.TH SEGATTACH 2.SH NAMEsegattach, segdetach, segfree \- map/unmap a segment in virtual memory.SH SYNOPSIS.B #include <u.h>.br.B #include <libc.h>.PP.ta \w'\fLlong 'u.Bvoid*	segattach(int attr, char *class, void *va, ulong len).PP.Bint	segdetach(void *addr).PP.Bint	segfree(void *va, ulong len).PP.SH DESCRIPTION.I Segattachcreates a new memory segment, adds itto the calling process's address space, and returns its lowest address.Segments belong to system-dependent classes.Segment classes.B memory(plain memory)and.B shared(shared memory)are available on all systems..PPShared segments are inherited by the children of the attaching processand remain untouched across a.IR fork (2).An.IR exec (2)will release a shared segment if it overlaps the segmentsin the file being.IR exec'ed ;otherwise the segment will be inherited..PPSome machines provide a segment class.BR lock .Lock segments allow access to special lock hardware providedby some multiprocessors, in particular the SGI Power Series machines..PPSystems may also provide interfaces to special hardware devices likeframe buffers through the.I segattachinterface.Device memory mapped by this method is typically uncached by default..PPIf the specified.I classis unknown,.I segattachdraws an error..PP.I Attrspecifies the new segment's attributes.The only attributes implemented on all classes of segment is.BR SG_RONLY ,which allows only read access on the segment, and.BR SG_CEXEC ,which causes the segment to be detached when the process does an.IR exec (2).Specific devices may implementattributes to control caching and allocation, but these will varybetween devices..PP.I Vaand.I lenspecify the position of the segment in the process's address space..I Vais rounded down to the nearest page boundary and.IB va + lenis rounded up.The system does not permit segments to overlap.If.I vais zero, the system will choose a suitable address..PP.I Segdetachremoves a segment from a process's address space. Memory used bythe segment is freed..I Addrmay be any address within the bounds of the segment..PPThe system will not permit the initial stack segment to be detachedfrom the address space..PP.I Segfreetells the system that it may free any physical memory within the span.RI [ va ,.IR 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..PPThe MIPS R2000 and R3000 have no hardware instructionsto implement locks.  The following method can be usedto build them from software.First, try to.I segattacha segment of class.BR lock .If this succeeds, the machine is an SGI Power Series andthe memory contains hardware locks.Each 4096-byte page has 64.B longwords at its beginning; each word implementsa test-and-set semaphore when read; the low bit of the wordis zero on success, one on failure.If the.I segattachfails, there is no hardware support but the operating systemhelps:Any.B COP3instruction will be trapped by the kernel and interpretedas a test-and-set.In the trap,.B R1points to a.BR long ;on return,.B R1is greater or equal zero on success, negative on failure.The following assembly language implements such a test-and-set..IP.EX.ta 8n +8n +8n +8n +8n +8n +8n/* *	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.EE.SH SOURCE.B /sys/src/libc/9syscall.SH SEE ALSO.IR lock (2),.IR segbrk (2),.IR segflush (2).br.BR /proc/*/segment.SH DIAGNOSTICSThese functions set.IR errstr ..I Segattachreturns.B (void*)-1on error..SH BUGSThere is a small fixed limit on the number of segments that may be attached,as well as a maximum segment size.