SEGATTACH(2) 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 call-
ing 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 attach-
ing 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 mul-
tiprocessors, 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
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SEGATTACH(2) SEGATTACH(2)
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 mem-
ory 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 ref-
erenced, it will be initialized as appropriate for the seg-
ment 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 oper-
ating 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 */
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)
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SEGATTACH(2) SEGATTACH(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 attached, as well as a maximum segment size.
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