C , how to create thread using pthread_create function

Question

I'm making a c file for a dispatch queue that gets a task and put it in to a queue which is the linked list. In order to do this, I need to create threads using

pthread_t cThread;
if(pthread_create(&cThread, NULL, work, param)){
    perror("ERROR creating thread.");
}

However I need to make another function that goes into 'work' and 'param' variable as parameters of create function. My friend told me that I just need to put any code in the work function that loops infinitely so the thread does not die.. Can anyone explain each parameter goes in to the pthread_create function- especially for work and param? I searched Google for this, but most of tutorials are so hard to understand the concept...

pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_create.html — miku
– miku, Commented Aug 9, 2011 at 2:55
yea i read that. I just want someone to explain me in sentences- what each parameter is for — Leanne
– Leanne, Commented Aug 9, 2011 at 3:03

Jens Gustedt · Accepted Answer · 2011-08-09 06:16:40Z

45

The four parameters to pthread_create are, in order:

A pointer to a pthread_t structure, which pthread_create will fill out with information on the thread it creates.
A pointer to a pthread_attr_t with parameters for the thread. You can safely just pass NULL most of the time.
A function to run in the thread. The function must return void * and take a void * argument, which you may use however you see fit. (For instance, if you're starting multiple threads with the same function, you can use this parameter to distinguish them.)
The void * that you want to start up the thread with. Pass NULL if you don't need it.

edited Aug 9, 2011 at 6:16

Jens Gustedt

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answered Aug 9, 2011 at 3:02

user149341

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Foo Bah · Accepted Answer · 2011-08-09 03:10:08Z

25

clarifying duskwuff's answer:

work parameter is a function pointer. The function should take one argument which is indicated as type void * and return value void *.

param is expected to be a pointer to the data that work will receive.

As an example, lets say you want to pass two int to the worker. Then, you can create something like this:

int *param = (int *)malloc(2 * sizeof(int));
param[0] = 123;
param[1] = 456;
pthread_create(&cThread, NULL, work, param);

Then your work function can convert the pointer type and grab the param data:

void *work(void * parm) {
    int *param = (int *)parm;
    int first_val = param[0];
    ....
}

You can do more complex stuff, like creating a struct with all of the data you need to pass.

answered Aug 9, 2011 at 3:10

Foo Bah

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3 Comments

Jens Gustedt Over a year ago

Please don't cast return values from malloc. Its return is void* and is always assignable to any data pointer. Same goes for parm.

lingjiankong Over a year ago

@Jens If you are using C++, then you must explicitly cast return values from malloc, or the code won't compile. If you are using C, then it is optional.

m4r35n357 Over a year ago

@lingjiankong the OP is labelled "c". I don't want to see c++ artifacts in a c answer. It is hard enough with the similar language names, let's not make it any more difficult!

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