CODEWITHSUNDEEP
ccastingpthreadsvoid-pointersfile-descriptor
Jared Beekman
Jared Beekman

Reputation: 320

Passing a file descriptor to a thread and using it in the function(homework)

My assignment is to do a stripped down multi-threaded mockup of a print server.

The function get_server's prototype is:

void *get_request(void *arg);

"The parameter arg points to an open file descriptor from where the request is to be read." So in testing it's recommended to use STDIN_FILENO, but the descriptor needs to be generic when all is said and done.

pthread_create(&tid, &attr, get_request, STDIN_FILENO);

Inside the function I'm trying to use arg, and can't change it from a void * to anything usable. For instance none of this works:

read(*arg, intvariable, sizeof(int)); // can't cast void * error
int fd = *arg; // can't cast void * error
int fd = *(int *)arg; // seg fault
int fd = *((int *)arg); // seg fault

int fd = atoi(arg); // seg fault
// yes I'm aware arg isn't a char* but that's 
// from the example code we were given

Upvotes: 1

Views: 6013

Answers (2)

chrisaycock
chrisaycock

Reputation: 37930

Your function expects a pointer to a value, but you are passing the value itself! So to follow the pointer scheme, you need to pass a pointer to a global variable that contains the value STDIN_FILENO.

Or alternatively, you can "cheat" by passing STDIN_FILENO like you're currently doing, and then cast the void* to a simple int (not a pointer!) and use it as is:

int fd = (int) arg;

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409176

You are on the right way:

void *get_request(void *arg)
{
    int fd = (int) arg;

    ...
}

However, it's not the recommended way. Instead create a variable and pass the address of that to the pthread_create call:

int fd = STDIN_FILENO;
pthread_create(&tid, &attr, get_request, &fd);

Then you use

void *get_request(void *arg)
{
    int fd = *((int *) arg);

    ...
}

Be careful though, so the scope of the variable used for the pthread_create call doesn't run out before the thread starts. Then you will have a pointer to unused memory. Put this variable at the top of the main function (if you are calling pthread_create in main).

Upvotes: 4

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