MPROTECT (2)
control allowable accesses to a region of memory
SYNOPSIS
#include <sys/mman.h>
int mprotect(const void *addr, size_t len, int prot);
DESCRIPTION
mprotect
controls how a section of memory may be accessed. If an access is
disallowed by the protection given it, the program receives a
SIGSEGV .
prot
is a bitwise-or of the following values:
PROT_NONE
The memory cannot be accessed at all.
PROT_READ
PROT_WRITE
The memory can be written to.
PROT_EXEC
The memory can contain executing code.
The new protection replaces any existing protection. For example, if the
memory had previously been marked PROT_READ, and mprotect
is then called with prot PROT_WRITE, it will no longer
be readable.
RETURN VALUE
On success,
mprotect
returns zero. On error, -1 is returned, and
errno
is set appropriately.
ERRORS
EINVAL
addr is not a valid pointer, or not a multiple of PAGESIZE.
EFAULT
The memory cannot be accessed.
EACCES
The memory cannot be given the specified access. This can happen,
for example, if you
mmap (2)
a file to which you have read-only access, then ask
mprotect
to mark it
PROT_WRITE .
ENOMEM
Internal kernel structures could not be allocated.
EXAMPLE
#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);
}
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
mprotect
can be used only on regions of memory obtained from
mmap (2).
SEE ALSO
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