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$FreeBSD: head/lib/libc/sys/shm_open.2 140505 2005-01-20 09:17:07Z ru $
.Dd March 24, 2000 .Dt SHM_OPEN 3 .Os .Sh NAME .Nm shm_open , shm_unlink .Nd "shared memory object operations" .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/types.h n sys/mman.h .Ft int .Fn shm_open "const char *path" "int flags" "mode_t mode" .Ft int .Fn shm_unlink "const char *path" .Sh DESCRIPTION The .Fn shm_open function opens (or optionally creates) a .Tn POSIX shared memory object named .Fa path . The .Fn shm_unlink function removes a shared memory object named .Fa path .
p In the .Fx implementation, .Tn POSIX shared memory objects are implemented as ordinary files. The .Fn shm_open and .Fn shm_unlink act as wrappers around the .Xr open 2 and .Xr unlink 2 routines, and .Fa path , .Fa flags , and .Fa mode arguments are as specified for those functions. The .Fa flags argument is checked to ensure that the access mode specified is not .Dv O_WRONLY (which is not defined for shared memory objects).
p In addition, the .Fx implementation causes .Fn mmap of a descriptor returned by .Fn shm_open to behave as if the .Dv MAP_NOSYNC flag had been specified to .Xr mmap 2 . (It does so by setting a special file flag using .Xr fcntl 2 . )
p The .Fn shm_unlink function makes no effort to ensure that .Fa path refers to a shared memory object. .Sh RETURN VALUES If successful, .Fn shm_open returns a non-negative integer; .Fn shm_unlink returns zero. Both functions return -1 on failure, and set .Va errno to indicate the error. .Sh COMPATIBILITY The .Fa path argument does not necessarily represent a pathname (although it does in this and most other implementations). Two processes opening the same .Fa path are guaranteed to access the same shared memory object if and only if .Fa path begins with a slash
q Ql / character.
p Only the .Dv O_RDONLY , .Dv O_RDWR , .Dv O_CREAT , .Dv O_EXCL , and .Dv O_TRUNC flags may be used in portable programs.
p The result of using .Xr open 2 , .Xr read 2 , or .Xr write 2 on a shared memory object, or on the descriptor returned by .Fn shm_open , is undefined. It is also undefined whether the shared memory object itself, or its contents, persist across reboots. .Sh ERRORS The .Fn shm_open and .Fn shm_unlink functions can fail with any error defined for .Fn open and .Fn unlink , respectively. In addition, the following errors are defined for .Fn shm_open : l -tag -width Er t Bq Er EINVAL The object named by .Fa path is not a shared memory object (i.e., it is not a regular file). t Bq Er EINVAL The .Fa flags argument to .Fn shm_open specifies an access mode of .Dv O_WRONLY . .El .Sh SEE ALSO .Xr mmap 2 , .Xr munmap 2 , .Xr open 2 , .Xr unlink 2 .Sh STANDARDS The .Fn shm_open and .Fn shm_unlink functions are believed to conform to .St -p1003.1b-93 . .Sh HISTORY The .Fn shm_open and .Fn shm_unlink functions first appeared in .Fx 4.3 . .Sh AUTHORS .An Garrett A. Wollman Aq wollman@FreeBSD.org (C library support and this manual page)
p .An Matthew Dillon Aq dillon@FreeBSD.org
q Dv MAP_NOSYNC