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Comment by rbranson

17 years ago

I don't think arrays are "converted" to pointers. Arrays are simply a cleaner way of doing pointer arithmetic and allocating large(r) blocks of the stack. Nothing is lost in this "conversion." The array never knows it's own dimensions beyond the time you declare it. It's up to the developer to keep track of that.

9 comments

rbranson

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agazso  17 years ago

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.

  • ori_b  17 years ago

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

  • rbranson  17 years ago

    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.

    • barrkel  17 years ago

      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.

    • nitrogen  17 years ago

      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])

ori_b  17 years ago

Arrays are converted to pointers on the first use. You can't ever use an array in C.