SEGATTACH(2) System Calls Manual SEGATTACH(2)
segattach, segdetach, segfree - map/unmap a segment in virtual memory
long 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 (as an integer).
Segments belong to system-dependent classes. Segment classes memory (plain memory) and shared (shared memory) are available on all sys-
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 par-
ticular 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 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 text and stack segments to be detached from the address space.
Segfree allows specific areas of a segment's memory to be returned to the system, but leaves that portion of the process's address space
valid, to be reconnected to initialized memory of the appropriate type if addressed again. Va and len are interpreted as in segattach but
need not refer to the entire segment.
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
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 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.
The return type of segattach is peculiar. Also, segattach returns -1 on error; beware that on some systems other negative values might be
There is a small fixed limit on the number of segments that may be attached, as well as a maximum segment size.