Is it possible for the following function to go into an infinite loop given an array and its size in C:

void foo( int* array , int array_size )
{
int index = 0;
for( ; index

Question

Is it possible for the following function to go into an infinite loop given an array and its size in C:

void foo( int* array , int array_size )
{
   int index = 0;
   for( ; index < array_size ; index++)
      array[ index ] = 0;
}

Paul · Accepted Answer

Bonus points if you consider the case of a buffer overflow.  If array_size is greater than the size of the buffer pointed to by array then the code may behave unpredictably.  In particular, if array is stack-allocated then the loop may lead to stack corruption, and depending on the calling code this may cause the program to loop infinitely.

int array[5];
int conditional = 0;
while( !conditional) {
  conditional = 1;
  foo(&array, 6);
}

It is not clear to me whether the loop in the foo function could loop infinitely.  If the index variable were somehow clobbered to 0 but the array_size variable remained greater than zero this could be possible.  Since array_size is higher in the stack than index they would likely both be clobbered to zero, causing the loop to terminate.  However, if foo was inlined and array_size a constant in the calling context it may get hardwired into the opcode and not stored on the stack.  In this case index could get zeroed out while array_size remained > 0, causing the loop in foo to not terminate.

Val V. · Answer

Actually, it's not possible. Typically in code questions, if they don't specify constraints on a variable, assume the worst. In this example, if array_size was negative, then the for loop would never run (i.g. index = 0, array_size = -1, then index < array_size would be false on the first go). Same goes if array_size is zero. If array_size is positive, then it is constrained since array_size can never equal infinity.

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