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Dd December 19, 2013
Dt SHM_OPEN 3
Os
Sh NAME
Nm shm_open , shm_unlink
Nd "shared memory object operations"
Sh LIBRARY
Lb librt
Sh SYNOPSIS
In sys/types.h
In sys/mman.h
In fcntl.h
Ft int
Fn shm_open "const char *name" "int flags" "mode_t mode"
Ft int
Fn shm_unlink "const char *name"
Sh DESCRIPTION
The
Fn shm_open
function creates or opens a
Tn POSIX
shared memory object named
Fa name .
The
Fa flags
argument contains a subset of the flags used by
Xr open 2 .
An access mode of either
Dv O_RDONLY
or
Dv O_RDWR
must be included in
Fa flags .
The optional flags
Dv O_CREAT ,
Dv O_EXCL ,
and
Dv O_TRUNC
may also be specified.
Pp
If
Dv O_CREAT
is specified,
then a new shared memory object named
Fa name
will be created if it does not exist.
In this case,
the shared memory object is created with mode
Fa mode
subject to the process' umask value.
If both the
Dv O_CREAT
and
Dv O_EXCL
flags are specified and a shared memory object named
Fa path
already exists,
then
Fn shm_open
will fail with
Er EEXIST .
Pp
Newly created objects start off with a size of zero.
If an existing shared memory object is opened with
Dv O_RDWR
and the
Dv O_TRUNC
flag is specified,
then the shared memory object will be truncated to a size of zero.
The size of the object can be adjusted via
Xr ftruncate 2
and queried via
Xr fstat 2 .
Pp
The new descriptor is set to close during
Xr execve 2
system calls;
see
Xr close 2
and
Xr fcntl 2 .
Pp
The
Fn shm_unlink
system call removes a shared memory object named
Fa path .
Sh RETURN VALUES
If successful,
Fn shm_open
returns a non-negative integer,
and
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
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
Pq Ql \&/
character.
Pp
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.
Pp
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 following errors are defined for
Fn shm_open :
Bl -tag -width Er
It Bq Er EACCES
The required permissions (for reading or reading and writing) are denied.
It Bq Er EEXIST
Dv O_CREAT
and
Dv O_EXCL
are specified and the named shared memory object does exist.
It Bq Er EFAULT
The
Fa path
argument points outside the process' allocated address space.
It Bq Er EINVAL
A flag other than
Dv O_RDONLY ,
Dv O_RDWR ,
Dv O_CREAT ,
Dv O_EXCL ,
or
Dv O_TRUNC
was included in
Fa flags ;
or the
Fa path
does not begin with a slash
Pq Ql \&/
character.
It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
It Bq Er ENAMETOOLONG
The entire pathname exceeded
Brq Dv PATH_MAX
characters.
It Bq Er ENFILE
The system file table is full.
It Bq Er ENOENT
Dv O_CREAT
is specified and the named shared memory object does not exist.
It Bq Er ENOTSUP
Not supported, most likely due to missing or incorrect
Pa /var/shm
mount.
El
Pp
The following errors are defined for
Fn shm_unlink :
Bl -tag -width Er
It Bq Er EACCES
The required permissions are denied.
Fn shm_unlink
requires write permission to the shared memory object.
It Bq Er EFAULT
The
Fa path
argument points outside the process' allocated address space.
It Bq Er ENAMETOOLONG
The entire pathname exceeded
Brq Dv PATH_MAX
characters.
It Bq Er ENOENT
The named shared memory object does not exist.
El
Sh SEE ALSO
Xr close 2 ,
Xr fstat 2 ,
Xr ftruncate 2 ,
Xr mmap 2 ,
Xr munmap 2
Sh STANDARDS
The
Fn shm_open
and
Fn shm_unlink
functions are expected to conform to
St -p1003.1b-93 .
Sh HISTORY
These functions first appeared in
Nx 7.0 .

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