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sdcfgy • yesterday at 8:44 AM • 2 replies • view on HN

When I write C I tend to always return any error code on the stack and pass the pointer to where I want the result to go. Makes managing memory easier. The owner gets to allocate.

Example:

  #define E_DIVZERO -100
  int divide(float a, float b, float *result) { 
      if (b == 0.0) return E_DIVZERO;
      *result = a / b;
  }

Replies

sparkie • yesterday at 5:46 PM

If you do:

    typedef struct {
        int status;
        float value;
    } div_result_t;

    div_result_t divide(float a, float b) {
        if (b = 0.0) return (div_result_t){ E_DIVZERO };
        return (div_result_t){ 0, a / b };
    }
On SYSV: The `status` gets returned in x0/rax, and `value` gets returned in x1/xmm0.

On MSVC: The caller allocates `sizeof(div_result_t)` on the stack, passes the the pointer to that space as an implicit first argument to `divide` in register x0/rcx. The function returns the same pointer as its result in x0/rax.

The latter is almost equivalent to you passing a pointer manually - but the compiler does it for you. In the former case, it's actually more efficient to use the struct as we don't even need to touch the stack - all arguments and returns are in registers (for structs up to two "eightbytes" - ie, 16-bytes).

IMO, if it intended to use automatic storage duration, and the size of the struct is small enough, returning by value is preferable - it performs better on SYSV, and basically automates what you would do manually on MSVC. If it is intended for manual storage duration, then yes, use pass and return by pointer explicitly.

quietbritishjim • yesterday at 10:46 AM

I agree, but you're missing the `return E_SUCCESS` at the end :-)

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