Content-type: text/html Man page of shm_open


Section: C Library Functions (3)
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shm_open - Open a shared memory object, creating the object if necessary (P1003.1b)  


#include <sys/mman.h>

int shm_open (
const char *name,
int oflag,
mode_t mode);  


Points to a name of the shared memory object.
Specifies flags that define file status and file access modes. This argument is constructed from the bitwise inclusive OR of zero or more of the flags defined in the <fcntl.h> header file.
Specifies the shared memory object's permission bits. This argument is used only when the shared memory object is created.


The shm_open function establishes a connection between a shared memory object and a file descriptor. It creates an open file descriptor that refers to the shared memory object and a file descriptor that refers to that open file descriptor. This file descriptor is used by other functions to refer to the shared memory object. The name points to a string naming a shared memory object. The name can be a pathname, in which case other processes referring to the same pathname refer to the same shared memory object.

Once a shared memory object is created, its state and all data associated with it persist until the shared memory is unlinked.

The shm_open function returns a file descriptor that is the lowest numbered file descriptor not currently open for that process. File status flags and access modes are set according to the oflag argument. These flags are defined in the <fcntl.h> header file and can have zero or more of the following values:

O_RDONLY - Open for read access only.
O_RDWR - Open for read or write access.
O_CREAT - Create the shared memory if the memory object does not exist already. The user ID and group ID of the shared memory object are identical to those of the calling process. The shared memory object's permission bits are set to the value of mode, except those set in the file mode creation mask of the process.
O_EXCL - Prevent the opening of a shared memory object if O_CREAT is set and the shared memory object already exists. Use this flag only in combination with O_CREAT.
O_TRUNC - Truncate the shared memory object to zero length if it is successfully opened for read or write access (O_RDWR).

The initial contents of the shared memory object are binary zeros.  


On a successful call to shm_open a nonnegative integer is returned that represents the lowest numbered unused file descriptor. The file descriptor points to the shared memory object. Otherwise, -1 is returned and errno is set to indicate the error.  


The shm_open function fails under the following conditions:

Permission to create the shared memory object is denied. The shared memory object exists and the permissions specified by oflag are denied. O_TRUNC is specified and write permission is denied.
O_CREAT and O_EXCL are set, but the named shared memory object already exists.
A signal has interrupted the shm_open operation.
The shm_open operation is not supported for the given name.
Too many file descriptors are currently in use by this process.
The length of the path argument exceeds PATH_MAX, or a pathname component is longer than NAME_MAX while _POSIX_NO_TRUNC is in effect.
Too many shared memory objects are currently open in the system.
O_CREAT is not set and the named shared memory object does not exist.
Memory space for creation of the new shared memory object is insufficient.


Functions: close(2), dup(2), exec(2), fcntl(2), fstat(2), mmap(2), umask(2), shm_unlink(3) delim off




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Time: 02:41:05 GMT, October 02, 2010