Comments on: Effective C Tip #7 – Use strongly typed function parameters

By: Anders

Anders — Fri, 06 May 2011 07:03:22 +0000

“ISO 8601 Data elements and interchange formats — Information interchange — Representation of dates and times”

By: Nigel Jones

Nigel Jones — Sat, 30 Apr 2011 00:01:12 +0000

That’s very interesting Jorg. I don’t recollect having seen this before.

By: Jörg Seebohn

Jörg Seebohn — Fri, 29 Apr 2011 15:26:55 +0000

The typedef SERIAL_STR is not standard:

You should declare it like:

typedef char SERIAL_STR[16] ;

Every array parameter is treated as a pointer parameter.

But there is a trick if you want to let the compiler determine the correct array size:

void test_func(SERIAL_STR * param)
{
printf(“sizeof(*param)=%u”, sizeof(*param)) ;
}

// prints sizeof(*param)=16

By: Nigel Jones

Nigel Jones — Fri, 29 Apr 2011 14:32:34 +0000

Hmmm, that may well explain it. I normally find GCC to be very good on issues such as this, so perhaps it’s something Microchip has done as a ‘special extension’?

By: Dirk

Dirk — Fri, 29 Apr 2011 13:48:10 +0000

I’m using the Microchip C compiler for dsPIC30 (based on gcc version 4.0.3).

By: Nigel Jones

Nigel Jones — Fri, 29 Apr 2011 12:42:34 +0000

The same thought had crossed my mind as well. I just dropped the code into my IAR ARM compiler. It gives the following errors on your typedef:
Error[Pe040]: expected an identifier test.c 114
Error[Pe065]: expected a “;” test.c 114
What compiler are you using?

By: Dirk

Dirk — Fri, 29 Apr 2011 11:53:03 +0000

But wouldn’t it be more reasonable to give a compile error? It seems wrong to me that they are different although they are “declared” in the same way.

By: Nigel Jones

Nigel Jones — Fri, 29 Apr 2011 10:14:57 +0000

I’m glad you find the articles useful. While a typedef allows you to create a shorthand for a data type, it doesn’t allow you to break the language rules. In C you can’t pass an array to a function by value – only by reference. Thus given that SERIAL_STR is typedef’d to be char[16], the compiler is simply converting your attempt to pass by value into a pass by reference – i.e. a pointer

By: Dirk

Dirk — Fri, 29 Apr 2011 09:55:33 +0000

Hi Nigel!
I learned a lot from your articles, thank you very much!
So after reading this article about strongly typed parameters, I stumbled upon a problem with typedefs as function parameters. See this code:

typedef char[16] SERIAL_STR;

void test_func(SERIAL_STR param)
{
SERIAL_STR local_var;
printf(“sizeof(param)=%u\n”,sizeof(param));
printf(“sizeof(SERIAL_STR)=%u\n”,sizeof(SERIAL_STR));
printf(“sizeof(local_var)=%u\n”,sizeof(local_var));
}

On a 16bit dsPIC the output is:
sizeof(param)=2
sizeof(SERIAL_STR)=16
sizeof(local_var)=16

When the typedef is a function parameter, it seems to be treated like a pointer by the compiler.

By: Nigel Jones

Nigel Jones — Sat, 29 Jan 2011 20:37:56 +0000

I tend to think of enumerated values as ‘typed constants’. Like you because they are constant I think they should be capitalized. The type side of enumerations I don’t usually worry about too much, in part because I have Lint set up to complain loudly about questionable type conversions involving enumerated types.