While working with Threads in C, I'm facing the warning
"warning: cast to pointer from integer of different size"
The code is as follows
#include<stdio.h>
#include<sys/types.h>
#include<stdlib.h>
#include<pthread.h>
void *print(void *id)
{
int a=10;
printf("My thread id is %ld\n",pthread_self());
printf("Thread %d is executing\n",id);
return (void *) 42;
}
int main()
{
pthread_t th[5];
int t;
int i;
int status;
void *ret;
for(i=0;i<5;i++)
{
status=pthread_create(&th[i],NULL,print,(void *)i); //Getting warning at this line
if(status)
{
printf("Error creating threads\n");
exit(0);
}
pthread_join(th[i],&ret);
printf("--->%d\n",(int *)ret);
}
pthread_exit(NULL);
}
Can anybody explain how to pass an integer to a function which receives (void * ) as a parameter?
This is a fine way to pass integers to new pthreads, if that is what you need. You just need to suppress the warning, and this will do it:
#include <stdint.h>
void *threadfunc(void *param)
{
int id = (intptr_t) param;
...
}
int i, r;
r = pthread_create(&thread, NULL, threadfunc, (void *) (intptr_t) i);
Discussion
This may offend your sensibilities, but it's very short and has no race conditions (as you'd have if you used &i). No sense in writing a few dozen lines of extra code just to get a bunch of numbered threads.
Data races
Here is a bad version with a data race:
#include <pthread.h>
#include <stdio.h>
#define N 10
void *thread_func(void *arg)
{
int *ptr = arg;
// Has *ptr changed by the time we get here? Maybe!
printf("Arg = %d\n", *ptr);
return NULL;
}
int main()
{
int i;
pthread_t threads[N];
for (i = 0; i < N; i++) {
// NO NO NO NO this is bad!
pthread_create(&threads[i], NULL, thread_func, &i);
}
for (i = 0; i < N; i++) {
pthread_join(threads[i], NULL);
}
return 0;
}
Now, what happens when I run it with the thread sanitizer?
(Also, check out how it prints "5" twice...)
==================
WARNING: ThreadSanitizer: data race (pid=20494)
Read of size 4 at 0x7ffc95a834ec by thread T1:
#0 thread_func /home/depp/test.c:9 (a.out+0x000000000a8c)
#1 <null> <null> (libtsan.so.0+0x000000023519)
Previous write of size 4 at 0x7ffc95a834ec by main thread:
#0 main /home/depp/test.c:17 (a.out+0x000000000b3a)
Location is stack of main thread.
Thread T1 (tid=20496, running) created by main thread at:
#0 pthread_create <null> (libtsan.so.0+0x0000000273d4)
#1 main /home/depp/test.c:18 (a.out+0x000000000b1c)
SUMMARY: ThreadSanitizer: data race /home/depp/test.c:9 thread_func
==================
Arg = 1
Arg = 2
Arg = 3
Arg = 4
Arg = 5
Arg = 6
Arg = 7
Arg = 8
Arg = 9
Arg = 5
ThreadSanitizer: reported 1 warnings
you can do something like this:
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <pthread.h>
struct th {
pthread_t thread;
int id;
int ret;
};
void *print(void *id) {
int a=10;
struct th *self = (struct th *) id;
printf("My thread id is %ld\n",pthread_self());
printf("Thread %d is executing\n",self->id);
self->ret = random();
return;
}
int main(void) {
struct th th[5];
int t;
int i;
int status;
void *ret;
for(i=0;i<5;i++) {
th[i].id = i;
status=pthread_create(&th[i].thread,NULL,print,&th[i]); //Getting warning at this line
if(status) {
printf("Error creating threads\n");
exit(0);
}
}
for (i=0;i<5;i++) {
pthread_join(th[i].thread,&ret);
printf("%d--->%d\n",th[i].id,th[i].ret);
}
pthread_exit(NULL);
}
will output:
My thread id is 4496162816
My thread id is 4497870848
My thread id is 4498944000
My thread id is 4498407424
Thread 0 is executing
Thread 1 is executing
My thread id is 4499480576
Thread 3 is executing
Thread 2 is executing
0--->1804289383
Thread 4 is executing
1--->846930886
2--->1714636915
3--->1681692777
4--->1957747793
passing a unique pointer to each thread wont race, and you can get/save any kind of information in the th struct
you can pass the int value as void pointer like (void *)&n where n is integer, and in the function accept void pointer as parameter like void foo(void *n);and finally inside the function convert void pointer to int like, int num = *(int *)n;. this way you won't get any warning.
change:
status=pthread_create(&th[i],NULL,print,(void *)i);
to:
status=pthread_create(&th[i],NULL,print,(reinterpret_cast<void*>(i));
The reinterpret_cast makes the int the size of a pointer and the warning will stop. Basically its a better version of (void *)i.
Related
I have found several codes using dynamic memory allocation method for passing an integer as the last argument, like this example:
void *foo(void *i) {
int a = *((int *) i);
free(i);
}
int main() {
pthread_t thread;
for ( int i = 0; i < 10; ++1 ) {
int *arg = malloc(sizeof(*arg));
if ( arg == NULL ) {
fprintf(stderr, "Couldn't allocate memory for thread arg.\n");
exit(EXIT_FAILURE);
}
*arg = i;
pthread_create(&thread, 0, foo, arg);
}
/* Wait for threads, etc */
return 0;
}
Otherwise, I found academic examples that use an array method, like this example passing a struct:
//Runnable example
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
struct my_Struct{
int index;
int value;
};
void* my_Func(void *received_struct){
struct my_Struct *struct_ptr = (struct my_Struct*) received_struct;
printf("index: %d | value: %d \n", struct_ptr->index, struct_ptr->value);
//Now use 'struct_ptr->index', 'struct_ptr->value' as you wish
}
int main(){
struct my_Struct mystruct_1[5];
printf("\nEnter 5 numbers:\n");
for (int i=0; i<5; i++){
scanf("%d", &mystruct_1[i].value);
mystruct_1[i].index = i;
}
pthread_t tid[5];
for(int i=0; i<5; i++){
pthread_create(&(tid[i]), NULL, my_Func, &mystruct_1[i]);
}
for (int i=0; i<5; i++){
pthread_join(tid[i], NULL);
}
}
//To run: gcc [C FILE].c -lpthread -lrt
// ./a.out
//Grepper Profile: https://www.codegrepper.com/app/profile.php?id=9192
What is the advantage of the dynamic memory allocation method?
Update: I found in the IBM i documentation, based on gcc, that they use memory allocation
The last parameter of pthread_create() is passed on the stack of the newly created thread. When it is an integer or anything else which the size is little or equal than the size of a pointer, the parameter can be passed as it is to pthread_create() with a cast. But it is not the recommended way to make it as it is not portable on machines where for example the integers do not have the same size as the pointers. Let's compile the following simple program:
#include <pthread.h>
#include <stdio.h>
void* thread_task(void* arg)
{
printf("Thread received parameter: %d\n", (int)arg);
return NULL;
}
int main(void)
{
pthread_t tid;
int i = 45;
pthread_create(&tid, NULL, thread_task, (void *)i);
pthread_join(tid, NULL);
return 0;
}
The gcc compiler raises the following warnings:
$ gcc pcreate.c -o pcreate -lpthread
pcreate.c: In function ‘thread_task’:
pcreate.c:6:45: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
printf("Thread received parameter: %d\n", (int)arg);
^
pcreate.c: In function ‘main’:
pcreate.c:15:43: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
pthread_create(&tid, NULL, thread_task, (void *)i);
Hence, using a pointer on a structure or a table entry or a dynamically allocated memory area is the way to make it portable. The preceding program could be fixed as:
#include <pthread.h>
#include <stdio.h>
void* thread_task(void* arg)
{
printf("Thread received parameter: %d\n", *((int *)arg));
return NULL;
}
int main(void)
{
pthread_t tid;
int t[1] = { 45 };
pthread_create(&tid, NULL, thread_task, (void *)t);
pthread_join(tid, NULL);
return 0;
}
The compiler no longer raises any warnings:
$ gcc pcreate.c -o pcreate -lpthread
$ ./pcreate
Thread received parameter: 45
When you only want to pass one integer you can just cast it to a pointer:
void *foo(void *i) {
int a = (int)i;
}
for ( int i = 0; i < 10; ++1 ) {
pthread_create(&thread, 0, foo, (void*)i);
}
This gives maximum efficiency and no lifetime concerns.
I'm having a trouble passing a struct pointer into a function because I'm a bit confused with those pointers and references. I want to modify the thread.thread_num value from the thread_startfunction.
#include <stdio.h>
#include <stdlib.h> //malloc, free
#include <pthread.h>
#define N 5
// void *malloc(size_t);
struct thread {
pthread_t thread_id;
int thread_num;
// int thread_sum;
};
void *thread_start(void *thread)
{
struct thread *my_data;
my_data = (struct thread *)thread;
printf("num T: %i\n", my_data->thread_num);
my_data->thread_num=4;
printf("num T: %i\n", my_data->thread_num);
return NULL;
}
int main(int argc, char *argv[])
{
int i;
struct thread pthread_data;
struct thread *thread = &pthread_data;
thread->thread_num=2;
pthread_create(&thread->thread_id, NULL, thread_start, (void *)&thread);
printf("num: %i\n",thread->thread_num);
pthread_exit(NULL);
return 0;
}
But the value that print the main doesn't change (2).
And then I want to create an array of thread struct, but I don't know how exactly do that:
I guess it should be something like this:
int main(int argc, char *argv[])
{
int i;
struct thread pthread_data;
struct thread *thread[N-1] = &pthread_data; // I don't know how to manage this.
for(i=0; i<N; i++)
{
thread->thread_num=i;
pthread_create(&thread[i]->thread_id, NULL, thread_start, (void *)&thread[i]);
printf("num %i: %i\n",i,thread[i]->thread_num);
}
pthread_exit(NULL);
return 0;
}
Any thoughts?
I recommend you to read http://www.advancedlinuxprogramming.com/alp-folder/alp-ch04-threads.pdf
And here what you wanted:
#define N 5
typedef struct thread {
pthread_t thread_id;
int thread_num;
// int thread_sum;
} ThreadData;
void *thread_start(void *thread)
{
ThreadData *my_data = (ThreadData*)thread;
//there is no guarantee that prints will be in order
// we will use its initial thread->num ,cause it differs for each thread
//plus you will see how threads will behave
int order=my_data->thread_num;
printf("%i) before num T: %i\n",order, my_data->thread_num);
my_data->thread_num=4;
printf("%i) after assignment num T: %i\n",order ,my_data->thread_num);
return NULL;
}
int main(int argc, char *argv[])
{
int i;
ThreadData thread[N];
for(i=0; i<N; i++)
{
thread[i].thread_num=i;
pthread_create(&(thread[i].thread_id), NULL, thread_start, (void *)(thread+i));
}
//wait for all threads
for (i = 0; i < N; i++)
pthread_join(thread[i].thread_id, NULL);
//print results of each thread
for (i = 0; i < N; i++)
printf(" %i)thread: number %i\n",i,thread[i].thread_num);
return 0;
}
I can see multiple mistakes in your code.
First you have your pointers wrong. In the first example, it is enough to pass &pthread_data to pthread_create, &thread is the address of the thread pointer, so you are passing struct thread ** to your function instead of struct thread *. In the second example you should use (void *) thread[i]. &thread[i] is again struct thread **.
When you want each thread to write to its own thread data, then you should make an array of thread data, so that each thread has its own piece, else you will run into race conditions.
As others have already pointed out, you should call pthread_join before calling printf to ensure that the main thread will wait for all worker threads.
Also note that if you are calling pthread_join from other function that the one that spawned those threads, you have to ensure that the array of tread data will not go out of scope (in this case it would be probably better to use malloc or a global array).
You can't. As soon as main exits, which it does when you call pthread_exit, pthread_data ceases to exist, since it's local to the main function. So it can't be modified.
I have the following functions :
void *foo(void *i) {
int a = (int) i;
}
int main() {
pthread_t thread;
int i;
pthread_create(&thread, 0, foo, (void *) i);
}
At compilation, there are some errors about casting ((void *) i and int a = (int) i). How can I pass an integer as the last argument of pthread_create properly?
Building on szx's answer (so give him the credit), here's how it would work in your for loop:
void *foo(void *i) {
int a = *((int *) i);
free(i);
}
int main() {
pthread_t thread;
for ( int i = 0; i < 10; ++1 ) {
int *arg = malloc(sizeof(*arg));
if ( arg == NULL ) {
fprintf(stderr, "Couldn't allocate memory for thread arg.\n");
exit(EXIT_FAILURE);
}
*arg = i;
pthread_create(&thread, 0, foo, arg);
}
/* Wait for threads, etc */
return 0;
}
On each iteration of the loop, you're allocating new memory, each with a different address, so the thing that gets passed to pthread_create() on each iteration is different, so none of your threads ends up trying to access the same memory and you don't get any thread safety issues in the way that you would if you just passed the address of i. In this case, you could also set up an array and pass the addresses of the elements.
You can allocate an int on the heap and pass it to pthread_create(). You can then deallocate it in your thread function:
void *foo(void *i) {
int a = *((int *) i);
free(i);
}
int main() {
pthread_t thread;
int *i = malloc(sizeof(*i));
pthread_create(&thread, 0, foo, (void *) i);
}
You should cast the address of i (rather than the value of i as you do now) in the last argument of pthread_create().
pthread_create(&thread, 0, foo, (void *) &i);
^ is missing
And the casting is wrong in your function too. It should be:
int a = *((int*) i);
If you intend to read the value, you should also initialize i to some value in main() as it's uninitialized now.
2 Use proper definition for main():
int main(void)
or int main(int argc, char *argv[]) or its equivalent.
Old question, but I faced the same problem today, and I decided not to follow this path.
My application was really about performance, so I chose to have this array of ints declared statically.
Since I don't know a lot of applications where your pthread_join / pthread_cancel is in another scope than your pthread_create, I chose this way :
#define NB_THREADS 4
void *job(void *_i) {
unsigned int i = *((unsigned int *) _i);
}
int main () {
unsigned int ints[NB_THREADS];
pthread_t threads[NB_THREADS];
for (unsigned int i = 0; i < NB_THREADS; ++i) {
ints[i] = i;
pthread_create(&threads[i], NULL, job, &ints[i]);
}
}
I find it more elegant, more efficient, and you don't have to worry about freeing since it only lives in this scope.
While this is an old question there is one option missing when all you need is to pass a positive integer like a descriptor: you can pass it directly as the address, while it it a hack it works well and avoid allocating anything :)
NOTE: the size of the integer must match the size of a pointer on your OS but nowadays most systems are native 64bits.
#include <pthread.h>
#include <inttypes.h>
#include <stdio.h>
void *_thread_loop(void *p)
{
uint64_t n = (uint64_t)p;
printf("received %llu\n", n);
return NULL;
}
int main(int argc, char const *argv[])
{
pthread_t read_thread_id;
uint64_t n = 42;
pthread_create(&read_thread_id, NULL, _thread_loop, (void *)n);
pthread_join(read_thread_id, NULL);
return 0;
}
I am attempting on creating multiple threads that each thread calculates a prime. I am trying to pass a second argument to a function using thread create. It keeps throwing up errors.
void* compute_prime (void* arg, void* arg2)
{
here is my main() with the create thread. &primeArray[i] after &max_prime is giving me the errors.
for(i=0; i< num_threads; i++)
{
primeArray[i]=0;
printf("creating threads: \n");
pthread_create(&primes[i],NULL, compute_prime, &max_prime, &primeArray[i]);
thread_number = i;
//pthread_create(&primes[i],NULL, compPrime, &max_prime);
}
/* join threads */
for(i=0; i< num_threads; i++)
{
pthread_join(primes[i], NULL);
//pthread_join(primes[i], (void*) &prime);
//pthread_join(primes[i],NULL);
//printf("\nThread %d produced: %d primes\n",i, prime);
printf("\nThread %d produced: %d primes\n",i, primeArray[i]);
sleep(1);
}
the error i get is:
myprime.c: In function âmainâ:
myprime.c:123: warning: passing argument 3 of âpthread_createâ from incompatible pointer type
/usr/include/pthread.h:227: note: expected âvoid * (*)(void *)â but argument is of type âvoid * (*)(void *, void *)â
myprime.c:123: error: too many arguments to function âpthread_createâ
It works fine if i take out the second argument.
You can only pass a single argument to the function that you are calling in the new thread. Create a struct to hold both of the values and send the address of the struct.
#include <pthread.h>
#include <stdlib.h>
typedef struct {
//Or whatever information that you need
int *max_prime;
int *ith_prime;
} compute_prime_struct;
void *compute_prime (void *args) {
compute_prime_struct *actual_args = args;
//...
free(actual_args);
return 0;
}
#define num_threads 10
int main() {
int max_prime = 0;
int primeArray[num_threads];
pthread_t primes[num_threads];
for (int i = 0; i < num_threads; ++i) {
compute_prime_struct *args = malloc(sizeof *args);
args->max_prime = &max_prime;
args->ith_prime = &primeArray[i];
if(pthread_create(&primes[i], NULL, compute_prime, args)) {
free(args);
//goto error_handler;
}
}
return 0;
}
In case of std::thread, the user can pass arguments to the thread function in the following method
std::thread(funcName,arg1,arg2);
for instance,
//for a thread function,
void threadFunction(int x,int y){
std::cout << x << y << std::endl;
}
// u can pass x and y values as below
std::thread mTimerThread;
mTimerThread = std::thread(threadFunction,1,12);
This is the code of Manakarse , everything is really good but you need a
pthread_join(thread[i],NULL)
just to be sure all of threads will successfully execute before end of main thread("main will "waiting" while all of threads aren't finished yet)
;
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct
{
int stop_flag;
char name[30];
} _process_data;
typedef struct
{
int meter_no;
int port_no;
} _process_control;
typedef struct
{
_process_data *process_data;
_process_control *process_control;
} compute_prime_struct;
void *compute_prime (void *args)
{
compute_prime_struct *actual_args = args;
printf("actual_args->process_data->stop_flag [%d]\n",actual_args->process_data->stop_flag);
printf("actual_args->process_data->name [%s]\n",actual_args->process_data->name);
printf("actual_args->process_control->meter_no [%d]\n",actual_args->process_control->meter_no);
printf("actual_args->process_control->port_no [%d]\n",actual_args->process_control->port_no);
free(actual_args);
return 0;
}
void fill_data(_process_data *process_data,_process_control *process_control)
{
process_data->stop_flag=1;
process_data->name[0]='P';
process_control->meter_no=6;
process_control->port_no=22;
pthread_t tid;
compute_prime_struct *args = malloc(sizeof (*args));
args->process_data = malloc(sizeof (*args->process_data));
args->process_control = malloc(sizeof (*args->process_control));
memcpy (args->process_data, process_data, sizeof (args->process_data));
memcpy (args->process_control, process_control, sizeof (*args->process_control));
if(pthread_create(&tid, NULL, compute_prime, args))
{
free(args);
printf("Error here");
}
sleep(1);
}
int main()
{
_process_data process_data;
_process_control process_control;
fill_data(&process_data,&process_control);
return 0;
}
This is my code. It's very simple.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *func(void *arg)
{
printf("ID=%d\n", *(int*)arg);
pthread_exit(NULL);
}
int main()
{
pthread_t pt[4];
int i;
for (i = 0; i < 4; i++)
{
int temp = i;
pthread_create(&pt[i], NULL, func, (void*)&temp);
}
sleep(1);
return 0;
}
I compiled it:
gcc p_test.c -lpthread
I ran it. It printed 2 2 3 3. I ran it again. It printed 2 3 3 2.
My problem is:
Why was 2 or 3 printed twice?
Why didn't it print 1 3 2 0 or any other results?
The major problem here is that you're taking the address of the local variable temp, and then using that pointer outside the scope of the variable - as soon as you exit one iteration of the loop, your pointer to temp becomes invalid and you must not dereference it.
You're passing a pointer to a temporary variable into the thread creation function and this temporary goes out of scope at the end of the loop block. It would seem to me that the temporary address is being reused by the compiler and so that when the threads are executing, they see the same address location.
If you do:
int *temp = malloc(sizeof(int));
*temp = i;
pthread_create(&pt[i], NULL, func, (void*)temp);
instead, you should see the results you expect.
In this case, the thread function needs to free the int after it is printed it to avoid a memory leak.
Also, it's better practice to pthread_join() the threads that you're waiting for rather than just sleep()ing.
because it prints temp, all threads shares the memory(why temp is "shared" is explained by TheJuice), so all threads "share" temp . Use a mutex or make temp a private variable.
Private Variables in Threads
Or you can use phtread_join like this:
int main()
{
pthread_t pt[4];
int i;
for (i =0 ; i < 4; i++)
{
pthread_create(&pt[i], NULL, func, (void*)&i);
pthread_join(pt[i],NULL);
}
//sleep(1);
return 0;
}
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *func(void* arg)
{
printf("ID=%d\n", (int)arg);
pthread_exit(NULL);
return 0;
}
int main()
{
pthread_t pt[4];
int i;
for (i =0 ; i < 4; i++)
{
pthread_create(&pt[i], NULL, func, (void*)i);
pthread_join(pt[i],NULL);
}
return 0;
}