C linux pthread needs sleep to work

Question

I'm writing a program in c on linux using gcc. If I'm not using that sleep statement it will print "thread created" 2,3 or 4 number of times randomly. Can anyone explain me this behavior? //the following code is just a sample, i know is not really useful to create a thread just to print a string :)

void* makeRequest(void* arg) {
    printf("Thread created\n");
}

int main(){
   pthread_t thr[10];
   for(i=0; i<3; i++){  
       pthread_create(&thr[i], 0, makeRequest, &i);
       sleep(1);
    }
 }

p.s. I included pthread.h and the compile option -pthread

Answer 1

You should join all threads that you create, ending main as you do exits the whole process.

Alternatively you could end your main with pthread_exit.

Answer 2

In addition to other remarks,

 pthread_create(&thr[i], 0, makeRequest, &i);

is incorrect, because i is a local variable, so &i is the same pointer on all your calls to pthread_create

You generally should make the data pointer to your thread routine -here the thread routine is makeRequest either a static pointer, or a unique pointer (unique for each thread); in practice, make it a pointer to some malloc-ed memoory.

A better practice would be declare some struct my_thread_data_st, to uniquely allocate it in the heap with

struct my_thread_data_st* td = malloc(sizeof(struct my_thread_data_st));
if (!td) perror("malloc td"), exit(EXIT_FAILURE);
memset (td, 0, sizeof(struct my_thread_data_st));
// fill td appropriately, then
pthread_create(&thr[i], 0, makeRequest, td);

Or you could have an array of e.g. int-s, e.g. int num[4];, initialize it appropriately, then pthread_create(&thr[i], 0, makeRequest, &num[i]);

Of course, if td is heap-allocated thru malloc, don't forget to free it at the appropriate time, e.g. after the thread ended (e.g. after having pthread_join-ed it). You might be also interested by Boehm's GC and use GC_malloc instead of malloc (then, don't bother about freeing memory, the GC will do it).

If the threads are accessing a shared data, you should serialize access to it with some [global or static] mutex (using pthread_mutex_lock & pthread_mutex_unlock)

Don't forget to call pthread_join on all your threads before exiting -e.g. returning from main.

I suggest reading some pthreads tutorial and some book on advanced linux programming.

Answer 3

You need:

for(i = 0; i < 3; i++)
{
   pthread_join(thr[i], NULL);
}

As you have written it, there is nothing waiting for your threads to finish before main returns and exits the whole process, killing the threads.

Answer 4

First use join: and no need to send &i if you don't use it.

print "thread created" 2,3 or 4 number of times randomly.

When main thread dies all child thread also die. so you need to wait in join.

randomly.: because we don't know in thread context switching which will get chance. it may be main-thread or child threads.

for(i=0; i<3; i++){  
       pthread_create(&thr[i], 0, makeRequest, &i);   
}

for(i=0; i<3; i++){  
       pthread_join(thr[i], NULL);
}

Second use -lpthread not -pthread

$ gcc file.c -lpthread -o output      

EDITED

-pthread Adds support for multithreading with the "pthreads" library. This option sets flags for both the preprocessor and linker: Jens Gustedt comment

Difference between -pthread and -lpthread