Just a minor nitpick, though: wouldn't it be possible for an application to catch the SIGSEGV and let the code proceed, making invalid the assumption made by gcc?
Uhh, it's even worse than that. Have a look at the following code:
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h> #include <sys/types.h> #include <sys/mman.h>
struct test { int code; }; int test_check_first(struct test *a) { int ret; if (!a) return -1; ret = a->code; return ret; } int test_check_last(struct test *a) { int ret; ret = a->code; if (!a) return -1; return ret; }
int main() { int i; struct test *nullmem = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0); if (nullmem == MAP_FAILED) { fprintf(stderr,"mmap: %s\n",strerror(errno)); exit(1); } for (i = 0; i < 2; i++) { nullmem[i].code = i; printf("nullmem[%d].code = %d\n",i,i); printf("test_check_first(&nullmem[%d]) = %d\n",i, test_check_first(&nullmem[i])); printf("test_check_last(&nullmem[%d]) = %d\n",i, test_check_last(&nullmem[i])); } munmap(nullmem,4096); exit(0); }
Without optimization:
king:~# gcc -o mmapnull mmapnull.c king:~# ./mmapnull nullmem[0].code = 0 test_check_first(&nullmem[0]) = -1 test_check_last(&nullmem[0]) = -1 nullmem[1].code = 1 test_check_first(&nullmem[1]) = 1 test_check_last(&nullmem[1]) = 1
With optimization:
BUG ==> ^^^king:~# gcc -O2 -o mmapnull mmapnull.c king:~# ./mmapnull nullmem[0].code = 0 test_check_first(&nullmem[0]) = -1 test_check_last(&nullmem[0]) = 0
nullmem[1].code = 1 test_check_first(&nullmem[1]) = 1 test_check_last(&nullmem[1]) = 1
This is on multiple platforms, including PPC Linux, X86 Linux, and PPC Mac OS X. All exhibit the exact same behavior and output. I think I'll probably go report a GCC bug now :-D
Dereferencing null pointers is relied upon by a number of various emulators and such, and is "platform-defined" in the standard, so since Linux allows mmap at NULL, GCC shouldn't optimize that case any differently.
Cheers, Kyle Moffett
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GCM/CS/IT/U d- s++: a18 C++++>$ UB/L/X/*++++(+)>$ P+++(++++)>$
L++++(+++) E W++(+) N+++(++) o? K? w--- O? M++ V? PS+() PE+(-) Y+
PGP+++ t+(+++) 5 X R? tv-(--) b++++(++) DI+ D+ G e->++++$ h!*()>++$ r !y?(-)
------END GEEK CODE BLOCK------
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
