Passing an int to a pthread - c

Question

Can't get the pthread's last parameter to pass through as an actual int. I would like to be able to access the 1 that is put in the last parameter of pthread_create. Any help would be greatly appreciated!
#include <pthread.h>
#include <unistd.h>
#include <malloc.h>
#define MAX 5
int arr[MAX];
int p0 = 0;
int p1 = 0;
int p2 = 0;
int p3 = 0;
int p4 = 0;
void *process(void *arg);
int main(int argc, char *argv[]) {
if(argc != 7) {
printf("must be 6 ints");
return -1;
}
int quantum = atoi(argv[1]);
p0 = atoi(argv[2]);
p1 = atoi(argv[3]);
p2 = atoi(argv[4]);
p3 = atoi(argv[5]);
p4 = atoi(argv[6]);
This below is the pthread I am referring to.
int *pointer0 = malloc(sizeof(*pointer0));
*pointer0 = p0;
/* intialize thread 1 */
pthread_t tid0;
pthread_attr_t attr0;
pthread_attr_init(&attr0);
pthread_create(&tid0, &attr0, process, 1);
pthread_join(tid0, NULL);
//if the time remaining of process is not zero, run process[i] again
return 0;
}
void *process(void *arg) {
//char **pointer = (char**) arg;
//int burst = **pointer;
int burst = 0;
//int i = atoi(arg);
printf("%ls", (int *)arg);
pthread_exit(NULL);
}

Answer 1

If you read the manual pages of pthread_create() function, you will notice that you are passing an int instead of a void* in the last argument.
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg);
To solve this problem, create a variable to store the int value and pass it through the pthread_create() function using its address. Then, in the process() function, cast the argument to an int.
int main () {
// (...)
int arg = 1;
pthread_create(&tid0, &attr0, process, &arg);
// (...)
return 0;
}
void *process (void *arg) {
int arg = *((int *) arg);
// (...)
return NULL;
}
Now, notice that the variable arg I just declared in the main() function will be always available, once the program only exits the main() function at the end. If the pthread_create() function is called inside another function, it is highly recommended that you allocate the memory dynamically to ensure it is always available even if the function is exited. Do not forget to free the memory before exiting the process() function.
int foo () {
// (...)
int *arg = malloc( sizeof(int) );
if ( !arg ) {
fprintf(stderr, "error: allocating memory");
return 0;
}
*arg = 1;
pthread_create(&tid0, &attr0, process, arg);
// (...)
return 0;
}
void *process (void *arg) {
int value = *((int *) arg);
// (...)
free(arg);
return NULL;
}
If you want to pass more variables trough the pthread_create() function, declare a struct and repeat the same process.
typedef struct {
char name[50];
char surname[50];
int age;
} person_t;
int foo () {
// (...)
person_t *arg = malloc( sizeof(person_t) );
if ( !arg ) {
fprintf(stderr, "error: allocating memory");
return 0;
}
sprintf( arg->name , "Miguel" );
sprintf( arg->name , "Carvalho" );
arg->age = 22;
pthread_create(&tid0, &attr0, process, arg);
// (...)
return 0;
}
void *process (void *arg) {
person_t person = *((person_t *) arg);
printf( "%s; %s; %d\n", person.name, person.surname, person.age);
// (...)
free(arg);
return NULL;
}