Arguments were passed wrong in pthread - linux

Question

I write a code to print out strings: "Thread 0" to "Thread 4" using pthread.
Here is my code:
Case 1:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void* parameter) {
long *i = (long *)parameter;
printf("Thread %ld\n", *i);
pthread_exit(0);
}
int main(int argc, char *argv[]) {
pthread_t threads[5];
long i = 0;
for (i = 0; i < 5; i++) {
pthread_create(&threads[i], 0, print_message_function, (void *)&i);
}
pthread_exit(NULL);
}
But the result is:
Thread 2
Thread 3
Thread 3
Thread 4
Thread 5
or:
Thread 0
Thread 0
Thread 0
Thread 0
Thread 0
It changed when I run it again. So I don't know why the values I passed are (2 to 5) or all (0) or ..... (many cases). I think my arguments I passed is from 0 to 4.
Case 2:
When I change to the new code:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void *parameter);
int main(int argc, char *argv[]) {
pthread_t threads[5];
int i = 0;
for (i = 0; i < 5; i++) {
char *msg = (char*)malloc(sizeof(char));
sprintf(msg, "Thread %d", i);
pthread_create(&threads[i], 0, print_message_function, (void *)msg);
}
}
void *print_message_function(void *parameter) {
printf("%s\n", (char *)parameter);
}
The result is:
Thread 1
Thread 0
Thread 3
Thread 2
Thread 4
Thread 4
It means the loop run 6 times! Why?

Answer 1

Change Case 1 to this:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void* parameter) {
long i = (long)parameter; // <<<
printf("Thread %ld\n", i); // <<<
pthread_exit(0);
}
int main(int argc, char *argv[]) {
pthread_t threads[5];
long i = 0;
for (i = 0; i < 5; i++) {
pthread_create(&threads[i], 0, print_message_function, (void *)i); // <<<
}
pthread_exit(NULL);
}
The reason that you were seeing inconsistent results before was because you were passing a pointer to each thread where each pointer was pointing at the same local variable, which you were then modifying.
In Case 2 you are mallocing only a single char and then trying to write a string to it. It should be fairly easy to fix.

Answer 2

Your case 2 approach is valid, however you need to fix the malloc part to allocate enough bytes. Change it to
char *msg = (char*)malloc(sizeof(char) * (strlen("Thread ") + 10));
// assuming i will take at most 9-digits (unlikely case)
For your case 1, you are passing address of i. But the value of i is changing thread function will get whatever value is there at that location when it tries to print. Also note that address of i may not be valid by the time thread function executes as its allocated on stack and will go away when main function returns.