MREMAP(2)

NAME
mremap - re-map a virtual memory address

LIBRARY
libc

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 for details.