http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907
--- Comment #1 from graham.stott at btinternet dot com 2012-10-12 10:24:30 UTC --- 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.
