How to return a value from pthread threads in C

Question

I am new to C and would like to play with threads a bit  I would like to return some value from a thread using pthread exit    My code is as follows    include  lt pthread h gt   include  lt stdio h gt   void  myThread        int ret   42     pthread exit  amp ret      int main        pthread t tid     void  status      pthread create  amp tid  NULL  myThread  NULL      pthread join tid   amp status       printf   d n    int  status          return 0      I would expect the program output  42 n  but it outputs a random number  How can I print the returned value   EDIT  According to first answers the problem is that I am returning pointer to local variable  What is the best practice of returning storing variables of multiple threads  A global hash table   Thanks in advance

User · Answer

Here is a correct solution  In this case tdata is allocated in the main thread  and there is a space for the thread to place its result    include  lt pthread h gt   include  lt stdio h gt   typedef struct thread data      int a     int b     int result     thread data   void  myThread void  arg       thread data  tdata  thread data   arg      int a tdata- gt a     int b tdata- gt b     int result a b      tdata- gt result result     pthread exit NULL      int main        pthread t tid     thread data tdata      tdata a 10     tdata b 32      pthread create  amp tid  NULL  myThread   void    amp tdata      pthread join tid  NULL       printf   d    d    d n   tdata a  tdata b  tdata result          return 0

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include lt stdio h gt   include lt pthread h gt  void  myprint void  x     int k      int   x    printf   n Thread created   value of k   d  n  k      k  11   pthread exit  void   k      int main      pthread t th1   int x  5   int  y   pthread create  amp th1 NULL myprint  void   amp x    pthread join th1  void   amp y    printf   n Exit value is   d  n  y

User · Answer

You ve returned a pointer to a local variable  That s bad even if threads aren t involved   The usual way to do this  when the thread that starts is the same thread that joins  would be to pass a pointer to an int  in a location managed by the caller  as the 4th parameter of pthread create  This then becomes the  only  parameter to the thread s entry-point  You can  if you like  use the thread exit value to indicate success    include  lt pthread h gt   include  lt stdio h gt   int something worked void           thread operation might fail  so here s a silly example        void  p   malloc 10       free p       return p   1   0     void  myThread void  result       if  something worked                int  result    42         pthread exit result        else          pthread exit 0           int main        pthread t tid     void  status   0     int result      pthread create  amp tid  NULL  myThread   amp result      pthread join tid   amp status       if  status    0           printf   d n  result        else          printf  thread failed n             return 0      If you absolutely have to use the thread exit value for a structure  then you ll have to dynamically allocate it  and make sure that whoever joins the thread frees it   That s not ideal  though

User · Answer

if you re uncomfortable with returning addresses and have just a single variable eg  an integer value to return  you can even typecast it into  void    before passing it  and then when you collect it in the main  again typecast it into  int   You have the value without throwing up ugly warnings

User · Answer

I think you have to store the number on heap  The int ret variable was on stack and was destructed at the end of execution of function myThread   void  myThread            int  ret   malloc sizeof int           if  ret    NULL                                       ret   42         pthread exit ret       Don t forget to free it when you don t need it     Another solution is to return the number as value of the pointer  like Neil Butterworth suggests

User · Answer

You are returning a reference to ret which is a variable on the stack

User · Answer

Question   What is the best practice of returning storing variables of multiple threads  A global hash table   This totally depends on what you want to return and how you would use it  If you want to return only status of the thread  say whether the thread completed what it intended to do  then just use pthread exit or use a return statement to return the value from the thread function    But  if you want some more information which will be used for further processing then you can use global data structure  But  in that case you need to handle concurrency issues by using appropriate synchronization primitives  Or you can allocate some dynamic memory  preferrably for the structure in which you want to store the data  and send it via pthread exit and once the thread joins  you update it in another global structure  In this way only the one main thread will update the global structure and concurrency issues are resolved  But  you need to make sure to free all the memory allocated by different threads

User · Answer

You are returning the address of a local variable  which no longer exists when the thread function exits  In any case  why call pthread exit  why not simply return a value from the thread function   void  myThread        return  void    42      and then in main   printf   d n   int status        If you need to return a complicated value such a structure  it s probably easiest to allocate it dynamically via malloc   and return a pointer  Of course  the code that initiated the thread will then be responsible for freeing the memory

[c] How to return a value from pthread threads in C?

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