Thread can't count, giving wrong result

Question

I wrote this piece of code

#include       /* Input/Output */
#include      /* General Utilities */
#include            


        

Answer 1

unsigned int cnt=0; is share-able among the threads and operation ++ is not atomically increase cnt. Two thread may read same values of cnt and increase, and overwrites cnt. You need to apply some concurrency control mechanism like semaphore, or mutex.

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If you would disassemble code using following command (suppose code name is thread1.c)

~$ gcc thread.c -lpthread -S  

The output assembly code name is thread1.s.

your wil find cnt++ is more then one instruction in low level in your code:

    movl    $0, -12(%ebp)
    jmp .L2
.L3:
    movl    cnt, %eax
    addl    $1, %eax
    movl    %eax, cnt
    addl    $1, -12(%ebp)
.L2:
    movl    NITERS, %eax

(1) cnt fist move to %eax
(2) then add one to %exc
(3) move %eax into cnt back

And due to thread context switching in between this line, same value of cnt is read by more than one threads. hence cnt++ is not atomic.

Note: Global variable are thread shareable like cnt, and local variable like i that you declared in count() is thread specific.

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I modified your code and imposed concurrency control using semaphore, now it would work fine.

Only modified code shown

#include     /* POSIX Threads */
#include 
unsigned int cnt=0;  /*Count  variable%*/
const int NITERS=1000;

sem_t mysem;

void count()
{
    int i=0;
    for(i=0; i

This will work good! some examples:

nms@NMS:~$ ./thread 
OK! cnt=2000
nms@NMS:~$ ./thread 
OK! cnt=2000
nms@NMS:~$ ./thread 
OK! cnt=2000