GNU/Linux man pages
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GNU/Linux |
RedHat 5.2(Apollo) |
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mprotect(2) |
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mprotect − control allowable accesses to a region of memory
#include <sys/mman.h>
int mprotect(const void *addr, size_t len, int prot);
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:
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PROT_NONE |
The memory cannot be accessed at all. |
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PROT_READ |
The memory can be read. |
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PROT_WRITE |
The memory can be written to. |
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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.
On success, mprotect returns zero. On error, −1 is returned, and errno is set appropriately.
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EINVAL |
addr is not a valid pointer, or not a multiple of PAGESIZE. | ||
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EFAULT |
The memory cannot be accessed. | ||
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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. | ||
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ENOMEM |
Internal kernel structures could not be allocated. |
#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);
}
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).
mmap(2)
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mprotect(2) | |