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Consequences of passing too few register parameters to a C function

Consequences of passing too few register parameters to a C function

aragonite

Comments (23)

  • bananamogul
    Raymend Chen has probably forgotten more about programming than I'll ever know, but aren't the first two blah() function examples either missing a } or have a superfluous { after the else?
  • CodeArtisan
    Until C23, you could declare a pointer to a procedure that takes an unspecified amount of any type arguments like this void foo( int (*f)() ) { f(1); f(1, "2" , 3.0); } https://godbolt.org/z/s6e5rnqv9If you compile with -std=c23, both gcc and clang will throw an error ( (*f)() is now the same as (*f)(void) )
  • charleslmunger
    I had fun exploiting this to detect the falling convention used by some code at runtime - there were two different options depending on OS version; one passed a jnienv* as the first param, the other did not. So if I called it with 0, I could tell which was being used based on whether the first argument was NULL or not. Only used for specific architectures with a defined ABI that behaved this way, of course.
  • anitil
    I had never considered the idea of passing too few register params so I didn't immediately think of the reuse problem. And I had no idea about Itanium's Not-a-thing bit! Always a good read from Raymond Chen.
  • _kst_
    It's not even possible to pass too few arguments to a function in C unless you go out of your way to write bad code.You can write a function declaration that's inconsistent with its definition in another translation unit. Declaring the function in a shared header file avoids this.You can use an old-style declaration that doesn't specify what parameters a function expects. Don't do that. Use prototypes.You can use a cast to convert a function pointer to an incompatible type, and call through the resulting pointer. Don't do that.You can call a function with no visible declaration if your compiler overly permissive or is operating in pre-C99 mode. Don't do that.
  • LelouBil
    Interesting that some CPUs have a calling convention "built-in"
  • hyperhello
    Do you really not ‘pass’ register parameters? How can anyone tell if you didn’t?
  • rurban
    Of which decade is this post? I cannot think of any modern architecture which still passes args on the stack.Itanium? Stone age
  • 9fwfj9r
    I regard this yet another unintuitive Itanium quirk that makes it failed.
  • marlburrow
    [flagged]