[Valgrind-users] mmap(2) returns 0x0 for <=4096 bytes

[Kristaps D. <kri...@bs...>]

Hello list,

I'm experiencing a very strange error with valgrind-3.10.1 on Mac OS X 
10.7.5 (via homebrew).  In short, mmap(2) returns 0x0 for a regular 
MAP_FILE of <=4096 bytes.  To reproduce, I use the following simple test:

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

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
	int i, fd;
	void *buf;
	struct stat st;

	for (i = 1; i < argc; i++) {
		if (-1 == (fd = open(argv[i], O_RDONLY, 0))) {
			perror(argv[i]);
			break;
		}
		if (-1 == fstat(fd, &st)) {
			perror(argv[i]);
			break;
		}
		buf = mmap(NULL, st.st_size,
			PROT_READ, MAP_SHARED, fd, 0);
		if (MAP_FAILED == buf) {
			perror(argv[i]);
			break;
		}
		printf("%s -> %p (%zu)\n",
			argv[i], buf, (size_t)st.st_size);
		munmap(buf, st.st_size);
		close(fd);
	}
	return(i < argc ? EXIT_FAILURE : EXIT_SUCCESS);
}

When I run it on perfectly ordinary files, at first it randomly returned 
0x0 instead of the usual mapped address, for example,

% valgrind ./a.out ../kcgi/*.xml
../kcgi/index.xml -> 0x338000 (12424)
../kcgi/template.xml -> 0x0 (188)
../kcgi/test.xml -> 0x0 (4)
../kcgi/version_0_4_2.xml -> 0x0 (488)
../kcgi/version_0_4_3.xml -> 0x0 (576)
../kcgi/version_0_4_4.xml -> 0x0 (291)

And now without valgrind...

% ./a.out ../kcgi/*.xml
../kcgi/index.xml -> 0x109ecf000 (12424)
../kcgi/template.xml -> 0x109ecf000 (188)
../kcgi/test.xml -> 0x109ecf000 (4)
../kcgi/version_0_4_2.xml -> 0x109ecf000 (488)
../kcgi/version_0_4_3.xml -> 0x109ecf000 (576)
../kcgi/version_0_4_4.xml -> 0x109ecf000 (291)

After running this on more files, I noticed that large files seemed ok; 
small files failed.  I bisected, and in the end:

% valgrind  ./a.out test*.xml
test1.xml -> 0x0 (617)
test2.xml -> 0x338000 (6170)
test3.xml -> 0x0 (3117)
test4.xml -> 0x338000 (4820)
test5.xml -> 0x0 (4096)
test6.xml -> 0x338000 (4097)

In short, I can reliably have a value of 0x0 returned for files of less 
than or equal to 4096 bytes.

Any thoughts?  valgrind itself doesn't report anything, and 
--trace-syscalls for the offending parts show nothing different for the 
"failed" cases.

Best,

Kristaps