On Sun, Aug 4, 2013 at 12:30 AM, Roland Mainz <[email protected]> wrote: > On Sun, Aug 4, 2013 at 12:04 AM, Dan Douglas <[email protected]> wrote: >> Is it specified what the value of x should be after this expression? >> >> x=0; : $((x+=x=1)) >> >> Bash, ksh93, mksh, posh say 1. zsh, dash, busybox say 2. Clang and gcc both >> throw warnings about it, but both plus icc agree on 2. > > Just curious: Is that x86-specific or is the result always the same on > other architectures, too ? Maybe there is something in ISO C1X/C99 > which actually defines or recommends a specific compiler behaviour. > Maybe we should ask the "clang" people about the "why ?" ...
pcc (Portable C Compiler) peole answered first: ---------- Forwarded message ---------- From: Szabolcs Nagy <[email protected]> Date: Sun, Aug 4, 2013 at 5:01 AM Subject: Re: [Pcc] Fwd: Why do all compilers return |x==2| for |x=0; x+=x=1; | despite being undefined behaviour ? To: Roland Mainz <[email protected]> Cc: [email protected] * Roland Mainz <[email protected]> [2013-08-04 01:11:47 +0200]: > Does any know why { gcc4.6, Sun Studio, icc, pcc } always return > |x==2| for |x=0; x+=x=1;| (see forwarded email below) despite gcc's > warning that it may be undefined behaviour ? Why does none of the > compilers return |1| ? Is there anything in the ISO C standard which > mandates this ? no, it is undefined behaviour in iso c, hence the warning (c99 6.5p2) the compilers happen to sequence the evaluation of x=1 and x on the left side of += the same way the program could of course output anything or crash or have the entire thing optimized away (i suspect a recent compiler with sufficiently high optimization level would do the later) ---- Bye, Roland -- __ . . __ (o.\ \/ /.o) [email protected] \__\/\/__/ MPEG specialist, C&&JAVA&&Sun&&Unix programmer /O /==\ O\ TEL +49 641 3992797 (;O/ \/ \O;)
