C++ Program crashes with array of structs declaration

Question

I can't figure out what's the problem with the following code, it just crashes without outputing anything to the screen:

#include <cstdlib>
#include <iostream>
using namespace std;

typedef struct {
    unsigned int recid;
    unsigned int num;
    char str[120];
    bool valid;
} record_t;

typedef struct {
    unsigned int blockid;
    unsigned int nreserved; 
    record_t entries[100];
    bool valid;
    unsigned char misc;
} block_t;

int main(){
    cout << "Before Buffer" << endl;
    block_t buffer[1000];
    cout << "After Buffer" << endl;
    return 0;
}

I tried Qt debugger and GBD and they just show segmentation fault and point at the start of the main function.

The size of each block_t element is 13,2 Kbs so the size of the buffer array should be around 13Mb. Maybe that's too much for a C-array?

If you think it is the size of the array, then have you tried smaller sizes? — Shafik Yaghmour
– Shafik Yaghmour, Commented Apr 29, 2013 at 1:57
I'm supposed to do external sorting on 100GB data given just this buffer I didn't think I could't store even 13MB in it... I must be missing sth... — Paris P
– Paris P, Commented Apr 29, 2013 at 2:01
Your array is probably bigger than your stack space. Use heap space (e.g. block_t** b = (block_t**)malloc(sizeof(block_t)*1000);). — user1621465
– user1621465, Commented Apr 29, 2013 at 2:03

taocp · Accepted Answer · 2013-04-29 02:01:02Z

3

block_t buffer[1000];

probably used all your stack space (requires larger than 1000* 100 *120 *1 Byte assume ASCII approximately equals 12MB, not considering other fields of those structs), therefore, you get a segmentation fault.

Try to use:

block_t * buffer = new block_t[1000];

or std::vector instead or increase your stack space to larger size if possible.

answered Apr 29, 2013 at 2:01

taocp

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1 Comment

Paris P Over a year ago

Thanks didn't know these 2 had that much difference other than being able to allocate in runtime

Michael Burr · Accepted Answer · 2013-04-29 02:01:21Z

2

Your buffer variable is about 13MB - too large for a stack allocation.

answered Apr 29, 2013 at 2:01

Michael Burr

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