pthread join and pthread exit

Question

I have a question about C concurrency programming   In the pthread library  the prototype of pthread join is  int pthread join pthread t tid  void   ret     and the prototype of pthread exit is   void pthread exit void  ret     So I am confused that  why pthread join takes the return value of the process as a pointer to a void pointer from reaped thread  but pthread exit only takes a void pointer from the exited thread  I mean basically they are all return values from a thread  why there is a difference in type

User · Accepted Answer

In pthread exit  ret is an input parameter  You are simply passing the address of a variable to the function   In pthread join  ret is an output parameter  You get back a value from the function  Such value can  for example  be set to NULL   Long explanation   In pthread join  you get back the address passed to pthread exit by the finished thread  If you pass just a plain pointer  it is passed by value so you can t change where it is pointing to  To be able to change the value of the pointer passed to pthread join  it must be passed as a pointer itself  that is  a pointer to a pointer

User · Answer

It because every time void pthread exit void  ret    will be called from thread function so which ever you want to return simply its pointer pass with pthread exit    Now at int pthread join pthread t tid  void   ret    will be always called from where thread is created so here to accept that returned pointer you need double pointer    i think this code will help you to understand this  include  lt stdio h gt   include  lt string h gt   include  lt pthread h gt   include  lt stdlib h gt   void  thread function void  ignoredInThisExample        char  a   malloc 10       strcpy a  quot hello world quot        pthread exit  void  a     int main         pthread t thread id      char  b       pthread create   amp thread id  NULL  amp thread function  NULL        pthread join thread id  void    amp b     here we are reciving one pointer                                          value so to use that we need double pointer      printf  quot b is  s n quot  b        free b      lets free the memory

User · Answer

The typical use is  void  ret   NULL  pthread t tid   something      change it suitably if  pthread join  tid   amp ret       handle error       do something with the return value ret

[c] pthread_join() and pthread_exit()

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