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.TH SEGATTACH 2
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.SH NAME
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segattach, segdetach, segfree \- map/unmap a segment in virtual memory
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.PP
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.ta \w'\fLlong 'u
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.B
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void* segattach(int attr, char *class, void *va, ulong len)
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.PP
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.B
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int segdetach(void *addr)
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.PP
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.B
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int segfree(void *va, ulong len)
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.PP
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.SH DESCRIPTION
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.I Segattach
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creates a new memory segment, adds it
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to the calling process's address space, and returns its lowest address.
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Segments belong to system-dependent classes.
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Segment classes
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.B memory
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(plain memory)
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and
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.B shared
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(shared memory)
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are available on all systems.
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.PP
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Shared segments are inherited by the children of the attaching process
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and remain untouched across a
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.IR fork (2).
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An
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.IR exec (2)
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will release a shared segment if it overlaps the segments
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in the file being
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.IR exec'ed ;
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otherwise the segment will be inherited.
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.PP
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Some machines provide a segment class
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.BR lock .
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Lock segments allow access to special lock hardware provided
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by some multiprocessors, in particular the SGI Power Series machines.
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.PP
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Systems may also provide interfaces to special hardware devices like
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frame buffers through the
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.I segattach
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interface.
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Device memory mapped by this method is typically uncached by default.
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.PP
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If the specified
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.I class
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is unknown,
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.I segattach
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draws an error.
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.PP
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.I Attr
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specifies the new segment's attributes.
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The only attributes implemented on all classes of segment is
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.BR SG_RONLY ,
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which allows only read access on the segment, and
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.BR SG_CEXEC ,
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which causes the segment to be detached when the process does an
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.IR exec (2).
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Specific devices may implement
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attributes to control caching and allocation, but these will vary
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between devices.
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.PP
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.I Va
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and
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.I len
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specify the position of the segment in the process's address space.
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.I Va
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is rounded down to the nearest page boundary and
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.IB va + len
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is rounded up.
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The system does not permit segments to overlap.
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If
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.I va
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is zero, the system will choose a suitable address.
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.PP
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.I Segdetach
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removes a segment from a process's address space. Memory used by
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the segment is freed.
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.I Addr
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may be any address within the bounds of the segment.
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.PP
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The system will not permit the initial stack segment to be detached
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from the address space.
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.PP
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.I Segfree
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tells the system that it may free any physical memory within the span
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.RI [ va ,
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.IR va+len ),
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but leaves that portion of the process's address space valid.
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The system will not free any memory outside that span,
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and may not free all or even any of the specified memory.
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If free'd memory is later referenced,
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it will be initialized as appropriate for the segment type.
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For example data and text segments will be read from the executable file,
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and bss segments will be filled with zero bytes.
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.PP
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The MIPS R2000 and R3000 have no hardware instructions
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to implement locks. The following method can be used
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to build them from software.
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First, try to
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.I segattach
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a segment of class
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.BR lock .
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If this succeeds, the machine is an SGI Power Series and
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the memory contains hardware locks.
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Each 4096-byte page has 64
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.B long
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words at its beginning; each word implements
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a test-and-set semaphore when read; the low bit of the word
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is zero on success, one on failure.
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If the
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.I segattach
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fails, there is no hardware support but the operating system
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helps:
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Any
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.B COP3
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instruction will be trapped by the kernel and interpreted
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as a test-and-set.
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In the trap,
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.B R1
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points to a
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.BR long ;
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on return,
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.B R1
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is greater or equal zero on success, negative on failure.
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The following assembly language implements such a test-and-set.
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.IP
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.EX
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.ta 8n +8n +8n +8n +8n +8n +8n
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/*
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* MIPS test and set
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*/
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TEXT tas(SB), $0
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MOVW R1, sema+0(FP) /* save arg on stack */
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btas:
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MOVW sema+0(FP), R1
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MOVB R0, 1(R1)
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NOR R0, R0, R0 /* NOP */
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WORD $(023<<26) /* MFC3 R0, R0 */
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BLTZ R1, btas
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RET
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.EE
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.SH SOURCE
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.B /sys/src/libc/9syscall
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.SH SEE ALSO
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.IR lock (2),
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.IR segbrk (2),
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.IR segflush (2)
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.br
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.BR /proc/*/segment
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.SH DIAGNOSTICS
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These functions set
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.IR errstr .
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.I Segattach
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returns
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.B (void*)-1
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on error.
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.SH BUGS
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There is a small fixed limit on the number of segments that may be attached,
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as well as a maximum segment size.
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