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SEGATTACH(2) System Calls Manual SEGATTACH(2)
segattach, segdetach, segfree - map/unmap a segment in virtual memory
void*segattach(int attr, char *class, void *va, ulong len)
int segdetach(void *addr)
int segfree(void *va, ulong len)
Segattach creates a new memory segment, adds it to the calling
process's address space, and returns its lowest address. Segments
belong 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
implemented 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 detached when the process does an exec(2). Specific devices may
implement 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 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
implements 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 instruc‐
tion 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
implements such a test-and-set.
* MIPS test and set
TEXT tas(SB), $0
MOVW R1, sema+0(FP) /* save arg on stack */
MOVW sema+0(FP), R1
MOVB R0, 1(R1)
NOR R0, R0, R0 /* NOP */
WORD $(023<<26) /* MFC3 R0, R0 */
BLTZ R1, btas
lock(2), segbrk(2), segflush(2)
These functions set errstr. Segattach returns (void*)-1 on error.
There is a small fixed limit on the number of segments that may be
attached, as well as a maximum segment size.