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Is it guaranteed safe/portable to use the address of a function parameter on a C89/C99-compliant compiler?

As an example, the AAPCS for 32-bit ARM uses registers r0-r3 for parameter passing if the function parameters meet specific size and alignment requirements. I would assume that using the address of a parameter passed through a register would yield unexpected results, but I ran a test on the ARM compiler I'm using and it appears to relocate these parameters to the stack if the code attempts to reference the addresses of these parameter. While it would appear safe in my particular application, I'm wondering if this is guaranteed across architectures (with an ANSI/ISO-compliant compiler) that can utilize registers directly to pass function parameters.

Do the standards define this behavior?

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    This is allowed under the ISO C standard Commented Dec 29, 2015 at 21:26
  • What M.M. said. I'm more curious how would the calling code which is possibly in a separate source file and compiled separately, know when to pass by register vs. pass by stack. Or does ARM always assume pass by register when the size/alignment requirements are met by the function signature (and the invoked code takes care of moving to stack)? Commented Dec 29, 2015 at 21:32
  • The arguments are passed via the suitable registers according to the ABI. Arguments whose address is taken within the body of the function are then migrated to automatic storage (aka on the stack). Commented Dec 29, 2015 at 21:40
  • Function arguments are normal variables inside the function. Unless you use the register storage specifier, the standard guarantees you can take their address. Commented Dec 29, 2015 at 22:37
  • @selbie: That does not change the interface, as that is defined by the ABI (for modern ARM the AAPCS). Internals are hidden inside the function itself. Commented Dec 29, 2015 at 22:47

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In C, the only lvalues you cannot take addresses of are bitfields (which cannot appear in function parameters) and variables or function parameters of register storage class. It is perfectly safe to take the address of a parameter, but keep in mind that arguments are passed by value, thus you must make sure that you don't use the address of a local variable or parameter once its life time ends.

Generally, the compiler has a pass where it checks which local variables and parameters are operands to unary & operators. These are then copied to a suitable piece of RAM when appropriate. The calling convention does not affect this.

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3 Comments

The second paragraph is technically correct for many implementations and a good idea to add here. However, one should emphasise, that argument passing and details how the address is taken is not covered by the C standard. There are architectures which allow CPU registers to be accessed through normal memory reads or which do not even use registers to pass arguments (some don't even have registers available), etc.
@Olaf I don't know why you need to emphasize this. Doesn't the wording “Generally...” make clear that there are platforms on which the mechanism is different? And even on platforms that map register in memory, at least if the address is passed to another function it cannot point to a register as the callee might decide to use this register for its own purpose.
I tried to emphasize that the C standard does not mandate it. "Generally" is questionable, as that is a matter of the platform and there are some platforms which don't behave that way. I would not bet which has more implementations keeping in mind the variety of CPU architectures (don't for get about the embedded world, which just starts to be streamlined by the Cortex-M - be it for good or bad). If such an aliased address is passed on is a different subject and requires other measures.

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