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Comment by agazso | Hacker News Reader

Here is an example that caused me a few bugs.

  // define a new type called md5_t
  typedef char md5_t[33];
  md5_t g_md5;
  // here sizeof(g_md5) == sizeof(md5_t)
  
  void f(md5_t md5)
  {
    // here sizeof(md5) == sizeof(char*)
  }

The type information definitely lost inside functions.

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Simple rule: Don't use sizeof on pointers. And don't typedef arrays to make them look like simple types.

A better simple rule: always use sizeof on typename. That always works therefore you can use typedef arrays too.

Totally agree. If you start relying on the subtle behaviour of sizeof, it's just asking for a world of trouble. It's so easy to get mixed up with whether it's going to return the length of the array, the size of the elements in the array, the total number of bytes the array is using or the size of the pointer. Come to think of it, I think that's completely scope dependent and depends on whether the array has lots its array-ness. I have no idea. :)

This works, sure, but if you do:

        char test[1024][32];
        printf("%i\n", sizeof(test));

You'll get 1024 * 32 (32,768). It didn't know that it's a 2D array. When you pass a stack-allocated array, it passes by pointer. It works this way because C is portable assembler.

There is no logical connection between the use of the `sizeof` operator and differentiating between 1D and 2D arrays, so "it didn't know that it's a 2D array" makes no sense.

Secondly, it's not really meaningful to talk about "pass by pointer". Parameters are generally either passed by value, by reference, or by name. C only does pass by value; to emulate pass by reference, you need to pass the address by value, as a pointer - or as an "array". But the only array type you can declare is missing the crucial dimension length aspect.

Finally, saying that C works the way it does because it's portable assembler does not have strong explaining power. Arrays are not usually machine-level concepts; pointers with appropriate declaration syntaxes for static and automatic block allocations could take their place in C. Since C does have a higher-level construct called an array, it's not a big leap to imagine it being better designed.

sizeof() is used to determine the actual in-memory size of something for use by memcpy/malloc/etc., not the number of elements. There are commonly-used array size macros that can help with this, such as

  #define ARRAY_SIZE(x) sizeof(x)/sizeof((x)[0])

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