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

#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)

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 memory) 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 particular 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 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 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. 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.
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