man-pages/man2/mprotect.2

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.\" Copyright (C) 1995 Michael Shields <shields@tembel.org>.
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.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1997-05-31 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2003-08-24 by Andries Brouwer <aeb@cwi.nl>
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.\"
.TH MPROTECT 2 2003-08-24 "Linux 2.4" "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
The function
.BR mprotect ()
specifies the desired protection for the memory page(s) containing
part or all of the interval [\fIaddr\fP,\fIaddr\fP+\fIlen\fP\-1].
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.
.\" FIXME
.\" Document MAP_GROWSUP and MAP_GROWSDOWN
.PP
The new protection replaces any existing protection.
For example, if the
memory had previously been marked \fBPROT_READ\fR, and
.BR mprotect ()
is then called with \fIprot\fR \fBPROT_WRITE\fR, it will no longer
be readable.
.SH "RETURN VALUE"
On success,
.BR mprotect ()
returns zero.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.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
.BR mprotect ()
to mark it
.BR PROT_WRITE .
.TP
.B EFAULT
The memory cannot be accessed.
.TP
.B EINVAL
\fIaddr\fR is not a valid pointer, or not a multiple of PAGESIZE.
.TP
.B ENOMEM
Internal kernel structures could not be allocated.
Or: addresses  in the range
.RI [ addr ,
.IR addr + len ]
are invalid for the address space of the process,
or specify one or more pages that are not mapped.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001.
.\" SVr4 defines an additional error
.\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
POSIX says that
.BR mprotect ()
can be used only on regions of memory obtained from
.BR mmap (2).
.SH NOTES
On Linux it is always legal to call
.BR mprotect ()
on any address in a process' address space (except for the
kernel vsyscall area).
In particular it can be used
to change existing code mappings to be writable.

Whether
.B PROT_EXEC
has any effect different from
.B PROT_READ
is architecture and kernel version dependent.
.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(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR mmap (2)