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?
#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);
}
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
king:~# gcc -O2 -o mmapnull mmapnull.cBUG ==> ^^^
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