segment – long lived memory segments

bind '#g' /mnt/segment


The segment device provides a 2–level file system representing long–lived sharable segments that processes may segattach(2). The name of the directory is the class argument to segattach.

New segments are created under the top level using create (see open(2)). The DMDIR bit must be set in the permissions. Remove(2)'ing the directory makes the segment no longer available for segattach. However, the segment will continue to exist until all processes using it either exit or segdetach it.

Within each segment directory are two files, data and ctl. Reading and writing data affects the contents of the segment. Reading and writing ctl retrieves and sets the segment's properties.

There is only one control message, which sets the segment's virtual address and length in bytes:
va address length
Address is automatically rounded down to a page boundary and length is rounded up to end the segment at a page boundary. The segment will reside at the same virtual address in all processes sharing it. When the segment is attached using segattach, the address and length arguments are ignored in the call; they are defined only by the va control message. Once the address and length are set, they cannot be reset.

Reading the control file returns a message of the same format with the segment's actual start address and length.

Opening data or reading ctl before setting the virtual address yields the error ``segment not yet allocated''.

The permissions check when segattaching is equivalent to the one performed when opening data with mode ORDWR.


Create a one megabyte segment at address 0x10000000:
% bind '#g' /mnt/segment
% mkdir /mnt/segment/example
% echo 'va 0x10000000 0x100000' > /mnt/segment/example/ctl

Put the string ``hi mom'' at the start of the segment:
% echo –n hi mom > /mnt/segment/example/data

Attach the segment to a process:
ulong va;
va = segattach(0, "example", 0, 0);


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