pset5 invalid reads/writes

Question

My speller programs compile and run with no issues however valgrind doesn't seem to think so. I initially used malloc and manually initialised each pointer to null and the boolean to false. I have since switched to calloc and i'm still getting the same errors. This is valgrind's output:

==16497== Memcheck, a memory error detector

==16497== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.

==16497== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info

==16497== Command: ./speller /home/cs50/pset5/texts/quote.txt

==16497== Parent PID: 3464

==16497==

==16497== Invalid read of size 4

==16497== at 0x80490AC: load (dictionary.c:78)

==16497== by 0x8048725: main (speller.c:45)

==16497== Address 0x71f4fe8 is not stack'd, malloc'd or (recently) free'd

==16497==

==16497== Invalid write of size 4

==16497== at 0x80490E4: load (dictionary.c:80)

==16497== by 0x8048725: main (speller.c:45)

==16497== Address 0x71f4fe8 is not stack'd, malloc'd or (recently) free'd

==16497==

==16497== Invalid read of size 4

==16497== at 0x80490EE: load (dictionary.c:83)

==16497== by 0x8048725: main (speller.c:45)

==16497== Address 0x71f4fe8 is not stack'd, malloc'd or (recently) free'd

==16497==

==16497==

==16497== HEAP SUMMARY:

==16497== in use at exit: 576 bytes in 3 blocks

==16497== total heap usage: 367,047 allocs, 367,044 frees, 41,109,744 bytes allocated

==16497==

==16497== 224 (112 direct, 112 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 3

==16497== at 0x402C109: calloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)

==16497== by 0x80490DD: load (dictionary.c:80)

==16497== by 0x8048725: main (speller.c:45)

==16497==

==16497== LEAK SUMMARY:

==16497== definitely lost: 112 bytes in 1 blocks

==16497== indirectly lost: 112 bytes in 1 blocks

==16497== possibly lost: 0 bytes in 0 blocks

==16497== still reachable: 352 bytes in 1 blocks

==16497== suppressed: 0 bytes in 0 blocks

==16497== Reachable blocks (those to which a pointer was found) are not shown.

==16497== To see them, rerun with: --leak-check=full --show-leak-kinds=all

==16497==

==16497== For counts of detected and suppressed errors, rerun with: -v

==16497== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)

Here's my code:

root = calloc(1, sizeof(node));
node* current = root;

for (int c = fgetc(dp); c != EOF; c = fgetc(dp))
{
    if (c != '\n')
    {
        int index = c - '\'';

        // This is line 78 (the first valgrind invalid read):
        if (current->children[index] == NULL)
        {
            // line 80 (invalid write)
            current->children[index] = calloc(1, sizeof(node));
        }
        // 83 (invalid read)
        current = current->children[index];
    }
    else
    {
        current->is_word = true;
        count++;
        current = root;
    }
}

if(!ferror(dp))
{
    loaded = true;
    return true;
}
else
{
    loaded = false;
    return false;
}
fclose(dp);

Answer 1

I would say that your problem has to do with the value of index, not calloc or malloc. Look at how you are setting index:

int index = c - '\'';

The result for any lower case letter will be index = c - 39, or 58 through 83. My guess is that your array runs from 0 to 26, so you would be trying to access invalid elements in the array in the node. Did you remove a block of code that tests for apostrophes by accident?

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