NAME

mremap — re-map a virtual memory address

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/mman.h>

void *
mremap(void *oldp, size_t oldsize, void *newp, size_t newsize, int flags);

DESCRIPTION

The mremap() function resizes the mapped range (see mmap(2)) starting at oldp and having size oldsize to newsize. The following arguments can be OR'ed together in the flags argument:

MAP_ALIGNED(n)

The allocation should be aligned to the given boundary, i.e. ensure that the lowest n bits of the address are zero. The parameter n should be the base 2 logarithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the platform's page size as returned by sysconf(3) with the _SC_PAGESIZE request.

MAP_FIXED

newp is tried and mremap() fails if that address can't be used as new base address for the range. Otherwise, oldp and newp are used as hints for the position, factoring in the given alignment.

MAP_REMAPDUP

Duplicate the mapping. Both address ranges reference the same pages, but can have different protection flags.

RETURN VALUES

mremap() returns the new address or MAP_FAILED, if the remap failed.

EXAMPLES

The following example program creates a two mappings for the same memory range, one RW- and one R-X.

#include <sys/types.h>
#include <sys/mman.h>

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <err.h>

static int
return_1(void)
{
	return 1;
}

static void
return_1_end(void)
{
}

static int
return_2(void)
{
	return 2;
}

static void
return_2_end(void)
{
}

int
main(int argc, char *argv[])
{
        void *maprw, *maprx;
	int rv;
	size_t page = (size_t)sysconf(_SC_PAGESIZE);

	// Create the first mapping that has no protections, but intended
	// protections only
	maprw = mmap(NULL, page,
	    PROT_MPROTECT(PROT_EXEC|PROT_WRITE|PROT_READ),
            MAP_ANON|MAP_PRIVATE, -1, 0);
	if (maprw == MAP_FAILED)
		err(EXIT_FAILURE, "mmap failed");

        // Create the second mapping for the same physical space, which
	// again has no protections.
	maprx = mremap(maprw, page, NULL, page, MAP_REMAPDUP);
	if (maprx == MAP_FAILED)
		err(EXIT_FAILURE, "mremap failed");

	// Set the first mapping read/write
        if (mprotect(maprw, page, PROT_READ|PROT_WRITE) == -1)
		err(EXIT_FAILURE, "mprotect(rw) failed");

	// Set the second mapping read/execute
        if (mprotect(maprx, page, PROT_READ|PROT_EXEC) == -1)
		err(EXIT_FAILURE, "mprotect(rx) failed");

#define XS(a) (size_t)((uintptr_t)(a ## _end) - (uintptr_t)(a))

	// Copy and run the first function
	memcpy(maprw, return_1, XS(return_1));
	__builtin___clear_cache(maprw, (void *)((uintptr_t)maprw + page));
	rv = ((int (*)(void))maprx)();
	printf("%d\n", rv);

	// Copy and run the second function
	memcpy(maprw, return_2, XS(return_2));
	__builtin___clear_cache(maprw, (void *)((uintptr_t)maprw + page));
	rv = ((int (*)(void))maprx)();
	printf("%d\n", rv);

	return EXIT_SUCCESS;
}

COMPATIBILITY

The semantics of mremap() differ from the one provided by glibc on Linux in that the newp argument was added and a different set of flags are implemented.

ERRORS

The mremap() function fails if:

[E2BIG]

A request to extend oldp failed because of address overflow.

[EINVAL]

If oldp or newp are not page aligned, or oldsize or newsize are not a multiple of the page size, or if oldp + oldsize or newp + newsize wrap around, or if an invalid alignment was requested in the flags argument, or if the request was to extend oldp and the extension address space either did not fit, was already occupied, or had set permissions.

[ENOENT]

If the oldp segmented was not already mapped.

[ENOMEM]

If there was either no space to allocate/move memory, or if a fixed allocation was requested in the flags that could not be accommodated.

SEE ALSO

mmap(2), munmap(2)

HISTORY

The mremap() system call appeared in NetBSD 5.0. It was based on the code that supports mremap() compatibility for Linux binaries.

BUGS

If a mapping is created by mmap(2) with MAP_PRIVATE and then duplicated by mremap with MAP_REMAPDUP, calling fork(2) destroys the coupling between the original and duplicate mapping. Workarounds:

• Create the original mapping with MAP_SHARED.
• Use minherit(2) with MAP_INHERIT_NONE on the writable mapping.
• Don't use fork(2); use posix_spawn(2).

See PR lib/55177 for details.