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lukas239955_stm1_stmicro_com
lukas239955_stm1_stmicro_com
Associate II
August 21, 2012
Question

char to short - unexpected casting behavior

  • August 21, 2012
  • 2 replies
  • 701 views
Posted on August 21, 2012 at 19:45

Hi all,

I use STM32F205xx for my project but I did the same results with STM32VL Discovery kit.

I debug in Atollic TrueSTUDIO 2.3.0 Lite and I tried TrueSTUDIO 3.1. too.

The problem is that code 

  char a = -50;

  short b = a;

does cast negative values correctly. Internal representation of char should be signed and therefore b variable should be -50 and not 206, which is the result.

I checked this in VisualStudio and the result was -50 as expected. Another brand controller does it as expected.

I checked also disassembly listing. The code for other controller uses signed extend of data for this cast. Atollic makes a simple copy without signed extension.

Do I understand something wrong or is it a problem of a compiler?

How to deal with it correctly? Is it about settings of the compiler?

Thank you for your reactions in advance
    This topic has been closed for replies.

    2 replies

    Tesla DeLorean
    Tesla DeLorean
    Guru
    August 21, 2012
    Posted on August 21, 2012 at 20:38

    Probably because GCC thinks 'char' is 'unsigned char' by default.

    The standard does not specify if plain 

    char

    is signed or unsigned. Check the values of CHAR_MIN and CHAR_MAX as defined in limits.h, and if __CHAR_UNSIGNED__ is defined or not.

    http://www.linuxtopia.org/online_books/an_introduction_to_gcc/gccintro_html

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    lukas239955_stm1_stmicro_com
    lukas239955_stm1_stmicro_comAuthor
    Associate II
    August 22, 2012
    Posted on August 22, 2012 at 12:29

    Thank you for the quick reply.

    Yes, I expected char to be specified as signed, but it is not.

    So I will continue using ''signed char'' type declaration instead of ''char''

    Btw. when calling a functions like this, the compiller warns me about difference in signednes.

    void function (char * c);

     

     

    unsigned char aaa;

     

    function (&aaa);

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