SIGTRAP when freeing a three dimensional array in C

Question

I am trying to free a three dimensional pointer array representing a bmp image and whle it compiles ok I get SIGTRAP signals in gdb when debugging. My error message is

warning: HEAP[bmpsample.exe]:
warning: Heap block at 0061FFB8 modified at 0061FFCC past requested size of c.
Program received signal SIGTRAP, Trace/ breakpoint trap. 0x7787704e5 in ntdll!TpWaitForAlpcCompletion()
from ntdll.dll

The error occurs when I free the array after loading in the values from the bmp file. My code is as follows.

Allocation:

int ***alloc3D(int xlen, int ylen, int zlen) {
int i, j, ***array;
if ((array = malloc(xlen*sizeof(int**)))==NULL) {
    perror("Error in first assignment of 3D malloc\n");
}
// Allocate pointers for each row
for (i = 0; i < xlen; i++) {
    if ((array[i] = malloc(ylen*sizeof(int*)))==NULL){
        perror("Error in second assignment of 3D malloc\n");
    }
    // Allocate pointer for each column in the row
    for (j=0; j < ylen; j++) {
        if((array[i][j] = malloc(zlen*sizeof(int)))==NULL) {
            perror("Error in third assignment of 3D malloc\n");
        }
    }
}

Filling the array

int ***readBitmap(FILE *inFile, BmpImageInfo info, int*** array) {
    // Pixels consist of unsigned char values red, green and blue
Rgb *pixel = malloc( sizeof(Rgb) );
int read, j, i;
for( j=0; j<info.height; j++ ) {
    read = 0;
    for( i=0; i<info.width; i++ ) {
        if( fread(&pixel, 1, sizeof(Rgb), inFile) != sizeof(Rgb) ) {
                printf( "Error reading pixel!\n" );
        }
        array[j][i][1] = (int)(pixel->red);
        array[j][i][2] = (int)(pixel->green);
        array[j][i][3] = (int)(pixel->blue);
        read += sizeof(Rgb);
    }

    if ( read % 4 != 0 ) {
        read = 4 - (read%4);
        printf( "Padding: %d bytes\n", read );
        fread( pixel, read, 1, inFile );
    }
}
free(pixel);

return array;

}

Deallocation

void dealloc3D(int*** arr3D,int l,int m)
{
    int i,j;

    for(i=0;i<l;i++)
    {
        for(j=0;j<m;j++)
        {
                free(arr3D[i][j]);
        }
        free(arr3D[i]);
    }
    free(arr3D);
}

I suscpect that the problem lies in casting the RGB values from unsigned char to int, but I see no other way of doing it. If I simply assign integer values to the alloced array there is no problem with freeing them.

Did you learn to correctly use gdb? Did you ask for the backtrace (using the bt command of gdb) when the SIGTRAP occur? Did you compile your code with gcc -Wall -g ? — Basile Starynkevitch
You are corrupting memory. Change fread(&pixel, 1, sizeof(Rgb), inFile) to fread(pixel, 1, sizeof(*pixel), inFile) and read += sizeof(Rgb); to read += sizeof(*pixel);. That way when and/or if the type of pixel changes there will be fewer things to change in your code. — JimR
@BasileStarynkevitch I am compiling with both -Wall and -g without errors and bt full shows me that dealloc3D is called with the correct address to the array and the correct height and width. I am however very new to GDB and I only know the most basic controls. — Simon Thordal
@JimR Thank you for the advice, changing &pixel to pixel did not help sadly. I should also mention that images are read correctly to the array (checked with Matlab). — Simon Thordal
I am such an idiot. I am starting my indices from 1 when assigning the color values. The only reason my program gives a readable output is because I am doing the exactly same thing in a later function that converts the image to grayscale, which led me to believe that error had to be found in the deallocation. Sorry for wasting your time. — Simon Thordal

Bo Persson Bo Persson · Accepted Answer · 2012-04-06T12:38:15

You have a problem with the first fread statement

fread(&pixel, 1, sizeof(Rgb), inFile)

which is reading into the pointer pixel, not into what pixel is pointing to. After that, any use of pixel is likely to corrupt the heap (or something else).

SIGTRAP when freeing a three dimensional array in C

1 Answers