SEGMENT(3)                                             SEGMENT(3)

     NAME
          segment - long lived memory segments

     SYNOPSIS
          bind '#g' /mnt/segment

          #g/seg1
          #g/seg1/ctl
          #g/seg1/data
          #g/seg2
          #g/seg2/ctl
          #g/seg2/data
           ...

     DESCRIPTION
          The segment device provides a 2-level file system represent-
          ing long-lived sharable segments that processes may
          segattach(2). The name of the directory is the class argu-
          ment 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 seg-
          ment.  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 type
          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.  Optionally, type can be specified as
          fixed to create a physically continuous segment with a fixed
          physical base address suitable for hardware DMA access.
          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 for-
          mat with the segment's actual start address and length. For
          fixed segments, the type and physical base address are
          appended.

     SEGMENT(3)                                             SEGMENT(3)

          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.

     EXAMPLE
          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);
          }

     SEE ALSO
          segattach(2)

     SOURCE
          /sys/src/9/port/devsegment.c