http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907
--- Comment #2 from yangzhe1990 at gmail dot com 2012-10-12 11:20:39 UTC --- No, p is not modified twice. p is modified once, *p is modified once. (In reply to comment #1) > Not a bug p is modified twice without a seqence point the result is undefined > > > > ________________________________ > From: yangzhe1990 at gmail dot com <gcc-bugzi...@gcc.gnu.org> > To: gcc-bugs@gcc.gnu.org > Sent: Friday, 12 October 2012, 11:10 > Subject: [Bug c/54907] New: post increasing a value pointed by p in > subexpression of an expression modifying p saves the increased value in the > wrong place > > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907 > > Bug #: 54907 > Summary: post increasing a value pointed by p in subexpression > of an expression modifying p saves the increased value > in the wrong place > Classification: Unclassified > Product: gcc > Version: 4.7.1 > Status: UNCONFIRMED > Severity: normal > Priority: P3 > Component: c > AssignedTo: unassig...@gcc.gnu.org > ReportedBy: yangzhe1...@gmail.com > > > #include <stdio.h> > > int main() { > char s[] = "axxxxx"; > char *p = s; > > printf("s = %s in the beginning.\n" > "p is pointed at the %d-th char.\n", s, p - s); > //p = p + (*p)++ * 3 + 2 - 'a' * 3; // (1) > p += (*p)++ * 3 + 2 - 'a' * 3; // (2) > printf("p is moved ahead by %d steps\n", p - s); > printf("s = %s after the operation.\n", s); > return 0; > } > > The expected result is "bxxxxx". But the output is "axbxxx". > > Maybe in the wrong code, when it saves the value, it lookups the address again > by *p, but p is modified in the expression. > > As discussed in stackoverflow, > http://stackoverflow.com/questions/12823663/would-p-p-p-3-c-cause-an-undefined-behavior?answertab=votes#tab-top > most people think it's a bug of gcc. > > Bug found in gcc 4.4.6, 4.7.1, g++ 4.4.6. g++ 4.7.1 produces the correct > result.
