POSIX shared-memory API
Several IPC mechanisms are available for POSIX systems, including shared memory and message passing. Here, we explore the POSIX API for shared memory.
POSIX shared memory is organized using memory-mapped files, which associate the region of shared memory with a file. A process must first create a shared-memory object using the shm_open() system call, as follows:
shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666); Parameters: name: The first parameter specifies the name of the shared-memory object. Processes that wish to access this shared memory must refer to the object by this name. O_CREAT | O_RDWR : The subsequent parameters specify that the shared-memory object is to be created if it does not yet exist (O_CREAT) and that the object is open for reading and writing (O_RDWR). The last parameter establishes the directory permissions of the shared-memory object.
A successful call to shm_open() returns an integer file descriptor for the shared-memory object. Once the object is established, the ftruncate() function is used to configure the size of the object in bytes. The call
sets the size of the object to 4, 096 bytes. Finally, the mmap() function establishes a memory-mapped file containing the shared-memory object. It also returns a pointer to the memory-mapped file that is used for accessing the shared-memory object.
Programs showing POSIX shared memory API for producer and consumer
How to use shared memory with Linux in C
I have a bit of an issue with one of my projects. I have been trying to find a well documented example of using shared memory with fork() but to no success. Basically the scenario is that when the user starts the program, I need to store two values in shared memory: current_path which is a char* and a file_name which is also char*. Depending on the command arguments, a new process is kicked off with fork() and that process needs to read and modify the current_path variable stored in shared memory while the file_name variable is read only. Is there a good tutorial on shared memory with example code (if possible) that you can direct me to?
You may consider using threads instead of processes. Then the whole memory is shared with no further tricks.
The answers below discuss both the System V IPC mechanism, shmget() et al. and also the pure mmap() approach with MAP_ANON (aka MAP_ANONYMOUS ) — though MAP_ANON is not defined by POSIX. There is also POSIX shm_open() and shm_close() for managing shared memory objects. […continued…]
[…continuation…] These have the same advantage that the System V IPC shared memory has — the shared memory object can persist beyond the lifetime of the process that creates it (until some process executes shm_unlink() ), whereas mechanisms using mmap() require a file and MAP_SHARED to persist the data (and MAP_ANON precludes persistence). There’s a complete example in the Rationale section of the specification of shm_open() .
6 Answers 6
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 #include #include 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 #include 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); >> This is why Linux is so frustrating for inexperienced devs. The man page doesn’t explain how to actually use it, and there is no sample code. 🙁
Haha I know what you mean, but it’s actually because we’re not used to reading manpages. When I learned to read them and got used to them, they became even more useful than lousy tutorials with particular demonstrations. I remember I got a 10/10 in my Operating Systems course using nothing but manpages for reference during the exam.
shmget is a really old-fashioned, and some would say deprecated, way to do shared memory. Better to use mmap and shm_open , plain files, or simply MAP_ANONYMOUS .
Well, this answer became popular for some reason, so I decided to make it worth the read. It only took 4 years
Here is an example for shared memory :
#include #include #include #include #include #include #define SHM_SIZE 1024 /* make it a 1K shared memory segment */ int main(int argc, char *argv[]) < key_t key; int shmid; char *data; int mode; if (argc >2) < fprintf(stderr, "usage: shmdemo [data_to_write]\n"); exit(1); >/* make the key: */ if ((key = ftok("hello.txt", 'R')) == -1) /*Here the file must exist */ < perror("ftok"); exit(1); >/* create the segment: */ if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)) == -1) < perror("shmget"); exit(1); >/* attach to the segment to get a pointer to it: */ if ((data = shmat(shmid, NULL, 0)) == (void *)-1) < perror("shmat"); exit(1); >/* read or modify the segment, based on the command line: */ if (argc == 2) < printf("writing to segment: \"%s\"\n", argv[1]); strncpy(data, argv[1], SHM_SIZE); >else printf("segment contains: \"%s\"\n", data); /* detach from the segment: */ if (shmdt(data) == -1) < perror("shmdt"); exit(1); >return 0; > - Use ftok to convert a pathname and a project identifier to a System V IPC key
- Use shmget which allocates a shared memory segment
- Use shmat to attache the shared memory segment identified by shmid to the address space of the calling process
- Do the operations on the memory area
- Detach using shmdt
However this code doesn’t handle deletion of shared memory. After program exits, one have to delete it manually via ipcrm -m 0.
These are includes for using shared memory
#include #include int shmid; int shmkey = 12222;//u can choose it as your choice int main() < //now your main starting shmid = shmget(shmkey,1024,IPC_CREAT); // 1024 = your preferred size for share memory // IPC_CREAT its a flag to create shared memory //now attach a memory to this share memory char *shmpointer = shmat(shmid,NULL); //do your work with the shared memory //read -write will be done with the *shmppointer //after your work is done deattach the pointer shmdt(&shmpointer, NULL); try this code sample, I tested it, source: http://www.makelinux.net/alp/035
#include #include #include int main () < int segment_id; char* shared_memory; struct shmid_ds shmbuffer; int segment_size; const int shared_segment_size = 0x6400; /* Allocate a shared memory segment. */ segment_id = shmget (IPC_PRIVATE, shared_segment_size, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); /* Attach the shared memory segment. */ shared_memory = (char*) shmat (segment_id, 0, 0); printf ("shared memory attached at address %p\n", shared_memory); /* Determine the segment's size. */ shmctl (segment_id, IPC_STAT, &shmbuffer); segment_size = shmbuffer.shm_segsz; printf ("segment size: %d\n", segment_size); /* Write a string to the shared memory segment. */ sprintf (shared_memory, "Hello, world."); /* Detach the shared memory segment. */ shmdt (shared_memory); /* Reattach the shared memory segment, at a different address. */ shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0); printf ("shared memory reattached at address %p\n", shared_memory); /* Print out the string from shared memory. */ printf ("%s\n", shared_memory); /* Detach the shared memory segment. */ shmdt (shared_memory); /* Deallocate the shared memory segment. */ shmctl (segment_id, IPC_RMID, 0); return 0; >This is good code, except I don't think it shows how to access the shared-memory segment by a client (by using shmget and shmat from a different process), which is kind of the whole point of shared memory. =(
#include #include #include #include /* * pvtmMmapAlloc - creates a memory mapped file area. * The return value is a page-aligned memory value, or NULL if there is a failure. * Here's the list of arguments: * @mmapFileName - the name of the memory mapped file * @size - the size of the memory mapped file (should be a multiple of the system page for best performance) * @create - determines whether or not the area should be created. */ void* pvtmMmapAlloc (char * mmapFileName, size_t size, char create) < void * retv = NULL; if (create) < mode_t origMask = umask(0); int mmapFd = open(mmapFileName, O_CREAT|O_RDWR, 00666); umask(origMask); if (mmapFd < 0) < perror("open mmapFd failed"); return NULL; >if ((ftruncate(mmapFd, size) == 0)) < int result = lseek(mmapFd, size - 1, SEEK_SET); if (result == -1) < perror("lseek mmapFd failed"); close(mmapFd); return NULL; >/* Something needs to be written at the end of the file to * have the file actually have the new size. * Just writing an empty string at the current file position will do. * Note: * - The current position in the file is at the end of the stretched * file due to the call to lseek(). * - The current position in the file is at the end of the stretched * file due to the call to lseek(). * - An empty string is actually a single '\0' character, so a zero-byte * will be written at the last byte of the file. */ result = write(mmapFd, "", 1); if (result != 1) < perror("write mmapFd failed"); close(mmapFd); return NULL; >retv = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0); if (retv == MAP_FAILED || retv == NULL) < perror("mmap"); close(mmapFd); return NULL; >> > else < int mmapFd = open(mmapFileName, O_RDWR, 00666); if (mmapFd < 0) < return NULL; >int result = lseek(mmapFd, 0, SEEK_END); if (result == -1) < perror("lseek mmapFd failed"); close(mmapFd); return NULL; >if (result == 0) < perror("The file has 0 bytes"); close(mmapFd); return NULL; >retv = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0); if (retv == MAP_FAILED || retv == NULL) < perror("mmap"); close(mmapFd); return NULL; >close(mmapFd); > return retv; >