XDGShm(5X) UNIX System V XDGShm(5X)
NAME
XDGShm - The X11 Data General Shared Memory Transport
DESCRIPTION
X11 includes an implementation of shared memory transport
for client/server communication when both the client and
server are executing on the same computer. This feature
provides performance improvements of up to 100% for many X
protocol requests.
The decision of whether to use shared memory or Unix-domain
sockets is made at both the X server and client levels. This
has been facilitated by command-line options in both Xlib
and the X server, as outlined below:
X Server- By default, the server will attempt to create
and attach to a shared memory segment, and in turn
support both shared memory and socket clients. The X
server then creates a shared memory segment that is (by
default) 1044480 bytes in size. If this fails, then
only socket clients will be supported.
The user can force the server to use shared memory and
exit in case of a shared memory creation/attachment
failure by entering the command line option "+DGShm"
when initiating the server.
The user can disable shared memory support at the
server altogether via the command line option "-DGShm"
during server initiation, in which case only socket
transfers will be supported for local clients.
The shared memory segment size can be modified via the
command line option "-DGShmsize n", where n is the
number of 4096-byte pages. This value must be at least
128. The maximum value it can take depends on the
maximum shared memory segment size a given system can
support, as defined above.
Xlib- The user may specify whether or not to use shared
memory for both the standard clients as well as any
clients built with the libX11.a provided via the
-display option. Specifically, "-display unix:0" will
force the client to use Unix-domain sockets. "-display
DGShm:0" will force the client to use shared memory. If
the server does not support shared memory, then the
client will fail to open the display, and exit.
Finally, "-display :0" causes Xlib to attempt to use
shared memory. If the server does not support it, the
client will utilize Unix-domain sockets.
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