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Discussion on: I'm an Expert in Memory Management & Segfaults, Ask Me Anything!

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liulk profile image
Likai Liu

My approach for this specific problem is to use a compiler that warns about unused return value, such as gcc or clang. I know that stdlib.h on Linux and Mac OS X already decorates realloc() with warn_unused_result attribute.

stackoverflow.com/a/2889601

But just naively setting p = realloc(p, ...) is also wrong, since if the allocation fails, p would be set to NULL but the original object is still allocated. The original pointer is lost and now a memory leak. Use reallocf() which frees the original memory if it could not be resized.

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codevault profile image
Sergiu Mureşan

That's a really nice feature, didn't know about it.

But wouldn't that mean data loss in case the memory can't be resized? Wouldn't that become an unrecoverable error?

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codemouse92 profile image
Jason C. McDonald Ask Me Anything

@liulk Ha, I completely forgot to mention Clang! It does indeed have the best warnings of any compiler I've used. I almost always compile with -Wall -Wextra -Wpedantic -Werror; that last one (as you know, although the reader might not) causes the build to fail on any warnings.

I also use cppcheck as part of my autoreview workflow, and resolve all linter warnings before committing to the production branch.

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liulk profile image
Likai Liu

@codevault You're right, reallocf() would just free the memory and cause data loss, so it would serve a different use case than realloc(). The more general solution would be to always use this pattern, which is more verbose:

void *q = realloc(p, new_size);
if (q == NULL) {
  // do error handling.
  return;
}
p = q;

I just find that in most of my use cases, I would end up freeing p in the error handling, so I would just use reallocf() which results in less verbose code.

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codevault profile image
Sergiu Mureşan

I see, that makes sense. I can see myself freeing the memory most of the time when reallocation fails.

Good to note. Thanks!