_create (&thread1_id, NULL, &char_print, &thread1_args);
// Create a new thread to print 200 o's.
thread2_args.character = 'o';
thread2_args.count = 200;
//more new code lines 2
param.sched_priority = 10;
pthread_attr_setschedparam(&pta, &param);
pthread_setschedparam(thread2_id, SCHED_OTHER, &param);
//end of more new code lines 2
pthread_create (&thread2_id, NULL,
&char_print, &thread2_args);
// main waits for the threads to complete
pthread_join(thread1_id, NULL);
pthread_join(thread2_id, NULL);
return 0;
}
At the end, I compile and try to run, but it appears an error:
Segmentation failed (core dumped)
Once again, I am new to c++ and my english is not very good, but I want to try to understand why this does not work. Any help is welcome!

Answer 1

When you call pthread_setschedparam the thread id variables haven't been initialized yet. So you're trying to change parameters on an indeterminate thread.
The easiest way to change the priority is to do it in the thread themselves.
Regarding uninitialized local variables, their values are indeterminate until explicitly initialized. Using uninitialized local variables leads to undefined behavior.
If you see the example in the pthread_setschedparam you see it being called with pthread_self to set the own threads priority. You can use this to either add a field in the structure you pass to the thread that contains the priority, or have a wrapper thread-function which sets the priority and then calls the actual thread function.

Answer 2

You should first call pthread_create (&thread1_id, NULL, &char_print, &thread1_args); to create thread thread1_id, then you can set this thread's priority. I modify the code and it works fine.
thread1_args.character = 'x';
thread1_args.count = 200;
pthread_create (&thread1_id, NULL, &char_print, &thread1_args);
//more new code lines
param.sched_priority = 0;
pthread_attr_setschedparam(&pta, &param);
pthread_setschedparam(thread1_id, SCHED_OTHER, &param);
// Create a new thread to print 200 o's.
thread2_args.character = 'o';
thread2_args.count = 200;
pthread_create (&thread2_id, NULL, &char_print, &thread2_args);
//more new code lines 2
param.sched_priority = 10;
pthread_attr_setschedparam(&pta, &param);
pthread_setschedparam(thread2_id, SCHED_OTHER, &param);
You can read this link:https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_MRG/2/html/Realtime_Reference_Guide/chap-Realtime_Reference_Guide-Priorities_and_policies.html.
I test this code, but the output is different each time.