Passing in an element in an array as a pointer in a function in C

Question

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);

I am trying to pass in an argument as a pointer through pthread_create.

typedef struct {
    int input;
    int threadNum;
} ARGS;

ARGS argument16[16];
for (x = 0; x < 16; x++) {
    printf("pthread16_created: %d\n", x);
    argument16[x].threadNum = 16;
    argument16[x].input = x % 4;
    pthread_create(&(threads16[x]), NULL, funct, argument16[x]);
}

This is giving me error: incompatible type for argument 4 of ‘pthread_create’

How do I pass in a ARG type pointer in a array of ARGS? I can't seem to figure it out. I know arrays are already a pointer, but not sure how to pointer to a certain element in the array. Thank you

&argument16[x] In other words, you want to pass the address of the element, not the element itself. — user3386109
– user3386109, Commented Nov 11, 2015 at 2:32

Jonathan Leffler · Accepted Answer · 2015-11-11 03:40:25Z

3

The final argument to pthread_create() is a pointer to void so an address typecast to void * is expected. So, like commenter said, the address of argument16[x] rather than the value which is what you passed. Otherwise known as rvalue.

Inside the thread function, you want a pointer of correct type so:

ARGS * ptr  = (   ARGS *  ) arg;

Is the expected way to type cast back.

If you aren't sure in the future use a pointer to same type and increment it per loop then pass by lvalue.

edited Nov 11, 2015 at 3:40

Jonathan Leffler

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answered Nov 11, 2015 at 3:15

daemondave

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