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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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mprotect(2) System Calls Manual mprotect(2)
mprotect, pkey_mprotect - set protection on a region of memory
Standard C library (libc, -lc)
#include <sys/mman.h>
int mprotect(size_t size;
void addr[size], size_t size, int prot);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
int pkey_mprotect(size_t size;
void addr[size], size_t size, int prot, int pkey);
mprotect() changes the access protections for the calling
process's memory pages containing any part of the address range in
the interval [addr, addr+size-1]. addr must be aligned to a page
boundary.
If the calling process tries to access memory in a manner that
violates the protections, then the kernel generates a SIGSEGV
signal for the process.
prot is a combination of the following access flags: PROT_NONE or
a bitwise OR of the other values in the following list:
PROT_NONE
The memory cannot be accessed at all.
PROT_READ
The memory can be read.
PROT_WRITE
The memory can be modified.
PROT_EXEC
The memory can be executed.
PROT_SEM (since Linux 2.5.7)
The memory can be used for atomic operations. This flag
was introduced as part of the futex(2) implementation (in
order to guarantee the ability to perform atomic operations
required by commands such as FUTEX_WAIT), but is not
currently used in on any architecture.
PROT_SAO (since Linux 2.6.26)
The memory should have strong access ordering. This
feature is specific to the PowerPC architecture (version
2.06 of the architecture specification adds the SAO CPU
feature, and it is available on POWER 7 or PowerPC A2, for
example).
Additionally (since Linux 2.6.0), prot can have one of the
following flags set:
PROT_GROWSUP
Apply the protection mode up to the end of a mapping that
grows upwards. (Such mappings are created for the stack
area on architectures—for example, HP-PARISC—that have an
upwardly growing stack.)
PROT_GROWSDOWN
Apply the protection mode down to the beginning of a
mapping that grows downward (which should be a stack
segment or a segment mapped with the MAP_GROWSDOWN flag
set).
Like mprotect(), pkey_mprotect() changes the protection on the
pages specified by addr and size. The pkey argument specifies the
protection key (see pkeys(7)) to assign to the memory. The
protection key must be allocated with pkey_alloc(2) before it is
passed to pkey_mprotect(). For an example of the use of this
system call, see pkeys(7).
On success, mprotect() and pkey_mprotect() return zero. On error,
these system calls return -1, and errno is set to indicate the
error.
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.
EINVAL addr is not a valid pointer, or not a multiple of the
system page size.
EINVAL (pkey_mprotect()) pkey has not been allocated with
pkey_alloc(2)
EINVAL Both PROT_GROWSUP and PROT_GROWSDOWN were specified in
prot.
EINVAL Invalid flags specified in prot.
EINVAL (PowerPC architecture) PROT_SAO was specified in prot, but
SAO hardware feature is not available.
ENOMEM Internal kernel structures could not be allocated.
ENOMEM Addresses in the range [addr, addr+size-1] are invalid for
the address space of the process, or specify one or more
pages that are not mapped. (Before Linux 2.4.19, the error
EFAULT was incorrectly produced for these cases.)
ENOMEM Changing the protection of a memory region would result in
the total number of mappings with distinct attributes
(e.g., read versus read/write protection) exceeding the
allowed maximum. (For example, making the protection of a
range PROT_READ in the middle of a region currently
protected as PROT_READ|PROT_WRITE would result in three
mappings: two read/write mappings at each end and a read-
only mapping in the middle.)
POSIX says that the behavior of mprotect() is unspecified if it is
applied to a region of memory that was not obtained via mmap(2).
On Linux, it is always permissible to call mprotect() on any
address in a process's address space (except for the kernel
vsyscall area). In particular, it can be used to change existing
code mappings to be writable.
Whether PROT_EXEC has any effect different from PROT_READ depends
on processor architecture, kernel version, and process state. If
READ_IMPLIES_EXEC is set in the process's personality flags (see
personality(2)), specifying PROT_READ will implicitly add
PROT_EXEC.
On some hardware architectures (e.g., i386), PROT_WRITE implies
PROT_READ.
POSIX.1 says that an implementation may permit access other than
that specified in prot, but at a minimum can allow write access
only if PROT_WRITE has been set, and must not allow any access if
PROT_NONE has been set.
Applications should be careful when mixing use of mprotect() and
pkey_mprotect(). On x86, when mprotect() is used with prot set to
PROT_EXEC a pkey may be allocated and set on the memory implicitly
by the kernel, but only when the pkey was 0 previously.
On systems that do not support protection keys in hardware,
pkey_mprotect() may still be used, but pkey must be set to -1.
When called this way, the operation of pkey_mprotect() is
equivalent to mprotect().
mprotect()
POSIX.1-2008.
pkey_mprotect()
Linux.
mprotect()
POSIX.1-2001, SVr4.
pkey_mprotect()
Linux 4.9, glibc 2.27.
The program below demonstrates the use of mprotect(). The program
allocates four pages of memory, makes the third of these pages
read-only, and then executes a loop that walks upward through the
allocated region modifying bytes.
An example of what we might see when running the program is the
following:
$ ./a.out
Start of region: 0x804c000
Got SIGSEGV at address: 0x804e000
Program source
#include <malloc.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
static char *buffer;
static void
handler(int sig, siginfo_t *si, void *unused)
{
/* Note: calling printf() from a signal handler is not safe
(and should not be done in production programs), since
printf() is not async-signal-safe; see signal-safety(7).
Nevertheless, we use printf() here as a simple way of
showing that the handler was called. */
printf("Got SIGSEGV at address: %p\n", si->si_addr);
exit(EXIT_FAILURE);
}
int
main(void)
{
int pagesize;
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sa.sa_sigaction = handler;
if (sigaction(SIGSEGV, &sa, NULL) == -1)
handle_error("sigaction");
pagesize = sysconf(_SC_PAGE_SIZE);
if (pagesize == -1)
handle_error("sysconf");
/* Allocate a buffer aligned on a page boundary;
initial protection is PROT_READ | PROT_WRITE. */
buffer = memalign(pagesize, 4 * pagesize);
if (buffer == NULL)
handle_error("memalign");
printf("Start of region: %p\n", buffer);
if (mprotect(buffer + pagesize * 2, pagesize,
PROT_READ) == -1)
handle_error("mprotect");
for (char *p = buffer ; ; )
*(p++) = 'a';
printf("Loop completed\n"); /* Should never happen */
exit(EXIT_SUCCESS);
}
mmap(2), sysconf(3), pkeys(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-06-28 mprotect(2)
Pages that refer to this page: clone(2), madvise(2), mmap(2), pkey_alloc(2), PR_SET_MM_START_CODE(2const), remap_file_pages(2), seccomp(2), sigaction(2), subpage_prot(2), syscalls(2), pthread_attr_setguardsize(3), pthread_attr_setstack(3), systemd.exec(5), pkeys(7), shm_overview(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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