There are two approaches: shmget
and mmap
. I'll talk about mmap
, since it's more modern and flexible, but you can take a look at man shmget
(or this tutorial) if you'd rather use the old-style tools.
The mmap()
function can be used to allocate memory buffers with highly customizable parameters to control access and permissions, and to back them with file-system storage if necessary.
The following function creates an in-memory buffer that a process can share with its children:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
void* create_shared_memory(size_t size) {
// Our memory buffer will be readable and writable:
int protection = PROT_READ | PROT_WRITE;
// The buffer will be shared (meaning other processes can access it), but
// anonymous (meaning third-party processes cannot obtain an address for it),
// so only this process and its children will be able to use it:
int visibility = MAP_SHARED | MAP_ANONYMOUS;
// The remaining parameters to `mmap()` are not important for this use case,
// but the manpage for `mmap` explains their purpose.
return mmap(NULL, size, protection, visibility, -1, 0);
}
The following is an example program that uses the function defined above to allocate a buffer. The parent process will write a message, fork, and then wait for its child to modify the buffer. Both processes can read and write the shared memory.
#include <string.h>
#include <unistd.h>
int main() {
char parent_message[] = "hello"; // parent process will write this message
char child_message[] = "goodbye"; // child process will then write this one
void* shmem = create_shared_memory(128);
memcpy(shmem, parent_message, sizeof(parent_message));
int pid = fork();
if (pid == 0) {
printf("Child read: %s\n", shmem);
memcpy(shmem, child_message, sizeof(child_message));
printf("Child wrote: %s\n", shmem);
} else {
printf("Parent read: %s\n", shmem);
sleep(1);
printf("After 1s, parent read: %s\n", shmem);
}
}
Best Answer
Essentially, pipes - whether named or anonymous - are used like message passing. Someone sends a piece of information to the recipient and the recipient can receive it. Shared memory is more like publishing data - someone puts data in shared memory and the readers (potentially many) must use synchronization e.g. via semaphores to learn about the fact that there is new data and must know how to read the memory region to find the information.
With pipes the synchronization is simple and built into the pipe mechanism itself - your reads and writes will freeze and unfreeze the app when something interesting happens. With shared memory, it is easier to work asynchronously and check for new data only once in a while - but at the cost of much more complex code. Plus you can get many-to-many communication but it requires more work again. Also, due to the above, debugging of pipe-based communication is easier than debugging shared memory.
A minor difference is that fifos are visible directly in the filesystem while shared memory regions need special tools like
ipcs
for their management in case you e.g. create a shared memory segment but your app dies and doesn't clean up after itself (same goes for semaphores and many other synchronization mechanisms which you might need to use together with shared memory).Shared memory also gives you more control over bufferring and resource use - within limits allowed by the OS it's you who decides how much memory to allocate and how to use it. With pipes, the OS controls things automatically, so once again you loose some flexibility but are relieved of much work.
Summary of most important points: pipes for one-to-one communication, less coding and letting the OS handle things, shared memory for many-to-many, more manual control over things but at the cost of more work and harder debugging.