array of char pointer segfaulting

Question

The code below works fine if factors is small but when it gets larger I get a segmentation fault.The first instance I have encountered is at a size of ~800.000. This should easily fit into memory or what am I missing? Appreciate any help.

char  *fgs_arr [facs().size()],  
      *true_arr[facs().size()],  
      *maps_arr[facs().size()];

Maybe I'm missing something: `facs().size()` doesn't look like a valid C syntax to me. — Aleks G, Feb 16 '12 at 18:34
You might want to provide more information about what environment you are developing in. — Fls'Zen, Feb 16 '12 at 18:36
Are you sure the segfault occurs here and not somewhere else? — Tudor, Feb 16 '12 at 18:36
@AleksG It's probably another language, but even in C, if `facs` return struct that has member `int (*size)()`, it's valid. — asaelr, Feb 16 '12 at 18:42
Sorry, actually it's C/C++ facs() returns a std::vector so this is not the issue. And I am pretty sure it's here — user1214505, Feb 16 '12 at 18:46
@user1214505: please don't mix the languages. That's like saying you have a question with the German/English language. They're completely different except in a few minor ways. — Mooing Duck, Feb 16 '12 at 19:00
It isn't valid C++ syntax either. C++ doesn't allow for variable-sized arrays. Maybe it's g++ or MSVC++. — Robᵩ, Feb 16 '12 at 19:49

score 3 · Accepted Answer · edited Feb 17 '12 at 17:25

3

You are running out of Stack (The place where locally declared memory goes). You'll either need to dynamically declare it (this goes to the heap which is much larger) or increase stack size.

edited Feb 17 '12 at 17:25

Aman Aggarwal

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answered Feb 16 '12 at 18:36

RussS

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I believe _dynamically_ declared memory goes on the stack. – calebds Feb 16 '12 at 18:38
1

nope. http://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap – RussS Feb 16 '12 at 18:39
Thanks, actually what I needed was: char **fgs_arr = new char*[facs().size()] – user1214505 Feb 16 '12 at 19:27

score 3 · Answer 2 · answered Feb 16 '12 at 18:37

3

Since vla is on the stack, and the stack is (in most case) 1MB, it should not easily fit into memory. Use malloc instead (and don't forget to free).

answered Feb 16 '12 at 18:37

asaelr

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score 0 · Answer 3 · answered Feb 16 '12 at 19:53

As you have discovered, you are running out of stack space. Switching to new is inelegant; it promotes memory leaks and other bugs. Try std::vector<char*>:

std::vector<char*> fgs_arr(facs().size());
std::vector<char*> true_arr(facs().size());
std::vector<char*> maps_arr(facs().size());

array of char pointer segfaulting

3 Answers3