Threads in C - Input and Output

Today, we will see only to pass values to a thread.  Retrieve value from thread.

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#include <stdio.h> #include <pthread.h> #include <unistd.h> #include <stdlib.h> //Thread to change value from 20 to 30 void * threadtak (void * poin) { int *inttak = (int *)poin; *inttak=30; return NULL; } //Thread to return a value 60 void * threadret () { int *ret=malloc(sizeof(int)); *ret=60; printf("%d\n",*ret); return ret; } int main() { pthread_t tak,ret; pthread_attr_t attr; pthread_attr_init(&attr); int vartak=20; pthread_create(&tak, &attr,threadtak, &vartak); pthread_join(tak,NULL); printf("Modified vartak = %d\n",vartak); int *varret; pthread_create(&ret, &attr,threadret, NULL); pthread_join(ret,(void *)&varret); printf("Returned varret=%d\n",*varret); return 0; }

Lets break the explanation into 2.

Passing the value

Line 29: Address of vartak is passed as argument
Line 9: As it is declared void pointer, it can take the address of integer/any other data type.
Line 10: Value at inttak is changed to 30
Line 11: Returned as NULL
Line 31: vartak is already changed and can be accessed as normal int

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# gcc -pthread -o a.out main.c # ./a.out Modified vartak = 30 60 Returned varret=60

Value is vartak is displayed as 30

Returning the value

Line 33: We try to retrieve the value from threadret into integer pointer.
Line 34: As we are not passing any argument to the thread, we keep it NULL
Line 15: The argument for the thread can be kept blank as no argument is passed
Line 17: We create a memory and assign the value 60
Line 20: Return the address of ret
Line 35: Join should have void **.  We keep it &varret.  Because, *varret is value. varret is the address of *varret. &varret is the address of varret.

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# gcc -pthread -o a.out main.c # ./a.out Modified vartak = 30 60 Returned varret=60

Value of varret is displayed as 60.