C Preprocessor output int at Build

Question

i want to see output of Preprocessor calculating. Only with strings it works so:

#define XSTR(x) STR(x)
#define STR(x) #x

#define BEGIN "test"

#pragma message ".text= " XSTR(BEGIN)

when i set BEGIN to 32/2 the output is: #pragma message: .text= 32/2.

What can i make to solve this? I don´t won´t solutions like this to search in the .lss file:

uint16_t x = BEGIN;
PORTB = x>>8;
PORTA = x;

Thank you.

What do you *expect* the output to be when you define `BEGIN` to `32/2`? — John Bollinger, Nov 02 '21 at 13:01
The C preprocessor will only calculate things inside `#if` conditions. You won't get it to calculate and print something (let's ignore abominations like BOOST_PP). See also: https://stackoverflow.com/questions/1560357/can-the-c-preprocessor-perform-integer-arithmetic — , Nov 02 '21 at 13:08
@JohnBollinger an .lss file is the Assembler Output file generated from AVR-GCC. The Value 32/2 is only an example. Later this calculate the .text section. — Umbrecht, Nov 02 '21 at 13:12
Do not use double quote when defining BEGIN : #define BEGIN 32/2 — Ptit Xav, Nov 02 '21 at 14:11
@PtitXav i don´t used double quote for calculations. Only for Strings — Umbrecht, Nov 02 '21 at 14:26
What preprocessor do you use ? With clang I get : #pragma message(“.text= 32/2”) — Ptit Xav, Nov 02 '21 at 14:35
Compiler Version = 5.4.0, Preprocessor i dont know. I use the build in from Atmel Studio 7. Does it work for you? I don´t found a way to solve it easily. — Umbrecht, Nov 02 '21 at 14:55

score 2 · Accepted Answer · answered Nov 04 '21 at 13:56

What can i make to solve this?

You have to implement math operations in the preprocessor. For example:

#define _div_1_over_1  1
#define _div_2_over_1  2
#define _div_3_over_1  3
// etc...
#define _div_30_over_2  15
#define _div_31_over_2  15
#define _div_32_over_2  16
// etc...
// etc. for every possible a_over_b combinations, many many 1000 lines
#define DIV(a, b)  _div_##a##_over_##b

After that, you finally can:

#define XSTR(x) STR(x)
#define STR(x) #x

#define BEGIN DIV(32, 2)

#pragma message ".text= " XSTR(BEGIN)

Let's say this is one way of implementing division in preprocessor. For other solutions, see BOOST_PP_DIV and P99_DIV from p99 library.

emacs drives me nuts · Answer 2 · 2021-11-04T17:27:34.427

1

The C preprocessor just performs text replacement(s).

The first opportunity to see folded constants is in the tree dumps with -fdump-tree-all and then inspect the *.t<nnn> dumps.

To get the asm output from the compiler proper, try -safe-temps -fverbose-asm which inserts C source intermingled as asm comments in the *.s file.

edited Nov 04 '21 at 17:27

answered Nov 04 '21 at 13:12

emacs drives me nuts

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C Preprocessor output int at Build

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