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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
mprotect(2) System Calls Manualmprotect(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;voidaddr[size], size_tsize, intprot);#define _GNU_SOURCE/* See feature_test_macros(7) */#include <sys/mman.h>int pkey_mprotect(size_t size;voidaddr[size], size_tsize, intprot, intpkey);
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 aSIGSEGV signal for the process.prot is a combination of the following access flags:PROT_NONEor 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 thefutex(2) implementation (in order to guarantee the ability to perform atomic operations required by commands such asFUTEX_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 theMAP_GROWSDOWNflag set). Likemprotect(),pkey_mprotect() changes the protection on the pages specified byaddr andsize. Thepkey argument specifies the protection key (seepkeys(7)) to assign to the memory. The protection key must be allocated withpkey_alloc(2) before it is passed topkey_mprotect(). For an example of the use of this system call, seepkeys(7).
On success,mprotect() andpkey_mprotect() return zero. On error, these system calls return -1, anderrno is set to indicate the error.
EACCESThe memory cannot be given the specified access. This can happen, for example, if yoummap(2) a file to which you have read-only access, then askmprotect() to mark itPROT_WRITE.EINVALaddr is not a valid pointer, or not a multiple of the system page size.EINVAL(pkey_mprotect())pkey has not been allocated withpkey_alloc(2)EINVALBothPROT_GROWSUPandPROT_GROWSDOWNwere specified inprot.EINVALInvalid flags specified inprot.EINVAL(PowerPC architecture)PROT_SAOwas specified inprot, but SAO hardware feature is not available.ENOMEMInternal kernel structures could not be allocated.ENOMEMAddresses 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 errorEFAULTwas incorrectly produced for these cases.)ENOMEMChanging 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 rangePROT_READin the middle of a region currently protected asPROT_READ|PROT_WRITEwould result in three mappings: two read/write mappings at each end and a read- only mapping in the middle.)
POSIX says that the behavior ofmprotect() is unspecified if it is applied to a region of memory that was not obtained viammap(2). On Linux, it is always permissible to callmprotect() 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. WhetherPROT_EXEChas any effect different fromPROT_READdepends on processor architecture, kernel version, and process state. IfREAD_IMPLIES_EXECis set in the process's personality flags (seepersonality(2)), specifyingPROT_READwill implicitly addPROT_EXEC. On some hardware architectures (e.g., i386),PROT_WRITEimpliesPROT_READ. POSIX.1 says that an implementation may permit access other than that specified inprot, but at a minimum can allow write access only ifPROT_WRITEhas been set, and must not allow any access ifPROT_NONEhas been set. Applications should be careful when mixing use ofmprotect() andpkey_mprotect(). On x86, whenmprotect() is used withprot set toPROT_EXECa 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, butpkey must be set to -1. When called this way, the operation ofpkey_mprotect() is equivalent tomprotect().
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 ofmprotect(). 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: 0x804e000Program 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 theman-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 fetched from ⟨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- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28mprotect(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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |