Strange malloc

This is a discussion on Strange malloc within the C Programming forums, part of the General Programming Boards category; I am allocating this 2d array and well, let me post the code first: Code: struct cell { int oldPos, ...

  1. #1
    Obsessed with C chrismiceli's Avatar
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    Strange malloc

    I am allocating this 2d array and well, let me post the code first:
    Code:
    struct cell {
            int oldPos, newPos;
    };
    int main(void) {
            int x, y;
            struct cell **world;
    
            world = malloc(LINES * sizeof (struct cell*));
            for(x = 0;x < COLS; x++) {
                    world[x] = malloc(COLS * sizeof (struct cell));
            }
            for(x = 0; x < COLS; x++) {
                    for(y = 0; y < LINES; y++) {
                            world[x][y].newPos = 0;
                    }
            }
            return 0;
    }
    I know that I am not checking return values or all of that good stuff, and LINES and COLS are defined in a header as say 80 and 24 respectively. When I run the program, it segfaults when x reaches 27 in the assignment loop, but using a debugger, I found world[28][0] was valid. It appears that every other element past 27 is valid? Here is the gdb output from some commands.
    Quote Originally Posted by gdb
    (gdb) print world[27][0]
    Cannot access memory at address 0x0
    (gdb) print world[28][0]
    $3 = {oldPos = 0, newPos = 0}
    (gdb) print world[29][0]
    Cannot access memory at address 0x0
    (gdb) print world[30][0]
    $4 = {oldPos = 0, newPos = 0}
    (gdb) print world[31][0]
    Cannot access memory at address 0x0
    (gdb) print world[32][0]
    $5 = {oldPos = 0, newPos = 0}
    (gdb) print world[33][0]
    Cannot access memory at address 0x0
    (gdb) print world[34][0]
    $6 = {oldPos = 0, newPos = 0}
    (gdb) print world[35][0]
    Cannot access memory at address 0x0
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  2. #2
    Yes, my avatar is stolen anonytmouse's Avatar
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    You've got your constants (LINES and COLS) all mixed up. Try replacing the constants temporarily with their values to better see the mix up.

  3. #3
    Obsessed with C chrismiceli's Avatar
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    Thanks, I got it because of you, I should have had this for the mallocing
    Code:
    */.../*
    world = malloc(LINES * sizeof (struct cell*));
            for(x = 0;x < LINES; x++) {
                    world[x] = malloc(COLS * sizeof (struct cell));
            }
    /*...*/
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  4. #4
    Yes, my avatar is stolen anonytmouse's Avatar
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    You also need to swaps LINES and COLS here.
    Code:
            for(x = 0; x < COLS; x++) {
                    for(y = 0; y < LINES; y++) {
                            world[x][y].newPos = 0;
                    }
            }

  5. #5
    Obsessed with C chrismiceli's Avatar
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    Yeah, I fixed it there too, but I didn't post it. I just wanted the memory to be finally allocated right. Thanks again.
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