Thread: array of N pointers

Hello, I need some help on this:

Isn't

int *array[3];

the same thing as

int *array = (int*) malloc(3*sizeof(int*)); ??

I used the first declaration to print the integers: *array[i] and it worked.

When I used the second one for N pointers, I get an error about invalid argument type of 'unary*' (have 'int').

Thank you.

Edit:

Maybe pasting the whole code it will help:

Code:

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>

void *myfunc(void *arg)
{
	int *x = (int*) arg;
	printf("O meu pid é %u, o meu tid é %u e recebi arg=%d \n", (unsigned) getpid(), (unsigned) pthread_self(), *x);	               
	return (void*) x;
}

int main (int argc, char* argv[])
{
	if (argc != 2)
	{
		perror("Falta de argumentos!\n");
		printf("Erro %d\n", errno);
		_exit(1);
	}	 
	int n = atoi(argv[1]), st, i; 
	pthread_t *tids = (pthread_t*) malloc(n*sizeof(pthread_t));
	int *retval = (int*) malloc(n*sizeof(int*));
	for(i=1; i<=n; i++)
	{	 
		st= pthread_create(&tids[i-1], NULL, myfunc, (void*) &i);
		if (st != 0) 
		{ 
			perror("Erro em pthread_create()\n"); 
			printf("Erro %d\n", errno);
			_exit(2); 
		}
	}
	
	for (i=1; i<=n; i++)
	{
		st = pthread_join(tids[i-1], (void**) &retval[i-1]); 
		if (st != 0) 
		{ 
			perror ("Erro em pthread_join()\n"); 
			printf("Erro %d\n", errno);
			_exit(3);
		}
		printf("Thread %u devolveu %d\n", (unsigned) tids[i-1], *retval[i-1]);
	}
	_exit(0);
}

Originally Posted by CLimaPT

Isn't

int *array[3];

the same thing as

int *array = (int*) malloc(3*sizeof(int*)); ??

No, they are not the same. The former declares an array of 3 pointers to int; the latter declares a pointer to int.

If you want a dynamic array of 3 pointers to int, you should write:

Code:

int **array = malloc(3 * sizeof(*array));

Originally Posted by CLimaPT

When I used the second one for N pointers, I get an error about invalid argument type of 'unary*' (have 'int').

What is the exact error message?

I get this:

f3exercicio1.c: In function ‘main’:
f3exercicio1.c:36: error: invalid type argument of ‘unary *’ (have ‘int’)

Oh, I think I get it, thank you once again. No cast needed by the way?

Originally Posted by CLimaPT

f3exercicio1.c: In function ‘main’:
f3exercicio1.c:36: error: invalid type argument of ‘unary *’ (have ‘int’)

Line 36 appears to be a blank line... so which line is line 36?

By the way, there is no need to cast a pointer to be a pointer to void. The conversion will happen implicitly. There is also no need to cast a void* to be the desired pointer type unless you are trying to be compatible with C++.

You should also get used to the idiom of starting your loop counters from 0 instead of 1 when you are using the loop counters to index an array.

Right. Since retval is a pointer to int, retval[i-1] is an int, so *retval[i-1] does not make sense. Perhaps you mean to make retval a pointer to pointer to int, as I demonstrated in post #3.