.\" -*- nroff -*- .\" Copyright (C) 2007 Michael Kerrisk .\" and Copyright (C) 1995 Michael Shields . .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and author of this work. .\" .\" Modified 1996-10-22 by Eric S. Raymond .\" Modified 1997-05-31 by Andries Brouwer .\" Modified 2003-08-24 by Andries Brouwer .\" Modified 2004-08-16 by Andi Kleen .\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and .\" a much improved example program. .\" .TH MPROTECT 2 2012-08-14 "Linux" "Linux Programmer's Manual" .SH NAME mprotect \- set protection on a region of memory .SH SYNOPSIS .nf .B #include .sp .BI "int mprotect(void *" addr ", size_t " len ", int " prot ); .fi .SH DESCRIPTION .BR mprotect () changes protection for the calling process's memory page(s) containing any part of the address range in the interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1]. .I addr must be aligned to a page boundary. If the calling process tries to access memory in a manner that violates the protection, then the kernel generates a .B SIGSEGV signal for the process. .PP .I prot is either .B PROT_NONE or a bitwise-or of the other values in the following list: .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 modified. .TP .B PROT_EXEC The memory can be executed. .\" FIXME .\" Document PROT_GROWSUP and PROT_GROWSDOWN .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 EINVAL \fIaddr\fP is not a valid pointer, or not a multiple of the system page size. .\" Or: both PROT_GROWSUP and PROT_GROWSDOWN were specified in 'prot'. .TP .B ENOMEM Internal kernel structures could not be allocated. .TP .B ENOMEM Addresses in the range .RI [ addr , .IR addr + len \-1] are invalid for the address space of the process, or specify one or more pages that are not mapped. (Before kernel 2.4.19, the error .BR EFAULT was incorrectly produced for these cases.) .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 the behavior of .BR mprotect () is unspecified if it is applied to a region of memory that was not obtained via .BR mmap (2). .SH NOTES On Linux it is always permissible to call .BR 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 .B PROT_EXEC has any effect different from .B PROT_READ is architecture- and kernel version-dependent. On some hardware architectures (e.g., i386), .B PROT_WRITE implies .BR PROT_READ . POSIX.1-2001 says that an implementation may permit access other than that specified in .IR prot , but at a minimum can only allow write access if .B PROT_WRITE has been set, and must not allow any access if .B PROT_NONE has been set. .SH EXAMPLE .\" sigaction.2 refers to this example .PP The program below 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: .in +4n .nf .RB "$" " ./a.out" Start of region: 0x804c000 Got SIGSEGV at address: 0x804e000 .fi .in .SS Program source \& .nf #include #include #include #include #include #include #include #define handle_error(msg) \\ do { perror(msg); exit(EXIT_FAILURE); } while (0) char *buffer; static void handler(int sig, siginfo_t *si, void *unused) { printf("Got SIGSEGV at address: 0x%lx\\n", (long) si\->si_addr); exit(EXIT_FAILURE); } int main(int argc, char *argv[]) { char *p; 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: 0x%lx\\n", (long) buffer); if (mprotect(buffer + pagesize * 2, pagesize, PROT_READ) == \-1) handle_error("mprotect"); for (p = buffer ; ; ) *(p++) = \(aqa\(aq; printf("Loop completed\\n"); /* Should never happen */ exit(EXIT_SUCCESS); } .fi .SH "SEE ALSO" .BR mmap (2), .BR sysconf (3) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.