CODEWITHSUNDEEP
c
Nowhy
Nowhy

Reputation: 2914

C enter a if block when the condition is false

A bitmap sort programme,in which the assign part is:

for(i = MAX/64-1;i >= 0;i--){
    for(j = 0;j < 64;j++){
        if(0 != (arr[i] & (1 << j))){
            *p++ = j + 64 * i;  
        }   
    }   
}

I gdb it,sometimes when the if condition returns 0 and the program still enter the block and execute the statement:
*p++ = j + 64 *i;
which cause segmentfault at last,I just don't understand under which circumstance would it happen

example:
when gdb status like this:
j=44, i=6250, arr[i]=4096 and print (arr[i] & (1 << j)) gives 0
but programme still enter the block

Upvotes: 2

Views: 120

Answers (2)

Paul R
Paul R

Reputation: 213190

My guess is that you have 32 bit ints but your array data type is int64_t. In which case you need to change:

    if(0 != (arr[i] & (1 << j))){

to:

    if(0 != (arr[i] & (1LL << j))){

since 1 << j is undefined for j >= sizeof(int) * CHAR_BIT (i.e. j >= 32 in your case).

Upvotes: 4

unwind
unwind

Reputation: 400109

1 << j is not very well-defined if it's int and j is greater than or equal to CHAR_BIT * sizeof (int). This might be a problem since you use j = 44 in your example.

Upvotes: 3

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