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Comment by tba | Hacker News Reader

No? It ensures that malloc didn't return a NULL pointer and the '&& "memory error"' is a common pattern to add a comment describing why an assert() statement failed.

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One problem with it is that on most compilers if you compile with optimizations it will remove the assertion. Now the code is no longer guarded against malloc failures and will just segfault. If the intent is to teach people how to handle malloc failures gracefully, it's not that great of an example.

Since virtually no production C code† can actually handle the case of a random malloc() call failing (just like with exceptions in C++, the code to reliably unwind an allocation failure depends on exactly where you're at in your allocation pattern), the simplest and most reliable way of handling malloc() returning NULL is to rig the program to abort.

You're right that assert() isn't the most reliable way to do this (programs have to work whether or not assert() is a no-op; that's the point of assert).

On most platforms, you can just rig malloc to do the abort itself instead of failing --- either with configuration or by preloading a wrapper malloc. Some very, very large shops do exactly this.

Another very common idiom is "xmalloc", which does the malloc/if/abort dance. But xmalloc() misses every place where libraries call malloc(); the most obvious example is strdup(), but the more pernicious issue is 3rd party code that can't know to use x-whatever().

Calling malloc and then immediately assert()'ing success is a reasonable shorthand. That exact same code can be made safe on any mainstream platform just by changing malloc's configuration.

† On general-purpose platforms; I know things are more complicated on embedded platforms.

Good point about the 3rd party libraries there, it's true that even if you do your own cleanup after manual mallocs, you can't do much about theirs.

Do you have an example of documentation that shows how to configure malloc to do some cleanup in the case of failure? Based on the Linux manpages I can't really see any obvious way to do this short of preloading some kind of malloc wrapper that's custom-written for the application.

I think it may depend on the distro (on Ubuntu, for instance, you can set MALLOC_CHECK_ to 2 to force aborts on errors --- but this also forces the use of a debugging malloc, and you should care about malloc performance).

So what I'd recommend is, grab Jason Evans' jemalloc package and use that (it's good enough for Facebook!).

Why do you think the compiler with optimizations will remove the assertion?

The compiler will realise that:

    assert(assert(line != NULL && longest != NULL && "memory error");

Is equivalent to:

    assert(line != NULL && longest != NULL);

But there doesn't seem anything wrong with that.

Production code is supposed to define away the assertions. It is an actual error to manage important program state with asserts.

(That doesn't make these particular asserts an error).

In support of tptacek's comment, an abbreviate version of what's in assert.h on my OSX system:

  #ifdef NDEBUG
  #define assert(e)   ((void)0)
  #else

  #define assert(e)  \
    ((void) ((e) ? 0 : __assert (#e, __FILE__, __LINE__)))
  #define __assert(e, file, line) \
    ((void)printf ("%s:%u: failed assertion `%s'\n", file, line, e), abort())

So, if NDEBUG ("no debug") is defined, which it is if optimizations are turned on, asserts become a no-op.

In the earlier chapters they setup a Makefile with full debugging on so these don't get removed. It's mostly just for this one exercise, and in the rest of the book they use a set of debug macros I wrote:

http://c.learncodethehardway.org/book/learn-c-the-hard-waych...

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