Question

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

main()
{
   int a=5;
   a= a++ + ++a + ++a + a++ + a++;
   printf("%d",a);
}

Answer 1

This is not defined.

You can find the Committee Draft from May 6, 2005 of the C-standard here (pdf)

See section 6.5 Expressions:

2 Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.

and the example:

71) This paragraph renders undefined statement expressions such as

i = ++i + 1;

a[i++] = i;