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C Problem?

Jim Leifker 

unsigned char x=1;
x=(x++)<<3;

Using uVision 6.12
Gives answer of 8
It shifts binary 1 left and doesn't increment.
(even when optimization set to level 0)

Using Micorsoft C++ 6.0
Gives answer of 9
It shifts binary 1 left then increments.


I was a little confused with both answers because I was thought that
the increment would happen first because of the ( ) around x++ then
shift left three giving 16 but that's not the case.

This gives me 16 in both Keil and MS.
x=(++x)<<3; works it gives 16


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  • 0 in reply to Michael Fielding

    This ambiguity is actually rather unrelated to the one discussed here earlier. The earlier examples had much more serious problems than yours --- they produced undefined behaviour, which means the compiler is allowed to explode the processor 123*pi hours after noon of the Tuesday following last year's Hallowe'en, if it so pleases.

    Your example is broken only because introduced an additional ingredient into the game: multiple (pseudo-)concurrent threads of control. C doesn't even try to address that type of problem in the language itself, beyond controlling access to individual objects via the volatile qualifier.

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  • 0 in reply to Michael Fielding

    This ambiguity is actually rather unrelated to the one discussed here earlier. The earlier examples had much more serious problems than yours --- they produced undefined behaviour, which means the compiler is allowed to explode the processor 123*pi hours after noon of the Tuesday following last year's Hallowe'en, if it so pleases.

    Your example is broken only because introduced an additional ingredient into the game: multiple (pseudo-)concurrent threads of control. C doesn't even try to address that type of problem in the language itself, beyond controlling access to individual objects via the volatile qualifier.

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