getting error with pthread mutex attributes - multithreading

I am getting problem when I am giving pthread_mutex_attr as a parameter while creating thread. If I pass attribute as NULL it is working fine, but not desired this NULL,
Here I am posting code, Please help me correct my mistakes and learn things.
pthread_mutex_t thread_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_t thread1;
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&thread_mutex, &attr);
ret = pthread_create(&thread1, NULL, my_func, (void *)message);
pthread_mutexattr_destroy(&attr);
pthread_mutex_destroy(&thread_mutex);
pthread_exit(NULL);
if I pass attr like this
ret = pthread_create(&thread1,&attr,upload_data,(void *)message);
This is giving segmentation fault.

I am not sure what kind of error you get as you never specified what exactly your problem is but I assume you can't compile your code.
From man pthread_create:
#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg);
This shows that pthread_create doesn't use pthread_mutex_attr but pthread_attr

Related

printf is producing the segmentation fault?

I'm learning threads and my code runs upto last print statement. Why it is giving segmentation fault at print? I think possible reason could be non-existant address passed as argument to print, but it is not the reason, I'm passing valid address.
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
void *thread (void *vargp) {
int arg = *((int*)vargp);
return &arg;
}
int main () {
pthread_t tid;
int thread_arg = 0x7ffdbc32fa34;
int *ret_value;
pthread_create(&tid, NULL, thread, &thread_arg);
pthread_join(tid, (void **)(&ret_value));
printf("hello\n");
printf("%X\n", *ret_value);
return 0;
}
It is giving following output:
hello
Segmentation fault (core dumped)
Is it because I'm returning an address of a local variable, which gets destroyed once thread is returned? I don't think so, because changing to following code is also giving me segmentation fault!
void *thread (void *vargp) {
int * arg = malloc(sizeof(int));
*arg = *((int*)vargp);
return &arg;
}
Is it because I'm returning an address of a local variable, which gets
destroyed once thread is returned?
Yes, it is.
I don't think so, because changing to following code is also giving me
segmentation fault!
This code is also returning the address of a local variable (return &arg;). Instead, you should be returning the pointer value that malloc() returned (return arg;):
void *thread (void *vargp)
{
int * arg = malloc(sizeof(int));
*arg = *((int*)vargp);
return arg;
}
You also should not be casting the address of ret_value to type void ** in main() - the variable is of type int * not void *, so it shouldn't be written to through a void ** pointer (although, in practice, this will usually work). Instead, you should be using void * variable to hold the return value, then either casting this value to int * or assigning it to a variable of type int *:
void *ret_value;
pthread_create(&tid, NULL, thread, &thread_arg);
pthread_join(tid, &ret_value);
printf("%X\n", *(int *)ret_value);

Shared Memory giving ambiguous results

I was trying to communicate between two processes using Shared Memory concept. But here, though I have pointed the shared memory addresses of different variables to different files, they seem to be connected. As soon as I alter value of one variable, the new value overwrites on other variable too, in this case, se1->val and se2->val are coming out to be connected. Can someone help why it's happening so?
#include<stdio.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<unistd.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#define s(t) scanf("%d",&t)
#define p(t) printf("%d ",t)
struct sem
{
int val;
int xy;
};
struct sem* se1;
struct sem* se2;
int main()
{
printf("You in P1\n");
key_t key1,key2;
key1=ftok("shmfile1",0);
key2=ftok("shmfile3",0);
int shmid1=shmget(key1, sizeof(struct sem),0644|IPC_CREAT);
int shmid2=shmget(key2, sizeof(struct sem),0644|IPC_CREAT);
se1=shmat(shmid1,NULL,0);
se2=shmat(shmid2,NULL,0);
se1->xy=4;
se2->xy=8;
se1->val=0;
se2->val=1;
int r=10;
while(r--)
{
printf("\nIn P1 process ");
while(se2->val==0);
se2->val--;
se1->xy=se2->xy+1;
se1->val++;
p(se1->xy);
p(se2->xy);
}
return 0;
}
It is expected se1->val and se2->val will lead to semaphore type results, but due to overwriting it's not happening!

need to know how to interrupt all pthreads

In Linux, I am emulating an embedded system that has one thread that gets messages delivered to the outside world. If some thread detects an insurmountable problem, my goal is to stop all the other threads in their tracks (leaving useful stack traces) and allow only the message delivery thread to continue. So in my emulation environment, I want to "pthread_kill(tid, SIGnal)" each "tid". (I have a list. I'm using SIGTSTP.) Unfortunately, only one thread is getting the signal. "sigprocmask()" is not able to unmask the signal. Here is my current (non-working) handler:
void
wait_until_death(int sig)
{
sigset_t mask;
sigemptyset(&mask);
sigaddset(&mask, sig);
sigprocmask(SIG_UNBLOCK, &mask, NULL);
for (;;)
pause();
}
I get verification that all the pthread_kill()'s get invoked, but only one thread has the handler in the stack trace. Can this be done?
This minimal example seems to function in the manner you want - all the threads except the main thread end up waiting in wait_until_death():
#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <unistd.h>
#define NTHREADS 10
pthread_barrier_t barrier;
void
wait_until_death(int sig)
{
sigset_t mask;
sigemptyset(&mask);
sigaddset(&mask, sig);
sigprocmask(SIG_UNBLOCK, &mask, NULL);
for (;;)
pause();
}
void *thread_func(void *arg)
{
pthread_barrier_wait(&barrier);
for (;;)
pause();
}
int main(int argc, char *argv[])
{
const int thread_signal = SIGTSTP;
const struct sigaction sa = { .sa_handler = wait_until_death };
int i;
pthread_t thread[NTHREADS];
pthread_barrier_init(&barrier, NULL, NTHREADS + 1);
sigaction(thread_signal, &sa, NULL);
for (i = 0; i < NTHREADS; i++)
pthread_create(&thread[i], NULL, thread_func, NULL);
pthread_barrier_wait(&barrier);
for (i = 0; i < NTHREADS; i++)
pthread_kill(thread[i], thread_signal);
fprintf(stderr, "All threads signalled.\n");
for (;;)
pause();
return 0;
}
Note that unblocking the signal in the wait_until_death() isn't required: the signal mask is per-thread, and the thread that is executing the signal handler isn't going to be signalled again.
Presumably the problem is in how you are installing the signal handler, or setting up thread signal masks.
This is impossible. The problem is that some of the threads you stop may hold locks that the thread you want to continue running requires in order to continue making forward progress. Just abandon this idea entirely. Trust me, this will only cause you great pain.
If you literally must do it, have all the other threads call a conditional yielding point at known safe places where they hold no lock that can prevent any other thread from reaching its next conditional yielding point. But this is very difficult to get right and is very prone to deadlock and I strongly advise not trying it.

Posix Threads with Mutex

I have started working on POSIX threads. I wrote a simple code.
My question is on Mutex.
Initializing the mutex inside threaded function gives wrong result. While initializing the mutex inside main function (before creation of threads) gives proper result. Why is that happening?
The count value is expected to be 200000 but it is showing some improper value < 200000.
Here is my code.
#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <malloc.h>
void *thread_handler (void *name);
unsigned long int count=0;
pthread_mutex_t lock;
void main () {
pthread_t thread_num[2];
pthread_attr_t attr;
pthread_attr_init (&attr);
int i;
for (i=0;i<2;i++) {
if (pthread_create (&thread_num[i],&attr,(void *) thread_handler,NULL)<0) {
printf ("\n Error in Creating the Threads");
}
}
for (i=0;i<2;i++) {
pthread_join(thread_num[i],NULL); //Waiting for the Thread to Exit
}
printf ("\n The value of count=%ld\n",count);
}
void *thread_handler (void *arg) {
int i;
if (pthread_mutex_init (&lock,NULL)!=0) {
printf ("\n Error in Initializing the Mutex");
}
pthread_mutex_lock (&lock);
for (i=0;i<100000;i++) {
count++;
}
pthread_mutex_unlock(&lock);
pthread_exit(NULL);
}
Thanks in Advance,
NDPrasad.
When you initialize the mutex inside the thread_handler function, it is initialized twice(because you create two threads that execute this function). It causes undefined behavior(which means that anything can happen).
A quote from here:
Attempting to initialize an already initialized mutex results in undefined behavior.

Can some one please explain the following code of threads?

So I am new to system programming and learning threads.What does the term posix means? I need help in understanding the following code.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function( void *ptr );
main()
{
pthread_t thread1, thread2;
const char *message1 = "Thread 1";
const char *message2 = "Thread 2";
int iret1, iret2;
iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
pthread_join( thread1, NULL);
pthread_join( thread2, NULL);
printf("Thread 1 returns: %d\n",iret1);
printf("Thread 2 returns: %d\n",iret2);
exit(0);
}
void *print_message_function( void *ptr )
{
char *message;
message = (char *) ptr;
printf("%s \n", message);
}
I dont understand the line: pthread_join( thread1, NULL);
and the function: void *print_message_function( void *ptr ).
Also, what does the variable iret returns?
It spawns two threads.
Saves their "return values" to variables previously declared.
Joins to threads, basically waits for them to stop.
Prints the variables
exits with status 0, normal successful exit
posix is a general standard for Unix like operating systems. E.g file structure etc
You should read posix documentation.
I dont understand the line: pthread_join( thread1, NULL); and the
function: void *print_message_function( void *ptr ).
pthread_join blocks until the thread terminates
void *print_message_function( void *ptr ) is a function returning void* and receiving a void* as parameter
Again, you should read posix documentation (and learn more C).

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