gcc -O segmentation fault - c

Question

Here is a code to insert a data in a binary trie. This code works perfectly if I compile it with a basic gcc main.c -o main .
/**
* Insert a new gateway in the tree, at the position corresponding to the
* subnet address.
*
* addr : Subnet address
* netmask : Subnet mask
* gw : gateway identifier
*
* return : void.
*/
void insertMyAlgo(unsigned int addr, unsigned int netmask, unsigned int gw)
{
struct node* noeud;
int i;
int maskBit = countMaskBit(netmask);
// Going down in the tree until next mask bit = 0.
noeud = arbre;
for (i = 31; i > 31 - maskBit; i--)
{
// Bit = 1, go down in the right child.
if ((addr >> i) & 0x1)
{
if (noeud->fd == NULL)
noeud->fd = allocNode();
noeud = noeud->fd;
}
// Bit = 0, go down in the left child.
else
{
if (noeud->fg == NULL)
noeud->fg = allocNode();
noeud = noeud->fg;
}
}
// Insert the gateway in the node corresponding to our subnet address.
noeud->gateway = gw;
}
I would like to use the -O option to optimize time spent in looking up the tree, finding a specific key. I get a segfault when I execute my main with this -O options.
Gdb gave me the following informations :
Program received signal SIGSEGV, Segmentation fault. insertMyAlgo
(addr=12288, netmask=<optimized out>, gw=3238068734)
at mainbinaireBench.c:125 125 if (noeud->fg == NULL)
(gdb) print noeud->fg Cannot access memory at address 0x8
So the error seems to be here :
// Bit = 0, go down in the left child.
else
{
if (noeud->fg == NULL)
noeud->fg = allocNode();
noeud = noeud->fg;
}
I really don't know why there is this error, and why the program works without this -O option. I would really like to make it works, if some of you guys could help me to understand, it would be very nice.
Thank you !

Answer 1

It is not possible to know what is wrong for certain without a Short Self Contained Example
But there are debugging thechniques which may help you in finding what the problem is.
Add several asserts to your code :
void insertMyAlgo(unsigned int addr, unsigned int netmask, unsigned int gw)
{
struct node* noeud;
int i;
int maskBit = countMaskBit(netmask);
// Going down in the tree until next mask bit = 0.
assert( arbre!=NULL );
noeud = arbre;
for (i = 31; i > 31 - maskBit; i--)
{
// Bit = 1, go down in the right child.
if ((addr >> i) & 0x1)
{
if (noeud->fd == NULL)
{
noeud->fd = allocNode();
assert( noeud->fd!=NULL );
assert( noeud->fd->fd==NULL );
assert( noeud->fd->fg==NULL );
}
noeud = noeud->fd;
}
// Bit = 0, go down in the left child.
else
{
if (noeud->fg == NULL)
{
noeud->fg = allocNode();
assert( noeud->fg!=NULL );
assert( noeud->fg->fd==NULL );
assert( noeud->fg->fg==NULL );
}
noeud = noeud->fg;
}
}
// Insert the gateway in the node corresponding to our subnet address.
noeud->gateway = gw;
}
If any of those asserts fail now you have a better idea of what is happening. If none fail you have reduced your search space, the problem is elsewhere.
You may even leave the asserts there after you have finished debugging. Just define NDEBUG for release builds and all the code in asserts will be omited.