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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
shm_open(3) Library Functions Manualshm_open(3)shm_open, shm_unlink - create/open or unlink POSIX shared memory objects
Real-time library (librt,-lrt)
#include <sys/mman.h>#include <sys/stat.h>/* For mode constants */#include <fcntl.h>/* For O_* constants */int shm_open(const char *name, intoflag, mode_tmode);int shm_unlink(const char *name);
shm_open() creates and opens a new, or opens an existing, POSIX shared memory object. A POSIX shared memory object is in effect a handle which can be used by unrelated processes tommap(2) the same region of shared memory. Theshm_unlink() function performs the converse operation, removing an object previously created byshm_open(). The operation ofshm_open() is analogous to that ofopen(2).name specifies the shared memory object to be created or opened. For portable use, a shared memory object should be identified by a name of the form/somename; that is, a null-terminated string of up toNAME_MAX(i.e., 255) characters consisting of an initial slash, followed by one or more characters, none of which are slashes.oflag is a bit mask created by ORing together exactly one ofO_RDONLYorO_RDWRand any of the other flags listed here:O_RDONLY Open the object for read access. A shared memory object opened in this way can bemmap(2)ed only for read (PROT_READ) access.O_RDWROpen the object for read-write access.O_CREAT Create the shared memory object if it does not exist. The user and group ownership of the object are taken from the corresponding effective IDs of the calling process, and the object's permission bits are set according to the low-order 9 bits ofmode, except that those bits set in the process file mode creation mask (seeumask(2)) are cleared for the new object. A set of macro constants which can be used to definemode is listed inopen(2). (Symbolic definitions of these constants can be obtained by including<sys/stat.h>.) A new shared memory object initially has zero length—the size of the object can be set usingftruncate(2). The newly allocated bytes of a shared memory object are automatically initialized to 0.O_EXCLIfO_CREATwas also specified, and a shared memory object with the givenname already exists, return an error. The check for the existence of the object, and its creation if it does not exist, are performed atomically.O_TRUNC If the shared memory object already exists, truncate it to zero bytes. Definitions of these flag values can be obtained by including<fcntl.h>. On successful completionshm_open() returns a new file descriptor referring to the shared memory object. This file descriptor is guaranteed to be the lowest-numbered file descriptor not previously opened within the process. TheFD_CLOEXECflag (seefcntl(2)) is set for the file descriptor. The file descriptor is normally used in subsequent calls toftruncate(2) (for a newly created object) andmmap(2). After a call tommap(2) the file descriptor may be closed without affecting the memory mapping. The operation ofshm_unlink() is analogous tounlink(2): it removes a shared memory object name, and, once all processes have unmapped the object, deallocates and destroys the contents of the associated memory region. After a successfulshm_unlink(), attempts toshm_open() an object with the samename fail (unlessO_CREATwas specified, in which case a new, distinct object is created).
On success,shm_open() returns a file descriptor (a nonnegative integer). On success,shm_unlink() returns 0. On failure, both functions return -1 and seterrno to indicate the error.
EACCESPermission toshm_unlink() the shared memory object was denied.EACCESPermission was denied toshm_open()name in the specifiedmode, orO_TRUNCwas specified and the caller does not have write permission on the object.EEXISTBothO_CREATandO_EXCLwere specified toshm_open() and the shared memory object specified byname already exists.EINVALThename argument toshm_open() was invalid.EMFILEThe per-process limit on the number of open file descriptors has been reached.ENAMETOOLONG The length ofname exceedsPATH_MAX.ENFILEThe system-wide limit on the total number of open files has been reached.ENOENTAn attempt was made toshm_open() aname that did not exist, andO_CREATwas not specified.ENOENTAn attempt was to made toshm_unlink() aname that does not exist.
For an explanation of the terms used in this section, seeattributes(7). ┌───────────────────────────────┬───────────────┬────────────────┐ │Interface│Attribute│Value│ ├───────────────────────────────┼───────────────┼────────────────┤ │shm_open(),shm_unlink() │ Thread safety │ MT-Safe locale │ └───────────────────────────────┴───────────────┴────────────────┘
POSIX leaves the behavior of the combination ofO_RDONLYandO_TRUNCunspecified. On Linux, this will successfully truncate an existing shared memory object—this may not be so on other UNIX systems. The POSIX shared memory object implementation on Linux makes use of a dedicatedtmpfs(5) filesystem that is normally mounted under/dev/shm.
POSIX.1-2008.
glibc 2.2. POSIX.1-2001. POSIX.1-2001 says that the group ownership of a newly created shared memory object is set to either the calling process's effective group ID or "a system default group ID". POSIX.1-2008 says that the group ownership may be set to either the calling process's effective group ID or, if the object is visible in the filesystem, the group ID of the parent directory.
The programs below employ POSIX shared memory and POSIX unnamed semaphores to exchange a piece of data. The "bounce" program (which must be run first) raises the case of a string that is placed into the shared memory by the "send" program. Once the data has been modified, the "send" program then prints the contents of the modified shared memory. An example execution of the two programs is the following: $./pshm_ucase_bounce /myshm & [1] 270171 $./pshm_ucase_send /myshm hello; HELLO Further detail about these programs is provided below.Program source: pshm_ucase.h The following header file is included by both programs below. Its primary purpose is to define a structure that will be imposed on the memory object that is shared between the two programs. #ifndef PSHM_UCASE_H #define PSHM_UCASE_H #include <semaphore.h> #include <stddef.h> #include <stdio.h> #include <stdlib.h> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) #define BUF_SIZE 1024 /* Maximum size for exchanged string */ /* Define a structure that will be imposed on the shared memory object */ struct shmbuf { sem_t sem1; /* POSIX unnamed semaphore */ sem_t sem2; /* POSIX unnamed semaphore */ size_t cnt; /* Number of bytes used in 'buf' */ char buf[BUF_SIZE]; /* Data being transferred */ }; #endif // include guardProgram source: pshm_ucase_bounce.c The "bounce" program creates a new shared memory object with the name given in its command-line argument and sizes the object to match the size of theshmbuf structure defined in the header file. It then maps the object into the process's address space, and initializes two POSIX semaphores inside the object to 0. After the "send" program has posted the first of the semaphores, the "bounce" program upper cases the data that has been placed in the memory by the "send" program and then posts the second semaphore to tell the "send" program that it may now access the shared memory. /* pshm_ucase_bounce.c Licensed under GNU General Public License v2 or later. */ #include <ctype.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <sys/mman.h> #include <unistd.h> #include "pshm_ucase.h" int main(int argc, char *argv[]) { int fd; char *shmpath; struct shmbuf *shmp; if (argc != 2) { fprintf(stderr, "Usage: %s /shm-path\n", argv[0]); exit(EXIT_FAILURE); } shmpath = argv[1]; /* Create shared memory object and set its size to the size of our structure. */ fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR, 0600); if (fd == -1) errExit("shm_open"); if (ftruncate(fd, sizeof(struct shmbuf)) == -1) errExit("ftruncate"); /* Map the object into the caller's address space. */ shmp = mmap(NULL, sizeof(*shmp), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (shmp == MAP_FAILED) errExit("mmap"); /* Initialize semaphores as process-shared, with value 0. */ if (sem_init(&shmp->sem1, 1, 0) == -1) errExit("sem_init-sem1"); if (sem_init(&shmp->sem2, 1, 0) == -1) errExit("sem_init-sem2"); /* Wait for 'sem1' to be posted by peer before touching shared memory. */ if (sem_wait(&shmp->sem1) == -1) errExit("sem_wait"); /* Convert data in shared memory into upper case. */ for (size_t j = 0; j < shmp->cnt; j++) shmp->buf[j] = toupper((unsigned char) shmp->buf[j]); /* Post 'sem2' to tell the peer that it can now access the modified data in shared memory. */ if (sem_post(&shmp->sem2) == -1) errExit("sem_post"); /* Unlink the shared memory object. Even if the peer process is still using the object, this is okay. The object will be removed only after all open references are closed. */ shm_unlink(shmpath); exit(EXIT_SUCCESS); }Program source: pshm_ucase_send.c The "send" program takes two command-line arguments: the pathname of a shared memory object previously created by the "bounce" program and a string that is to be copied into that object. The program opens the shared memory object and maps the object into its address space. It then copies the data specified in its second argument into the shared memory, and posts the first semaphore, which tells the "bounce" program that it can now access that data. After the "bounce" program posts the second semaphore, the "send" program prints the contents of the shared memory on standard output. /* pshm_ucase_send.c Licensed under GNU General Public License v2 or later. */ #include <fcntl.h> #include <stddef.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <unistd.h> #include "pshm_ucase.h" int main(int argc, char *argv[]) { int fd; char *shmpath, *string; size_t len; struct shmbuf *shmp; if (argc != 3) { fprintf(stderr, "Usage: %s /shm-path string\n", argv[0]); exit(EXIT_FAILURE); } shmpath = argv[1]; string = argv[2]; len = strlen(string); if (len > BUF_SIZE) { fprintf(stderr, "String is too long\n"); exit(EXIT_FAILURE); } /* Open the existing shared memory object and map it into the caller's address space. */ fd = shm_open(shmpath, O_RDWR, 0); if (fd == -1) errExit("shm_open"); shmp = mmap(NULL, sizeof(*shmp), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (shmp == MAP_FAILED) errExit("mmap"); /* Copy data into the shared memory object. */ shmp->cnt = len; memcpy(&shmp->buf, string, len); /* Tell peer that it can now access shared memory. */ if (sem_post(&shmp->sem1) == -1) errExit("sem_post"); /* Wait until peer says that it has finished accessing the shared memory. */ if (sem_wait(&shmp->sem2) == -1) errExit("sem_wait"); /* Write modified data in shared memory to standard output. */ if (write(STDOUT_FILENO, &shmp->buf, len) == -1) errExit("write"); if (write(STDOUT_FILENO, "\n", 1) == -1) errExit("write"); exit(EXIT_SUCCESS); }close(2),fchmod(2),fchown(2),fcntl(2),fstat(2),ftruncate(2),memfd_create(2),mmap(2),open(2),umask(2),shm_overview(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17shm_open(3)Pages that refer to this page:ipcrm(1), execve(2), memfd_create(2), mmap2(2), mmap(2), truncate(2), umask(2), sem_init(3), sem_post(3), tmpfs(5), file-hierarchy(7), sem_overview(7), shm_overview(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |