CODEWITHSUNDEEP
cglobal-variablesshared-memorydining-philosopher
CodeMonkey
CodeMonkey

Reputation: 368

Making an array of ints visible to child processes using mmap

I am working on the dining philosopher's problem and can't use threads so I need to use shared memory to make the array of chopsticks and philosophers visible to all child processes. I am trying to use mmap, however, my use of mmap is incorrect and I am unsure how to fix it since my argument is an array of ints not an int as per this question. Should I be using a different function to make it in shared memory?

(declared at the global scope)
int chopsticks[5];
int sizeOfSticks=sizeof(int)*5;
void* map = mmap(0,sizeOfSticks,PROT_READ|PROT_WRITE, MAP_SHARED,chopsticks,0);

Upvotes: 2

Views: 4247

Answers (1)

Ulfalizer
Ulfalizer

Reputation: 4752

The second-to-last argument to mmap() is a file descriptor, for cases where you're mapping a file into memory. If you just need to allocate some shared memory, then do this instead:

int *chopsticks;

...

chopsticks = mmap(NULL, N_CHOPSTICKS*sizeof(int),
                  PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED,
                  -1, 0);

MAP_ANONYMOUS means the mapping has no corresponding file. Passing 0 for fd is likely to work too, but -1 might be more portable (see e.g. the mmap(2) man page on Linux).

The mapping will be inherited by fork(2)'d child processes.

As a small style side note, you could write N_CHOPSTICKS*sizeof(*chopsticks) instead of N_CHOPSTICKS*sizeof(int). That way the size will still be correct even if you change the type of chopsticks.

Upvotes: 3

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