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.\" -*- nroff -*-
.\"
.\" Copyright (C) 1995 Michael Shields <shields@tembel.org>.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and author of this work.
.\"
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Sat May 31 15:32:03 MET 1997 by Andries Brouwer <aeb@cwi.nl>
.\"
.TH MPROTECT 2 "1997-05-31" "Linux 2.0" "Linux Programmer's Manual"
.SH NAME
mprotect \- control allowable accesses to a region of memory
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.sp
\fBint mprotect(const void *\fIaddr\fB, size_t \fIlen\fB, int \fIprot\fB);
.fi
.SH DESCRIPTION
.B mprotect
controls how a section of memory may be accessed. If an access is
disallowed by the protection given it, the program receives a
.BR SIGSEGV .
.PP
.I prot
is a bitwise-or of the following values:
.TP 1.1i
.B PROT_NONE
The memory cannot be accessed at all.
.TP
.B PROT_READ
The memory can be read.
.TP
.B PROT_WRITE
The memory can be written to.
.TP
.B PROT_EXEC
The memory can contain executing code.
.PP
The new protection replaces any existing protection. For example, if the
memory had previously been marked \fBPROT_READ\fR, and \fBmprotect\fR
is then called with \fIprot\fR \fBPROT_WRITE\fR, it will no longer
be readable.
.SH RETURN VALUE
On success,
.B mprotect
returns zero. On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
\fIaddr\fR is not a valid pointer, or not a multiple of PAGESIZE.
.TP
.B EFAULT
The memory cannot be accessed.
.TP
.B EACCES
The memory cannot be given the specified access. This can happen,
for example, if you
.BR mmap (2)
a file to which you have read-only access, then ask
.B mprotect
to mark it
.BR PROT_WRITE .
.TP
.B ENOMEM
Internal kernel structures could not be allocated.
.SH EXAMPLE
.nf
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>
#include <limits.h> /* for PAGESIZE */
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif
int
main(void)
{
char *p;
char c;
/* Allocate a buffer; it will have the default
protection of PROT_READ|PROT_WRITE. */
p = malloc(1024+PAGESIZE-1);
if (!p) {
perror("Couldn't malloc(1024)");
exit(errno);
}
/* Align to a multiple of PAGESIZE, assumed to be a power of two */
p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));
c = p[666]; /* Read; ok */
p[666] = 42; /* Write; ok */
/* Mark the buffer read-only. */
if (mprotect(p, 1024, PROT_READ)) {
perror("Couldn't mprotect");
exit(errno);
}
c = p[666]; /* Read; ok */
p[666] = 42; /* Write; program dies on SIGSEGV */
exit(0);
}
.fi
.SH "CONFORMING TO"
SVr4, POSIX.1b (formerly POSIX.4). SVr4 defines an additional error
code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
POSIX.1b says that
.B mprotect
can be used only on regions of memory obtained from
.BR mmap (2).
.SH SEE ALSO
.BR mmap (2)
|
