C++ – Memory management and realloc

cmemory-leaksmemory-management

I'm going through my program with valgrind to hunt down memory leaks. Here's one that I'm not sure what to do with.

==15634== 500 (224 direct, 276 indirect) bytes in 2 blocks are definitely lost in loss record 73 of 392
==15634==    at 0x4007070: realloc (vg_replace_malloc.c:429)
==15634==    by 0x807D5C2: hash_set_column(HASH*, int, char const*) (Hash.cpp:243)
==15634==    by 0x807BB15: LCD::PluginDiskstats::PluginDiskstats() (PluginDiskstats.cpp:102)
==15634==    by 0x806E021: LCD::Evaluator::Evaluator() (Evaluator.cpp:27)
==15634==    by 0x8066A87: LCD::LCDControl::LCDControl() (LCDControl.h:16)
==15634==    by 0x80667F5: main (Main.cpp:8)

Here's the code:

/* add an entry to the column header table */
void hash_set_column(HASH * Hash, const int number, const char *column)
{
    if (Hash == NULL)
        return;

    Hash->nColumns++;
    Hash->Columns = (HASH_COLUMN *)realloc(Hash->Columns, Hash->nColumns * sizeof(HASH_COLUMN)); // line 243
    Hash->Columns[Hash->nColumns - 1].key = strdup(column);
    Hash->Columns[Hash->nColumns - 1].val = number;

    qsort(Hash->Columns, Hash->nColumns, sizeof(HASH_COLUMN), hash_sort_column);

}

Should I be doing something here in regards to memory management?

Best Answer

The problem is that if realloc() fails the function will return NULL but the original block will still be allocated. However, you've just overwritten the pointer to that block and can't free (or use) it anymore.

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

C++ – How to set, clear, and toggle a single bit

Setting a bit

Clearing a bit

Toggling a bit

Checking a bit

Changing the nth bit to x

Related Topic