mprotect.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)mprotect.2	8.1	9/11/90
.TH mprotect 2
.\" Created by depp -- 14 Mar 86
.\"
.SH Name
mprotect \- memory protection control
.SH Syntax
.nf
.ft B
#include <sys/mman.h>
#include <sys/types.h>
.PP
.ft B
int mprotect (addr, len, prot)
caddr_t addr;
int len, prot;
.fi
.SH Description
.NXR "mprotect system call"
.NXR "data memory" "changing protection"
The
.PN mprotect
system call
changes the protection of portions of
an application program's data memory.
Protection is performed on page cluster boundaries.
The default protection for data memory on process invocation is user
READ/WRITE.
The
.I addr
argument is the beginning address
of the data block and must fall on a page cluster
boundary.
.PP
The 
.I len
argument is the length of the data block, in bytes.
The length of the block is rounded up to a cluster boundary, and the 
size of the block to be protected is returned.
.PP
The
.I prot
argument is the requested protection for the block of memory.
Protection values affect only the user process.
Protection values are defined in <mman.h> as:
.EX 0
/* protections are chosen from these bits, ORed together */
#define PROT_READ       0x1     /* pages can be read */
#define PROT_WRITE      0x2     /* pages can be written */
#define PROT_EXEC       0x4     /* pages can be executed */
.EE
Setting the
.I prot
argument to zero (0)
indicates that the process cannot reference the memory
block, without causing a fault.
.PP
A protected page faults if the protection is violated, and a SIGBUS
signal is issued.
If the process has a handler defined for the SIGBUS signal, the 
.I code
parameter, described in 
.MS sigvec 2
and
.MS signal 3 ,
is used to pass in the virtual address that faulted.
.SH Restrictions
The page cluster size may change in future versions of ULTRIX. 
As a result, 
.PN getpagesize 
should be used to determine the correct
.I len
argument, and 
.PN sbrk 
or
.PN malloc 
should be used to determine the correct
.I addr
argument.
.PP
If the user handles a SIGBUS signal, the signal handler must either
abort the process or correct the condition that caused the protection 
fault (SIGBUS).
If some corrective action is not taken, an infinite loop results
because the faulting instruction is restarted.
If the user permits the default SIGBUS handler to be used, the process
aborts if a referenced page causes a fault.
.PP
The VAX architecture makes the following implications;
PROT_WRITE implies
(PROT_WRITE | PROT_READ | PROT_EXEC), and PROT_READ implies
(PROT_READ | PROT_EXEC).
.PP
Only the application can change the 
.PN mprotect
call's 
private data space.  This means that attempts to change text, shared 
memory, or stack space causes a EACCES failure.
.SH Return Values
Upon successful completion,
the size of the protected memory block, in bytes,
is returned.
Otherwise, a value of \-1 is returned and 
.I errno
is set to indicate the error.
.SH Diagnostics
The
.PN mprotect
call fails under the following conditions:
.TP 15
[EALIGN]
The
.I addr
argument is not on a cluster boundary.
.TP 15
[EINVAL]
The
.I prot
argument is not a valid protection mask.
.TP 15
[EACCES]
The memory block is not fully contained within private data space.
.SH See Also
getpagesize(2), sbrk(2), sigvec(2), malloc(3), signal(3)