What treatment can a pointer undergo and still be valid? - c

Question

Which of the following ways of treating and trying to recover a C pointer, is guaranteed to be valid?
1) Cast to void pointer and back
int f(int *a) {
void *b = a;
a = b;
return *a;
}
2) Cast to appropriately sized integer and back
int f(int *a) {
uintptr_t b = a;
a = (int *)b;
return *a;
}
3) A couple of trivial integer operations
int f(int *a) {
uintptr_t b = a;
b += 99;
b -= 99;
a = (int *)b;
return *a;
}
4) Integer operations nontrivial enough to obscure provenance, but which will nonetheless leave the value unchanged
int f(int *a) {
uintptr_t b = a;
char s[32];
// assume %lu is suitable
sprintf(s, "%lu", b);
b = strtoul(s);
a = (int *)b;
return *a;
}
5) More indirect integer operations that will leave the value unchanged
int f(int *a) {
uintptr_t b = a;
for (uintptr_t i = 0;; i++)
if (i == b) {
a = (int *)i;
return *a;
}
}
Obviously case 1 is valid, and case 2 surely must be also. On the other hand, I came across a post by Chris Lattner - which I unfortunately can't find now - saying something similar to case 5 is not valid, that the standard licenses the compiler to just compile it to an infinite loop. Yet each case looks like an unobjectionable extension of the previous one.
Where is the line drawn between a valid case and an invalid one?
Added based on discussion in comments: while I still can't find the post that inspired case 5, I don't remember what type of pointer was involved; in particular, it might have been a function pointer, which might be why that case demonstrated invalid code whereas my case 5 is valid code.
Second addition: okay, here's another source that says there is a problem, and this one I do have a link to. https://www.cl.cam.ac.uk/~pes20/cerberus/notes30.pdf - the discussion about pointer provenance - says, and backs up with evidence, that no, if the compiler loses track of where a pointer came from, it's undefined behavior.

Answer 1

According to the C11 draft standard:
Example 1
Valid, by §6.5.16.1, even without an explicit cast.
Example 2
The intptr_t and uintptr_t types are optional. Assigning a pointer to an integer requires an explicit cast (§6.5.16.1), although gcc and clang will only warn you if you don’t have one. With those caveats, the round-trip conversion is valid by §7.20.1.4. ETA: John Bellinger brings up that the behavior is only specified when you do an intermediate cast to void* both ways. However, both gcc and clang allow the direct conversion as a documented extension.
Example 3
Safe, but only because you’re using unsigned arithmetic, which cannot overflow, and are therefore guaranteed to get the same object representation back. An intptr_t could overflow! If you want to do pointer arithmetic safely, you can convert any kind of pointer to char* and then add or subtract offsets within the same structure or array. Remember, sizeof(char) is always 1. ETA: The standard guarantees that the two pointers compare equal, but your link to Chisnall et al. gives examples where compilers nevertheless assume the two pointers do not alias each other.
Example 4
Always, always, always check for buffer overflows whenever you read from and especially whenever you write to a buffer! If you can mathematically prove that overflow cannot happen by static analysis? Then write out the assumptions that justify that, explicitly, and assert() or static_assert() that they haven’t changed. Use snprintf(), not the deprecated, unsafe sprintf()! If you remember nothing else from this answer, remember that!
To be absolutely pedantic, the portable way to do this would be to use the format specifiers in <inttypes.h> and define the buffer length in terms of the maximum value of any pointer representation. In the real world, you would print pointers out with the %p format.
The answer to the question you intended to ask is yes, though: all that matters is that you get back the same object representation. Here’s a less contrived example:
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
int i = 1;
const uintptr_t u = (uintptr_t)(void*)&i;
uintptr_t v;
memcpy( &v, &u, sizeof(v) );
int* const p = (int*)(void*)v;
assert(p == &i);
*p = 2;
printf( "%d = %d.\n", i, *p );
return EXIT_SUCCESS;
}
All that matter are the bits in your object representation. This code also follows the strict aliasing rules in §6.5. It compiles and runs fine on the compilers that gave Chisnall et al trouble.
Example 5
This works, same as above.
One extremely pedantic footnote that will never ever be relevant to your coding: some obsolete esoteric hardware has one’s-complement or sign-and-magnitude representation of signed integers, and on these, there might be a distinct value of negative zero that might or might not trap. On some CPUs, this might be a valid pointer or null pointer representation distinct from positive zero. And on some CPUs, positive and negative zero might compare equal.
PS
The standard says:
Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.
Furthermore, if the two array objects are consecutive rows of the same multidimensional array, one past the end of the first row is a valid pointer to the start of the next row. Therefore, even a pathological implementation that deliberately sets out to cause as many bugs as the standard allows could only do so if your manipulated pointer compares equal to the address of an array object, in which case the implementation might in theory decide to interpret it as one-past-the-end of some other array object instead.
The intended behavior is clearly that the pointer comparing equal both to &array1+1 and to &array2 is equivalent to both: it means to let you compare it to addresses within array1 or dereference it to get array2[0]. However, the Standard does not actually say that.
PPS
The standards committee has addressed some of these issues and proposes that the C standard explicitly add language about pointer provenance. This would nail down whether a conforming implementation is allowed to assume that a pointer created by bit manipulation does not alias another pointer.
Specifically, the proposed corrigendum would introduce pointer provenance and allow pointers with different provenance not to compare equal. It would also introduce a -fno-provenance option, which would guarantee that any two pointers compare equal if and only if they have the same numeric address. (As discussed above, two object pointers that compare equal alias each other.)

Answer 2

1) Cast to void pointer and back
This yields a valid pointer equal to the original. Paragraph 6.3.2.3/1 of the standard is clear on this:
A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.
2) Cast to appropriately sized integer and back
3) A couple of trivial integer operations
4) Integer operations nontrivial enough to obscure provenance, but which will nonetheless leave the value unchanged
5) More indirect integer operations that will leave the value unchanged
[...] Obviously case 1 is valid, and case 2 surely must be also. On the other hand, I came across a post by Chris Lattner - which I unfortunately can't find now - saying case 5 is not valid, that the standard licenses the compiler to just compile it to an infinite loop.
C does require a cast when converting either way between pointers and integers, and you have omitted some of those in your example code. In that sense your examples (2) - (5) are all non-conforming, but for the rest of this answer I'll pretend the needed casts are there.
Still, being very pedantic, all of these examples have implementation-defined behavior, so they are not strictly conforming. On the other hand, "implementation-defined" behavior is still defined behavior; whether that means your code is "valid" or not depends on what you mean by that term. In any event, what code the compiler might emit for any of the examples is a separate matter.
These are the relevant provisions of the standard from section 6.3.2.3 (emphasis added):
An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.
Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.
The definition of uintptr_t is also relevant to your particular example code. The standard describes it this way (C2011, 7.20.1.4/1; emphasis added):
an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer.
You are converting back and forth between int * and uintptr_t. int * is not void *, so 7.20.1.4/1 does not apply to these conversions, and the behavior is implementation-defined per section 6.3.2.3.
Suppose, however, that you convert back and forth via an intermediate void *:
uintptr_t b = (uintptr_t)(void *)a;
a = (int *)(void *)b;
On an implementation that provides uintptr_t (which is optional), that would make your examples (2 - 5) all strictly conforming. In that case, the result of the integer-to-pointer conversions depends only on the value of the uintptr_t object, not on how that value was obtained.
As for the claims you attribute to Chris Lattner, they are substantially incorrect. If you have represented them accurately, then perhaps they reflect a confusion between implementation-defined behavior and undefined behavior. If the code exhibited undefined behavior then the claim might hold some water, but that is not, in fact, the case.
Regardless of how its value was obtained, b has a definite value of type uintptr_t, and the loop must eventually increment i to that value, at which point the if block will run. In principle, the implementation-defined behavior of a conversion from uintptr_t directly to int * could be something crazy such as skipping the next statement (thus causing an infinite loop), but such behavior is entirely implausible. Every implementation you ever meet will either fail at that point, or store some value in variable a, and then, if it didn't crash, it will execute the return statement.

Answer 3

Because different application fields require the ability to manipulate pointers in different ways, and because the best implementations for some purposes may be totally unsuitable for some others, the C Standard treats the support (or lack thereof) for various kinds of manipulations as a Quality of Implementation issue. In general, people writing an implementation for a particular application field should be more familiar than the authors of the Standard with what features would be useful to programmers in that field, and people making a bona fide effort to produce quality implementations suitable for writing applications in that field will support such features whether the Standard requires them to or not.
In the pre-Standard language invented by Dennis Ritchie, all pointers of a particular type that were identified the same address were equivalent. If any sequence of operations on a pointer would end up producing another pointer of the same type that identified the same address, that pointer would--essentially by definition--be equivalent to the first. The C Standard specifies some situations, however, where pointers can identify the same location in storage and be indistinguishable from each other without being equivalent. For example, given:
int foo[2][4] = {0};
int *p = foo[0]+4, *q=foo[1];
both p and q will compare equal to each other, and to foo[0]+4, and to foo[1]. On the other hand, despite the fact that evaluation of p[-1] and q[0] would have defined behavior, evaluation of p[0] or q[-1] would invoke UB. Unfortunately, while the Standard makes clear that p and q would not be equivalent, it does nothing to clarify whether performing various sequences of operations on e.g. p will yield a pointer that is usable in all cases where p was usable, a pointer that's usable in all cases where either p or q would be usable, a pointer that's only usable in cases where q would be usable, or a pointer that's only usable in cases where both p and q would be usable.
Quality implementations intended for low-level programming should generally process pointer manipulations other than those involving restrict pointers in a way that would yield a pointer that would be usable in any case where a pointer that compares equal to it could usable. Unfortunately, the Standard provides no means by which a program can determine if it's being processed by a quality implementations that is suitable for low-level programming and refuse to run if it isn't, so most forms of systems programming must rely upon quality implementations processing certain actions in documented manner characteristic of the environment even when the Standard would impose no requirements.
Incidentally, even if the normal constructs for manipulating pointers wouldn't have any way of creating pointers where the principles of equivalence shouldn't apply, some platforms may define ways of creating "interesting" pointers. For example, if an implementation that would normally trap operations on null pointers were run on in an environment where it might sometimes be necessary to access an object at address zero, it might define a special syntax for creating a pointer that could be used to access any address, including zero, within the context where it was created. A "legitimate pointer to address zero" would likely compare equal to a null pointer (even though they're not equivalent), but performing a round-trip conversion to another type and back would likely convert what had been a legitimate pointer to address zero into a null pointer. If the Standard had mandated that a round-trip conversion of any pointer must yield one that's usable in the same way as the original, that would require that the compiler omit null traps on any pointers that could have been produced in such a fashion, even if they would far more likely have been produced by round-tripping a null pointer.
Incidentally, from a practical perspective, "modern" compilers, even in -fno-strict-aliasing, will sometimes attempt to track provenance of pointers through pointer-integer-pointer conversions in such a way that pointers produced by casting equal integers may sometimes be assumed incapable of aliasing.
For example, given:
#include <stdint.h>
extern int x[],y[];
int test(void)
{
if (!x[0]) return 999;
uintptr_t upx = (uintptr_t)x;
uintptr_t upy = (uintptr_t)(y+1);
//Consider code with and without the following line
if (upx == upy) upy = upx;
if ((upx ^ ~upy)+1) // Will return if upx != upy
return 123;
int *py = (int*)upy;
*py += 1;
return x[0];
}
In the absence of the marked line, gcc, icc, and clang will all assume--even when using -fno-strict-aliasing, that an operation on *py can't affect *px, even though the only way that code could be reached would be if upx and upy held the same value (meaning that both px and py were produced by casting the same uintptr_t value). Adding the marked line causes icc and clang to recognize that px and py could identify the same object, but gcc assumes that the assignment can be optimized out, even though it should mean that py would be derived from px--a situation a quality compiler should have no trouble recognizing as implying possible aliasing.
I'm not sure what realistic benefit compiler writers expect from their efforts to track provenance of uintptr_t values, given that I don't see much purpose for doing such conversions outside of cases where the results of conversions may be used in "interesting" ways. Given compiler behavior, however, I'm not sure I see any nice way to guarantee that conversions between integers and pointers will behave in a fashion consistent with the values involved.