harvey/sys/man/2/segattach

.TH SEGATTACH 2
.SH NAME
segattach, 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
.B
void*	segattach(int attr, char *class, void *va, ulong len)
.PP
.B
int	segdetach(void *addr)
.PP
.B
int	segfree(void *va, ulong len)
.PP
.SH DESCRIPTION
.I 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
.B memory
(plain memory)
and
.B shared
(shared memory)
are available on all systems.
.PP
Shared segments are inherited by the children of the attaching process
and remain untouched across a
.IR fork (2).
An
.IR exec (2)
will release a shared segment if it overlaps the segments
in the file being
.IR exec'ed ;
otherwise the segment will be inherited.
.PP
Some machines provide a segment class
.BR lock .
Lock segments allow access to special lock hardware provided
by some multiprocessors, in particular the SGI Power Series machines.
.PP
Systems may also provide interfaces to special hardware devices like
frame buffers through the
.I segattach
interface.
Device memory mapped by this method is typically uncached by default.
.PP
If the specified
.I class
is unknown,
.I segattach
draws an error.
.PP
.I Attr
specifies 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 implement
attributes to control caching and allocation, but these will vary
between devices.
.PP
.I Va
and
.I len
specify the position of the segment in the process's address space.
.I Va
is rounded down to the nearest page boundary and
.IB va + len
is rounded up.
The system does not permit segments to overlap.
If
.I va
is zero, the system will choose a suitable address.
.PP
.I Segdetach
removes a segment from a process's address space. Memory used by
the segment is freed.
.I Addr
may be any address within the bounds of the segment.
.PP
The system will not permit the initial stack segment to be detached
from the address space.
.PP
.I Segfree
tells 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.
.PP
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
.I segattach
a segment of class
.BR lock .
If this succeeds, the machine is an SGI Power Series and
the memory contains hardware locks.
Each 4096-byte page has 64
.B 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
.I segattach
fails, there is no hardware support but the operating system
helps:
Any
.B COP3
instruction will be trapped by the kernel and interpreted
as a test-and-set.
In the trap,
.B R1
points to a
.BR long ;
on return,
.B R1
is 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 DIAGNOSTICS
These functions set
.IR errstr .
.I Segattach
returns
.B (void*)-1
on error.
.SH BUGS
There is a small fixed limit on the number of segments that may be attached,
as well as a maximum segment size.