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#include <limits.h>
#include <stdio.h>
...
    lval = lval * -1; // this is okay with -fwrapv
    printf("wrap lval  %ld\n", lval);
}

$ gcc -Wall -Wextra -Woverflow -fwrapv -fsanitize=undefined t.c -o t &
+& ./t
t.c: In function ‘main’:
...
 LONG_MAX  9223372036854775807
-LONG_MAX -9223372036854775808
wrap lval  9223372036854775807