XDGShm(5X) X11 5.4R3.00 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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