NAME
mremap
—
re-map a virtual memory
address
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/mman.h>
void *
mremap
(void
*oldp, size_t
oldsize, void
*newp, size_t
newsize, int
flags);
DESCRIPTION
Themremap
()
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, -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
HISTORY
The mremap
() system call appeared in
NetBSD 5.0. It was based on the code that supports
mremap
() compatibility for Linux binaries.