Question

I have this piece of code in a function. I want to print the value of y here.

if (x1 < 0 || y1 < 0) {

    // Vertical lign outside of layer
    if (dx == 0 && y1 < 0) {
        return GKIT_NOERR;
    }

    float m = dy / dx;
    float t = y1 - m * x1;
    float x = -t / m;
    float y = m * x + t;

    printf("Hello %s. You are %f years old.\n", "Niklas", y);
}

But I get a segmentation fault. It works with no value at all to be printed as float. I can change that to %d or similar, which works fine.

    int val = (int) y;
    printf("Hello %s. You are %d years old.\n", "Niklas", val);

Any idea where the Segfault comes from?

Edit: Complete function.

// coding: ascii
// author: Niklas Rosenstein
// e-mail: [e-mail]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "gkit/defines.h"
#include "gkit/utils.h"
#include "gkit/graphicslayer.h"

#define SWAP_IF_NECCESSARY(x1, y1, x2, y2)  \
    if (x2 < x1 && y2 < y1) {               \
        int temp = x2;                      \
        x2 = x1;                            \
        x1 = temp;                          \
        temp = y2;                          \
        y2 = y1;                            \
        y1 = temp;                          \
    }

/* Based on Bresenhams line algorithm. */
int gk_GraphicsLayer_drawLine(gk_GraphicsLayer* layer, gk_Color* color,
                              int x1, int y1, int x2, int y2,
                              gk_ColorBlendProc blend, gk_float opacity) {
    SWAP_IF_NECCESSARY(x1, y1, x2, y2);

    float dx = x2 - x1;
    float dy = y2 - y1;
    float cx = x1;
    float cy = y1;

    // Figure out where to start in case x1 or y1 are outside of the layer.
    if (x1 < 0 || y1 < 0) {

        // Vertical lign outside of layer
        if (dx == 0 && y1 < 0) {
            return GKIT_NOERR;
        }

        // The function's slope (m)
        // ------------------------
        float m = dy / dx;


        // Find the y-axis intersection (t)
        // -------------------------------
        // y = mx + t   =>
        // y - mx = t

        float t = y1 - m * x1;

        // Compute the root of the function (N)
        // ------------------------------------
        // 0 = mx + t   =>
        // mx = -t      =>
        // x = -t / m

        float x = -t / m;
        float y = m * x + t;

        printf("Hello %s. You are %f years old.\n", "Niklas", y);
    }


    int incx = GKIT_SIGNUM(dx);
    int incy = GKIT_SIGNUM(dy);
    if (dx < 0) { dx = -dx; }
    if (dy < 0) { dy = -dy; }

    int pdx, pdy;
    int ddx, ddy;
    int es, el;

    ddx = incx;
    ddy = incy;

    if (dx > dy) {
        pdx = incx;
        pdy = 0;
        es = dy;
        el = dx;
    }
    else {
        pdx = 0;
        pdy = incy;
        es = dx;
        el = dy;
    }

    float err = el / 2.0;

    #define SET_PIXEL(x, y) \
        do { \
        gk_Color* c = GKIT_GRAPHICSLAYER_ACCESSPIXEL(layer, (int)x, (int)y); \
        if (blend != Null) {                \
            gk_Color t = *c;                \
            blend(color, &t, c, opacity);   \
        }                                   \
        else {                              \
            *c = *color;                    \
        } } while (0)

    SET_PIXEL(cx, cy);

    int t;
    for (t=0; t < el; t++) {
        err -= es;
        if (err < 0) {
            err += el;
            cx += ddx;
            cy += ddy;
        }
        else {
            cx += pdx;
            cy += pdy;
        }
        SET_PIXEL(cx, cy);
    }

    #undef SET_PIXEL

    return GKIT_NOERR;
}

Edit: Complete stack trace:

#0 0xb7e68cb0   ___printf_fp(fp=0xb7fc3a20, info=0xbffff684, args=0xbffff6f8) (printf_fp.c:844)
#1 0xb7e63ab0   _IO_vfprintf_internal(s=0xb7fc3a20, format=<optimized out>, ap=0xbffff750 "\001") (vfprintf.c:1623)
#2 0xb7e6cc2f   __printf(format=0x8049da0 "Hello %s. You are %f years old.\n") (printf.c:35)
#3 0x8049143    gk_GraphicsLayer_drawLine(layer=0x804d008, color=0xbffff810, x1=-20, y1=-10, x2=49, y2=200, blend=0, opacity=0) (/home/niklas/git/c-gkit/gkit/graphicslayer.c:180)
#4 0x8049ba4    test_drawLine() (/home/niklas/git/c-gkit/main.c:46)
#5 0x8049c80    main() (/home/niklas/git/c-gkit/main.c:68)

Edit: Please note that printf() does work when putting it after or before the if-clause. I.e. Something like

    printf("Foo: %f\n", 1.0);
    // Figure out where to start in case x1 or y1 are outside of the layer.
    if (x1 < 0 || y1 < 0) {

        // Vertical lign outside of layer
        if (dx == 0 && y1 < 0) {
            return GKIT_NOERR;
        }

does work, but moving the printf() inside the if-clause yields a segmentation fault.

Update: According to T.E.D.‘s answer, I’ve tested around a little and this is what came out:

The problem seem the be the outcome of the comparison operations (<). I can do

if (True) { printf("%f", 53.3); }

but I can’t do

if (x1 < 0 || y1 < 0) { printf("%f", 53.3); }
// nor
if (x1 < 0) { printf("%f", 53.3); }
// nor
int x_smaller = x1 < 0;
if (x_smaller) { printf("%f", 53.3); }

Interesting is, that this works:

int x_smaller = x1 < 0;
int y_smaller = y1 < 0;
x_smaller = y_smaller = 1;
if (x_smaller || y_smaller) { printf("%f", 53.3); }

---

Conclusion: The outcome of the operations x1 < 0 and y1 < 0 tested in the if-clause make printf() fail. The questions are:

1. DAFUQ? Why is this happening?
2. How can I fix it?

---

If you are interested in the whole code, I don’t mind sharing it. It’s on github. It’s a Code::Blocks project. The only include-path must be to the parent-directory of the gkit folder.

Answer 1

Thanks to unkulunkulu, who provided his help in the chat, we were able to find the issue.

I couldn’t believe that previous calls to gk_GraphicsLayer_drawLine could influence the behaviour of following calls, but exactly this was the case. In my main() function, I called the function three times. The first call recieved accidently values that did also reach out of bounds of the pixel-array of gk_GraphicsLayer. The third call was the one who finally crashed the program.

This also explains why exiting the function after the if-clause (where the error appeared in) fixed the segfault. It was because it prevented the function from accessing memory it shouldn’t access.

Summary: Writing to the memory of an invalid address is so much dangerous, it can even trigger a completely other function to fail and lead you to miss-asumptions. Unfortunately, one doesn’t recieve Segmentation Fault errors when the invalid address is still in range of the memory that was supplied by the OS for your application.