Windows 7 64 bit c++ malloc fail

Question

I have a small c++ programme that fails to allocate more than 2880 with malloc. The statement:

void* tmpptr = malloc(2881);

crashes, whilst

void* tmpptr = malloc(2880);

does not. Every time!

I am using MinGW and compiling with

g++ -std=c++0x -pedantic -Wall -Wextra -g -D_GLIBCXX_DEBUG -static-libgcc -static-libstdc++

I know that the use of malloc is discouraged in C++ and I am planning to rewrite this anyway, but I would still want to know why this is not working. The same code has been working when I compiled it with gcc.

Update: This is the main function calls:

image * img = readPPM("pic/pic.ppm");
bw_image * sky = skyline(img, ref);
cont * lin = contour(sky, 0); // <-- chash
...

And the function starts with:

#include <cstdlib>
cont * contour(const bw_image * img, const char wrap) {
    int test = 2880;
    void* ptr1 = malloc(test);
    void* ptr2 = malloc(test);
...

Now the first malloc will work but not the second one. If I change test = 1440;, same result. But; for test = 140; already the first malloc will fail.

I've tried the code as a stand alone:

int main(int argc, char *argv[]) {
    int size = 2881;
    void* tmpptr;
    printf("Allocating, %d\n", size);
    tmpptr = malloc(size);
    printf("Allocated %d bytes successfully\n", size);
}

and it works without problems, so it seems to be something in main doing it.

rem_artifacts look like this

void rem_artifacts(bw_image * sky) {
    for (int y = 0; y < sky->y; ++y) for (int x = 0; x < sky->x; ++x) {
        int xp = x - 1, xn = x + 1;
        if (xp < 0) xp = sky->x - 1;
        if (xn == sky->x) xn = 0;
        int c = sky->data[x][y];  // this is wrong
        if (
            (y == 0 || sky->data[x][y-1] != c) &&  // and this
            (y == sky->y-1 || sky->data[x][y+1] != c) && // and this
            sky->data[xp][y] != c &&  // and this
            sky->data[xn][y] !=c  // and this
        ) sky->data[x][y] = !c; // and this
    }
}
bw_image * skyline(const image * img, const image * ref) {
    double tilt = 114.0 - 90.0;
    double pi = 3.14159265358979323846;
    double chang = 360.0 / 2.0 / pi;
    //double sint = sin(tilt / chang);
    //double cost = cos(tilt / chang);
    bw_image * sky = (bw_image*)malloc(sizeof(bw_image));
    sky->x = img->x;
    sky->y = img->y;    //  
    double cos30 = sqrt(3)/2;
    int lim0 = (int)((double)(img->y) / 2.0 + (double)(img->x) * tan(tilt/chang) * cos30);
    sky->data = (char**)malloc(sizeof(char*) * sky->y);
    for (int y = 0; y < sky->y; ++y) {
        sky->data[y] = (char*)malloc(sizeof(char) * sky->x);
        for (int x = 0; x < sky->x; ++x)
            sky->data[y][x] = !(y < lim0 && colour_dist_sq(img->data[y][x], ref->data[y][x]) < 5000.0);
    }
    rem_artifacts(sky);
    return sky;
}

And it crashes in the `malloc` call? You checked in a debugger? — Some programmer dude, Mar 27 '13 at 13:28
Can you show the entire small program? *Something Bad* is happening somewhere else. — Drew Dormann, Mar 27 '13 at 13:28
Also, read this post if you haven't. http://stackoverflow.com/questions/7480655/how-to-troubleshoot-crashes-in-malloc — Shane Hsu, Mar 27 '13 at 13:29
somehow this comes to my mind: http://www.codinghorror.com/blog/2008/03/the-first-rule-of-programming-its-always-your-fault.html — Najzero, Mar 27 '13 at 13:39
Machine: Samsung R530, no warnings when compiling (except for the unused dummy pointers). — petter, Mar 27 '13 at 13:58
@petter It may help you to provide a **short, complete** example of the problem or to step through your code in a debugger. Your program is either corrupting heap memory or allocating unreasonable amounts of memory somewhere. Possibly in the functions you're not showing. — Drew Dormann, Mar 27 '13 at 14:06
Ok. I see that you have included in the file where you have your function implementation. Actually there is no malloc in cstdlib but there is std::malloc there. And this is quite strange you have no warning about it. I'm not sure that this will help, but you can try to replace malloc with std::malloc. — JustAnotherCurious, Mar 27 '13 at 14:15
There doesn't seem to be a difference if a use just `malloc` or `std::malloc`. — petter, Mar 27 '13 at 14:30
And the `malloc` calls is on lines 22, 25, 26 and 27 in the file `skyline.cpp`? In the function `rem_artifacts`? According to the log, you do bad stuff there. — Some programmer dude, Mar 27 '13 at 14:31
@petter You seem to need a tool like **valgrind**. Take a minute to read through this. http://stackoverflow.com/questions/413477 — Drew Dormann, Mar 27 '13 at 14:44
Please show us how you declare and allocate `sky->data` and the sub-arrays of `sky->data`. — Some programmer dude, Mar 27 '13 at 14:51
I'd suspect an `sprintf` buffer overrun. Try switching to use bounds-checked `snprintf` and see if it goes away. — Nathan Ernst, Mar 27 '13 at 18:59

score 2 · Accepted Answer · answered Mar 27 '13 at 19:47

Thank you everybody,

It turns out that I was writing outside the allocated memory (a bit of a x and y confusion). The weird thing is though that the programme has been working until now (24/7 for many months) and doing what I have been expecting of it. It could though explain some weird things I haven't been able to explain.

Now I will anyway rewrite the code with standard C++ memory allocation.

score 0 · Answer 2 · answered Mar 27 '13 at 14:46

It looks to me like your heap is getting corrupted somewhere else in your program (probably during that big loop in main. The first thing you can do is start turning on warnings -Wall and -Wnon-virtual-dtor at a very minimum and then fixing them.

If you have access to Linux your best bet is to run this under valgrind and be amazed as it tells you exactly where you're overwriting memory.

Alternately if you have access to Purify ($$$) on Windows that may be able to help you as well. There are also memory checking malloc libraries available you may be able to use.

If none of those tools are available you're going to have to do diagnostics yourself by trying to remove sections of code and see if it still crashes or not.

The flags `-Wall` and `-Wnon-virtual-dtor` do not warn about anything (except unused variables). Will try it in debian (but I guess I won't have the same problem there). Anyway I need it to run in Windows in the end. — petter, Mar 27 '13 at 14:54

score 0 · Answer 3 · answered Mar 27 '13 at 15:00

If I would venture a guess, it's that you don't properly allocate sky->data or its sub-pointers.

This has to be done in two steps:

First allocate sky->data:
```
sky->data = new char*[some_x_size];
```

Then you have to allocate all sub-array separately:

for (int x = 0; x < some_x_size; x++)
    sky->data[x] = new char[some_y_size];

You can of course use malloc instead of new.

When freeing the data, you do it the opposite way: First free each sub-array in a loop, then the main array.

Windows 7 64 bit c++ malloc fail

3 Answers3