Undefined behavior when working with partially initialized struct in C90

Question

Let's consider the following code:

struct M {
  unsigned char a;
  unsigned char b;
};

void pass_by_value(struct M);

int main() {
  struct M m;
  m.a = 0;
  pass_by_value(m);
  return 0;
}

In the function pass_by_value m.b is initialized before used. However, since m is passed by value the compiler copies it to the stack already. No variable has storage class register here. a and b are of type unsigned char.

Does that have to be considered UB in C90? (Please note: I am specifically asking for C90)

This question is very similar to Returning a local partially initialized struct from a function and undefined behavior, but actually the other way around.

Answer 1

The C 1990 standard (and the C 199 standard) does not contain the sentence that first appears in C 2011 that makes the behavior of using some uninitialized values undefined.

C 2011 6.3.2.1 2 includes:

… If the lvalue has an incomplete type and does not have array type, the behavior is undefined. If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.

The whole of the corresponding paragraph in C 1990, clause 6.2.2.1, second paragraph, is:

Except when it is the operand of the sizeof operator, the unary & operator, the ++ operator, the -- operator, or the left operand of the . operator or an assignment operator, an lvalue that does not have array type is converted to the value stored in the designated object (and is no longer an lvalue). If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the lvalue has an incomplete type and does not have array type, the behavior is undefined.

Therefore, the behavior of the code in the question would seem to be defined, inasmuch that it passes the value stored in the structure.

In the absence of explicit statements in the standard, common practice helps guide interpretation. It is perfectly normal not to initialize all members of a structure yet to expect the structure to represent useful data, and therefore the behavior of using the structure as a value must be defined if at least one of its members is initialized. The equivalent question for C 2011 contains mention (from a C defect report) of the standard struct tm in one of its answers. The struct tm may be used to represent a specific date by filling in all of date fields (year, month, day of month) and possibly the time fields (hour, minute, second, even Daylight Savings Time indication) but leaving the day of week and day of year fields uninitialized.

In defining undefined behavior in 3.16, the 1990 standard does say it is “Behavior, upon use … of indeterminately valued objects, for which this International Standard imposes no requirements.” And 6.5.7 says “… If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate…” However, a structure with automatic storage duration in which one member, but not another, has been initialized is neither fully initialized nor not initialized. Given the intended uses of structures, I would say we should not consider use of the value of a partially initialized structure to be subject to being made undefined by 3.16.

Answer 2

Under C90, if an object held Indeterminate Value, each and every bit could independently be zero or one, regardless of whether or not they would in combination represent a valid bit pattern for the object's type. If an implementation specified the behavior of attempting to read each and every one of the 2ⁿ individual possible bit patterns an object could hold, the behavior of reading an object with Indeterminate Value would be equivalent to reading the value of an arbitrarily chosen bit pattern. If there were any bit patterns for which an implementation did not specify the effect of an attempted read, then the effects of trying to read an object that might hold such bit patterns would be likewise unspecified.

Code generation efficiency could be improved in some cases by specifying the behavior of uninitialized objects more loosely, in a way which would not otherwise be consistent with sequential program execution as specified but would nonetheless meet program requirements. For example, given something like:

struct foo { short dat[16]; } x,y,z;
void test1(int a, int b, int c, int d)
{
  struct foo temp;
  temp.dat[a] = 1;
  temp.dat[b] = 2;
  temp.dat[c] = 3;
  temp.dat[d] = 4;
  x=temp;
  y=temp;
}
void test2(int a, int b, int c, int d)
{
  test1(a,b,c,d);
  z=x;
}

If client code only cares about the values of x and y that correspond to values of temp that were written, efficiency might be improved, while still meeting requirements, if the code were rewritten as:

void test1(int a, int b, int c, int d)
{
  x.dat[a] = 1;
  y.dat[a] = 1;
  x.dat[b] = 2;
  y.dat[b] = 1;
  x.dat[c] = 3;
  y.dat[c] = 1;
  x.dat[d] = 4;
  y.dat[d] = 1;
}

The fact that the original function test1 doesn't do anything to initialize temp suggests that it won't care about what is yielded by any individual attempt to read it. On the other hand, nothing within the code for test2 would imply that client code wouldn't care about whether all members of x held the same values as corresponding values of y. Thus, such an inference would more likely be dangerous there.

The C Standard makes no attempt to define behavior in situations where an optimization might yield program behavior which, although useful, would be inconsistent with sequential processing of non-optimized code. Instead, the principle that optimizations must never affect any defined behavior is taken to imply that the Standard must characterize as Undefined all actions whose behavior would be visibly affected by optimization, leaving implementor discretion the question of what aspects of behavior should or should not be defined in what circumstances. Ironically, the only time the Standard's laxity with regard to this behavior would allow more efficient code generation outside contrived scenarios would be in cases where implementations treat the behavior as at least loosely defined, and programmers are able to exploit that. If a programmer had to explicitly initialize all elements of temp to avoid having the compiler behave in completely nonsensical fashion, that would eliminate any possibility of optimizing out the unnecessary writes to unused elements of x and y.