Strange values while initializing array using designated initializers

Question

When I initialize the array below all the output looks ok except for values[3]. For some reason values[3] initialized as values[0]+values[5]

Answer 1

It looks like you are subject to unspecified behavior here, since the order of evaluation of the initialization list expressions is unspecified, from the draft C99 standard section 6.7.8:

The order in which any side effects occur among the initialization list expressions is unspecified.¹³³⁾

and note 133 says:

In particular, the evaluation order need not be the same as the order of subobject initialization.

As far as I can tell, the normative text that backs up note 133 would be from section 6.5:

Except as specified later [...] the order of evaluation of subexpressions and the order in which side effects take place are both unspecified.

and we can see that an intializer is a full-expression from 6.8 (emphasis mine):

A full expression is an expression that is not part of another expression or of a declarator. Each of the following is a full expression: an initializer; [...]

After looking back at one of my old C++ answers that covered sequence points within an initializer and which places the full-expression in a different place then I originally concluded, I realized the grammar in 6.7.8 contained initializer twice:

initializer:
    assignment-expression
    { initializer-list }
    { initializer-list , }
initializer-list:
    designationopt initializer
    initializer-list , designationopt initializer

I originally did not notice this and thought the statement on full-expressions applied to the top element in the above grammar.

I now believe like C++ the full-expression applies to each initializer within the initializer-list which make my previous analysis incorrect.

Defect report 439 confirmed my suspicion that this was indeed the case, it contains the following example:

#include 

#define ONE_INIT      '0' + i++ % 3
#define INITIALIZERS      [2] = ONE_INIT, [1] = ONE_INIT, [0] = ONE_INIT

int main()
{
    int i = 0;
    char x[4] = { INITIALIZERS }; // case 1
    puts(x);
    puts((char [4]){ INITIALIZERS }); // case 2
    puts((char [4]){ INITIALIZERS } + i % 2); // case 3
}

and it says:

In every use of the INITIALIZERS macro, the variable i is incremented three times. In cases 1 and 2, there is no undefined behavior, because the increments are in expressions that are indeterminately sequenced with respect to one another, not unsequenced.

so each intializer within INITIALIZERS is a full-expression.

Since this defect report is against C11 it is worth noting that C11 is more verbose then C99 in the normative text on this issue and it says:

The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified.¹⁵²⁾

There is undefined behavior in the case where the following expressions are evaluated before the respective elements in values are assigned to:

 values[0] + values[5]

or:

 values[5]/10

This is undefined behavior since using an indeterminate value invokes undefined behavior.

In this specific case the simplest work-around would be to perform the calculations by hand:

int values[10] = { 
    [0]=197,[2]=-100,[5]=350,
    [3]= 197 + 350,
    [9]= 350/10
};

There are other alternatives such as doing the assignments to element 3 and 9 after the initialization.