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Re: [Bug-glpk] -Wstringop-overflow in function _glp_wclique1


From: Andrew Makhorin
Subject: Re: [Bug-glpk] -Wstringop-overflow in function _glp_wclique1
Date: Sun, 01 Oct 2017 10:17:30 +0300

Hi Heinrich,

Thank you for your bug report.

> compiling GLPK 4.63 on arm64 with gcc 7.2 gives the following warning:
> 
> misc/wclique1.c: In function ‘_glp_wclique1’:
> misc/wclique1.c:121:7: warning: ‘memset’: specified size between
> 18446744071562067968 and 18446744073709551615 exceeds maximum object
> size 9223372036854775807 [-Wstringop-overflow=]
>        memset(&skip[1], 0, sizeof(char) * n);
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> misc/wclique1.c:123:7: warning: ‘memset’: specified size between
> 18446744071562067968 and 18446744073709551615 exceeds maximum object
> size 9223372036854775807 [-Wstringop-overflow=]
>        memset(&d_flag[1], 0, sizeof(char) * n);
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 

Please ignore this warning message. The compiler makes a wrong
conclusion that n declared as int can be negative and being converted to
size_t which is unsigned long can result in a huge value (for example,
(size_t)-1 results in 2^64-1). This however cannot happen due to a check
xassert(n >= 0); in line 90.

> It can be avoided by first converting to unsigned int:
> 
> 120       /* initially all vertices are unmarked */
> 121       memset(&skip[1], 0, sizeof(char) * (unsigned int)n);
> 122       /* clear flags of all vertices */
> 123       memset(&d_flag[1], 0, sizeof(char) * (unsigned int)n);
> 

Then on a 32-bit platform there will be a similar message, because
converting -1 to unsigned int gives 2^32-1.


Best regards,

Andrew Makhorin




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