Mail Archives: cygwin/2005/02/24/18:18:15
Hello,
There is something strange about mmap(addr, ..., MAP_FIXED) under
Cygwin. For a reason, it always fails if addr is not on a 16xPAGE_SIZE
boundary. Here is a short demo:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/mman.h>
#define NRUNS 20
#define CHUNK_SIZE 4096
#define START_ADDR 0x640000
int main(void)
{
unsigned int i, pagesize, prot, flags;
void *addr, *maddr;
pagesize = getpagesize();
prot = PROT_READ | PROT_WRITE | PROT_EXEC;
flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED;
for (i = 0; i < NRUNS; i++) {
addr = (void *) START_ADDR + pagesize*i;
maddr = mmap(addr, CHUNK_SIZE, prot, flags, 0, 0);
if (maddr != MAP_FAILED) {
fprintf(stderr, "OK: %p -> %p\n", addr, maddr);
} else {
fprintf(stderr, "FAIL: %p (%s)\n", addr, strerror(errno));
}
}
exit(0);
}
And the output is:
OK: 0x640000 -> 0x640000
FAIL: 0x641000 (Permission denied)
FAIL: 0x642000 (Permission denied)
FAIL: 0x643000 (Permission denied)
FAIL: 0x644000 (Permission denied)
FAIL: 0x645000 (Permission denied)
FAIL: 0x646000 (Permission denied)
FAIL: 0x647000 (Permission denied)
FAIL: 0x648000 (Permission denied)
FAIL: 0x649000 (Permission denied)
FAIL: 0x64a000 (Permission denied)
FAIL: 0x64b000 (Permission denied)
FAIL: 0x64c000 (Permission denied)
FAIL: 0x64d000 (Permission denied)
FAIL: 0x64e000 (Permission denied)
FAIL: 0x64f000 (Permission denied)
OK: 0x650000 -> 0x650000
FAIL: 0x651000 (Permission denied)
FAIL: 0x652000 (Permission denied)
FAIL: 0x653000 (Permission denied)
On the other hand, if 'MAP_FIXED' is removed, mmap() happily uses the
address hint in all cases:
OK: 0x640000 -> 0x640000
OK: 0x641000 -> 0x641000
OK: 0x642000 -> 0x642000
OK: 0x643000 -> 0x643000
OK: 0x644000 -> 0x644000
OK: 0x645000 -> 0x645000
OK: 0x646000 -> 0x646000
OK: 0x647000 -> 0x647000
OK: 0x648000 -> 0x648000
OK: 0x649000 -> 0x649000
OK: 0x64a000 -> 0x64a000
OK: 0x64b000 -> 0x64b000
OK: 0x64c000 -> 0x64c000
OK: 0x64d000 -> 0x64d000
OK: 0x64e000 -> 0x64e000
OK: 0x64f000 -> 0x64f000
OK: 0x650000 -> 0x650000
OK: 0x651000 -> 0x651000
OK: 0x652000 -> 0x652000
OK: 0x653000 -> 0x653000
Any insight?
Regards,
Evgeny
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