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SEGATTACH(2)                  System Calls Manual                 SEGATTACH(2)



NAME
       segattach, segdetach, segfree - map/unmap a segment in virtual memory

SYNOPSIS
       #include <u.h>
       #include <libc.h>

       void*segattach(int attr, char *class, void *va, ulong len)

       int  segdetach(void *addr)

       int  segfree(void *va, ulong len)

DESCRIPTION
       Segattach  creates  a  new  memory  segment,  adds  it  to  the calling
       process's address space, and returns its lowest address.  Segments  be‐
       long  to  system-dependent classes.  Segment classes memory (plain mem‐
       ory) and shared (shared memory) are available on all systems.

       Shared segments are inherited by the children of the attaching  process
       and  remain  untouched  across  a  fork(2).   An exec(2) will release a
       shared segment if it overlaps the segments in the file  being  exec'ed;
       otherwise the segment will be inherited.

       Some machines provide a segment class lock.  Lock segments allow access
       to special lock hardware provided by some multiprocessors, in  particu‐
       lar the SGI Power Series machines.

       Systems  may  also  provide interfaces to special hardware devices like
       frame buffers through the segattach interface.  Device memory mapped by
       this method is typically uncached by default.

       If the specified class is unknown, segattach draws an error.

       Attr  specifies  the new segment's attributes.  The only attributes im‐
       plemented on all classes of segment is SG_RONLY, which allows only read
       access on the segment, and SG_CEXEC, which causes the segment to be de‐
       tached when the process does an exec(2).  Specific devices  may  imple‐
       ment  attributes to control caching and allocation, but these will vary
       between devices.

       Va and len specify the position of the segment in the process's address
       space.   Va  is rounded down to the nearest page boundary and va+len is
       rounded up.  The system does not permit segments to overlap.  If va  is
       zero, the system will choose a suitable address.

       Segdetach removes a segment from a process's address space. Memory used
       by the segment is freed.  Addr may be any address within the bounds  of
       the segment.

       The  system  will  not  permit the initial stack segment to be detached
       from the address space.

       Segfree tells the system that it may free any  physical  memory  within
       the span [va, va+len), but leaves that portion of the process's address
       space valid.  The system will not free any memory  outside  that  span,
       and  may  not  free all or even any of the specified memory.  If free'd
       memory is later referenced, it will be initialized as  appropriate  for
       the segment type.  For example data and text segments will be read from
       the executable file, and bss segments will be filled with zero bytes.

       The MIPS R2000 and R3000 have no  hardware  instructions  to  implement
       locks.   The  following method can be used to build them from software.
       First, try to segattach a segment of class lock.  If this succeeds, the
       machine  is an SGI Power Series and the memory contains hardware locks.
       Each 4096-byte page has 64 long words at its beginning; each  word  im‐
       plements a test-and-set semaphore when read; the low bit of the word is
       zero on success, one on failure.  If the segattach fails, there  is  no
       hardware  support  but the operating system helps: Any COP3 instruction
       will be trapped by the kernel and interpreted as  a  test-and-set.   In
       the  trap,  R1 points to a long; on return, R1 is greater or equal zero
       on success, negative on failure.  The following assembly  language  im‐
       plements such a test-and-set.

              /*
               *      MIPS test and set
               */
                      TEXT    tas(SB), $0
                      MOVW    R1, sema+0(FP)  /* save arg on stack */
              btas:
                      MOVW    sema+0(FP), R1
                      MOVB    R0, 1(R1)
                      NOR     R0, R0, R0      /* NOP */
                      WORD    $(023<<26)      /* MFC3 R0, R0 */
                      BLTZ    R1, btas
                      RET

SOURCE
       /sys/src/libc/9syscall

SEE ALSO
       lock(2), segbrk(2), segflush(2)
       /proc/*/segment

DIAGNOSTICS
       These functions set errstr.  Segattach returns (void*)-1 on error.

BUGS
       There  is a small fixed limit on the number of segments that may be at‐
       tached, as well as a maximum segment size.



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