Why is the same argv[] printed twice in the two threads? - c

Question

I just started programming in multiple threading and tinkering with some templates around, I produced this possibly horrible code:
#include <stdio.h>
#include <pthread.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
typedef struct __myarg_t //argument struct?
{
int a;
char *b;
} myarg_t;
typedef struct __myret_t //return thread struct?
{
int x;
char *y;
} myret_t;
char cat[100] = ""; //used for concatenation of argv
void *mythread(void *arg)
{
myarg_t *m = (myarg_t *)arg;
int print_index = 0;
printf("THREAD ID: %lu\n", pthread_self());
for (print_index = 0; print_index < m->a; print_index++)
{
printf("Printing %d th character %c\n", print_index, *(m->b + print_index));//spelling the words
}
myret_t *rfin = malloc(sizeof(myret_t));
rfin->x = 1;
strcat(cat, m->b);//concatenating argv with
strcat(cat, " "); //a space to cat
rfin->y = cat;//reassigning the new sentence to rfin
return (void *)rfin;
}
int main(int argc, char *argv[])
{
if (argc == 1)
{
printf("Enter a word as argument before commandline.\nExiting...\n");
exit(0);
}
int rc = 0;
pthread_t p[argc - 1]; //total threads
myret_t *m; //return thread
myarg_t args; //argument thread?
int i = 1;
while (i < argc) //creating threads:
{
printf("THREAD:\t%d\n", i);
args.a = strlen(argv[i]);
args.b = (char *)malloc(strlen(argv[i]));
strcpy(args.b, argv[i]);
pthread_create(&p[i - 1], NULL, mythread, &args);
i++;
}
i = 1;
while (i < argc) // Wait for threads to complete
{
pthread_join(p[i - 1], (void **)&m);
i++;
}
printf("returned %d %s\n", m->x, m->y); //end concatenation result for myret_t *m;
return 0;
}
so when I execute it:
gcc -g task2.c -o thread -Wall -pthread
./thread hello there
It gives me the following as the result:
THREAD: 1
THREAD: 2
THREAD ID: 139848453601024
Printing 0 th character t
Printing 1 th character h
Printing 2 th character e
Printing 3 th character r
Printing 4 th character e
THREAD ID: 139848445208320
Printing 0 th character t
Printing 1 th character h
Printing 2 th character e
Printing 3 th character r
Printing 4 th character e
returned 1 there there
Instead of hello there. Is there some problem with the way I am concatenating? Should I be using another thread to do it?
Should I lock the concatenation part of the code because different threads are using it at the same time?
Any idea as to where I am going wrong? Any help/advice/criticism is more than welcome.

Answer 1

You have a race condition in your code -- you pass a pointer to args to the first thread, and then the main() function immediately overwrites the member-variables in the object that the pointer points to, while the first thread is running in parallel, so the first thread (usually) doesn't get a chance to see the original args.a or args.b values before they got overwritten. To avoid this, you could allocate a separate args struct for each thread, instead:
myarg_t * args = (myarg_t *) malloc(sizeof(myarg_t));
args->a = strlen(argv[i]);
args->b = (char *)malloc(strlen(argv[i])+1); // +1 for NUL terminator byte
strcpy(args->b, argv[i]);
pthread_create(&p[i - 1], NULL, mythread, args);