u_int64_t array

Question

I'm trying to do this:

int main(void){
    u_int64_t NNUM = 2<<19;
    u_int64_t list[NNUM], i;

    for(i = 0; i < 4; i++){ 
        list[i] = 999;
    }
}

Why am I getting segfault at my Ubuntu 10.10 64 bits (gcc 4.6.1)?

The correct name is `uint64_t`, not `u_int64_t`. The latter is a legacy name that does not exist on standard C and is preserved in POSIX `sys/types.h` only for compatibility with legacy software. — R.. GitHub STOP HELPING ICE, Mar 30 '12 at 23:57

sth · Accepted Answer · 2012-03-30T23:58:33.593

5

You try to create a very large array on the stack. This leads to a stack overflow.

Try allocating the array on the heap instead. For example:

// Allocate memory
u_int64_t *list = malloc(NNUM * sizeof(u_int64_t));

// work with `list`
// ...

// Free memory again
free(list);

edited Mar 30 '12 at 23:58

answered Mar 30 '12 at 23:39

sth

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What do you mean with heap? Sorry for the ignorance :) – Frederico Schardong Mar 30 '12 at 23:41
You can accomplish this by making it a global (declared outside of `main()`) or making it `static` (which keeps it local to main). – tomlogic Mar 30 '12 at 23:43
Or by allocating the array using malloc. – Lawrence Velázquez Mar 30 '12 at 23:50
No way to do something like this inside main? u_int64_t NNUM = 2<<19; static u_int64_t list[NNUM]; – Frederico Schardong Mar 30 '12 at 23:52
@FredericoSchardong That’s the second suggestion tomlogic made. – Lawrence Velázquez Mar 30 '12 at 23:59
@LawrenceVelázquez I'm asking because gcc returns: error: storage size of ‘list’ isn’t constant – Frederico Schardong Mar 31 '12 at 00:04

score 2 · Answer 2 · answered Mar 30 '12 at 23:44

You declare NNUM = 2*2^19 == 2<<19 == 1048576.

and try to allocate on the stack 64 bits * 1048576 = num of bits* num of cells. It is 8.5 MegaBytes, it is just too much for allocation on the stack, you can try to allocate it on the heap and check if it really works using the return value of malloc.

heap VS. stack

score 0 · Answer 3 · answered Mar 31 '12 at 02:48

0

Your program requires a minimum stack size of 1048576, if you check with 'ulimit -s', it is most likely less than that. you can try 'ulimit -s 16384' and then re-execute again.

answered Mar 31 '12 at 02:48

pizza

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u_int64_t array

3 Answers3