I would like to know architectures which violate the assumptions I've listed below. Also, I would like to know if any of the assumptions are false for all architectures (that is, if any of them are just completely wrong).
sizeof(int *) == sizeof(char *) == sizeof(void *) == sizeof(func_ptr *)
The in-memory representation of all pointers for a given architecture is the same regardless of the data type pointed to.
The in-memory representation of a pointer is the same as an integer of the same bit length as the architecture.
Multiplication and division of pointer data types are only forbidden by the compiler. NOTE: Yes, I know this is nonsensical. What I mean is - is there hardware support to forbid this incorrect usage?
All pointer values can be casted to a single integer. In other words, what architectures still make use of segments and offsets?
Incrementing a pointer is equivalent to adding sizeof(the pointed data type) to the memory address stored by the pointer. If p is an int32* then p+1 is equal to the memory address 4 bytes after p.
I'm most used to pointers being used in a contiguous, virtual memory space. For that usage, I can generally get by thinking of them as addresses on a number line. See Stack Overflow question Pointer comparison.
I can't give you concrete examples of all of these, but I'll do my best.
sizeof(int *) == sizeof(char *) == sizeof(void *) == sizeof(func_ptr *)
I don't know of any systems where I know this to be false, but consider:
Mobile devices often have some amount of read-only memory in which program code and such is stored. Read-only values (const variables) may conceivably be stored in read-only memory. And since the ROM address space may be smaller than the normal RAM address space, the pointer size may be different as well. Likewise, pointers to functions may have a different size, as they may point to this read-only memory into which the program is loaded, and which can otherwise not be modified (so your data can't be stored in it).
So I don't know of any platforms on which I've observed that the above doesn't hold, but I can imagine systems where it might be the case.
The in-memory representation of all pointers for a given architecture is the same regardless of the data type pointed to.
Think of member pointers vs regular pointers. They don't have the same representation (or size). A member pointer consists of a this pointer and an offset.
And as above, it is conceivable that some CPU's would load constant data into a separate area of memory, which used a separate pointer format.
The in-memory representation of a pointer is the same as an integer of the same bit length as the architecture.
Depends on how that bit length is defined. :)
An int on many 64-bit platforms is still 32 bits. But a pointer is 64 bits.
As already said, CPU's with a segmented memory model will have pointers consisting of a pair of numbers. Likewise, member pointers consist of a pair of numbers.
Multiplication and division of pointer data types are only forbidden by the compiler.
Ultimately, pointers data types only exist in the compiler. What the CPU works with is not pointers, but integers and memory addresses. So there is nowhere else where these operations on pointer types could be forbidden. You might as well ask for the CPU to forbid concatenation of C++ string objects. It can't do that because the C++ string type only exists in the C++ language, not in the generated machine code.
However, to answer what you mean, look up the Motorola 68000 CPUs. I believe they have separate registers for integers and memory addresses. Which means that they can easily forbid such nonsensical operations.
All pointer values can be casted to a single integer.
You're safe there. The C and C++ standards guarantee that this is always possible, no matter the memory space layout, CPU architecture and anything else. Specifically, they guarantee an implementation-defined mapping. In other words, you can always convert a pointer to an integer, and then convert that integer back to get the original pointer. But the C/C++ languages say nothing about what the intermediate integer value should be. That is up to the individual compiler, and the hardware it targets.
Incrementing a pointer is equivalent to adding sizeof(the pointed data type) to the memory address stored by the pointer.
Again, this is guaranteed. If you consider that conceptually, a pointer does not point to an address, it points to an object, then this makes perfect sense. Adding one to the pointer will then obviously make it point to the next object. If an object is 20 bytes long, then incrementing the pointer will move it 20 bytes, so that it moves to the next object.
If a pointer was merely a memory address in a linear address space, if it was basically an integer, then incrementing it would add 1 to the address -- that is, it would move to the next byte.
Finally, as I mentioned in a comment to your question, keep in mind that C++ is just a language. It doesn't care which architecture it is compiled to. Many of these limitations may seem obscure on modern CPU's. But what if you're targeting yesteryear's CPU's? What if you're targeting the next decade's CPU's? You don't even know how they'll work, so you can't assume much about them. What if you're targeting a virtual machine? Compilers already exist which generate bytecode for Flash, ready to run from a website. What if you want to compile your C++ to Python source code?
Staying within the rules specified in the standard guarantees that your code will work in all these cases.
I don't have specific real world examples in mind but the "authority" is the C standard. If something is not required by the standard, you can build a conforming implementation that intentionally fails to comply with any other assumptions. Some of these assumption are true most of the time just because it's convenient to implement a pointer as an integer representing a memory address that can be directly fetched by the processor but this is just a consequent of "convenience" and can't be held as a universal truth.
Not required by the standard (see this question). For instance, sizeof(int*) can be unequal to size(double*). void* is guaranteed to be able to store any pointer value.
Not required by the standard. By definition, size is a part of representation. If the size can be different, the representation can be different too.
Not necessarily. In fact, "the bit length of an architecture" is a vague statement. What is a 64-bit processor, really? Is it the address bus? Size of registers? Data bus? What?
It doesn't make sense to "multiply" or "divide" a pointer. It's forbidden by the compiler but you can of course multiply or divide the underlying representation (which doesn't really make sense to me) and that results in undefined behavior.
Maybe I don't understand your point but everything in a digital computer is just some kind of binary number.
Yes; kind of. It's guaranteed to point to a location that's a sizeof(pointer_type) farther. It's not necessarily equivalent to arithmetic addition of a number (i.e. farther is a logical concept here. The actual representation is architecture specific)
For 6.: a pointer is not necessarily a memory address. See for example "The Great Pointer Conspiracy" by Stack Overflow user jalf:
Yes, I used the word “address” in the comment above. It is important to realize what I mean by this. I do not mean “the memory address at which the data is physically stored”, but simply an abstract “whatever we need in order to locate the value. The address of i might be anything, but once we have it, we can always find and modify i."
And:
A pointer is not a memory address! I mentioned this above, but let’s say it again. Pointers are typically implemented by the compiler simply as memory addresses, yes, but they don’t have to be."
Some further information about pointers from the C99 standard:
6.2.5 §27 guarantees that void* and char* have identical representations, ie they can be used interchangably without conversion, ie the same address is denoted by the same bit pattern (which doesn't have to be true for other pointer types)
6.3.2.3 §1 states that any pointer to an incomplete or object type can be cast to (and from) void* and back again and still be valid; this doesn't include function pointers!
6.3.2.3 §6 states that void* can be cast to (and from) integers and 7.18.1.4 §1 provides apropriate types intptr_t and uintptr_t; the problem: these types are optional - the standard explicitly mentions that there need not be an integer type large enough to actually hold the value of the pointer!
sizeof(char*) != sizeof(void(*)(void) ? - Not on x86 in 36 bit addressing mode (supported on pretty much every Intel CPU since Pentium 1)
"The in-memory representation of a pointer is the same as an integer of the same bit length" - there's no in-memory representation on any modern architecture; tagged memory has never caught on and was already obsolete before C was standardized. Memory in fact doesn't even hold integers, just bits and arguably words (not bytes; most physical memory doesn't allow you to read just 8 bits.)
"Multiplication of pointers is impossible" - 68000 family; address registers (the ones holding pointers) didn't support that IIRC.
"All pointers can be cast to integers" - Not on PICs.
"Incrementing a T* is equivalent to adding sizeof(T) to the memory address" - true by definition. Also equivalent to &pointer[1].
I don't know about the others, but for DOS, the assumption in #3 is untrue. DOS is 16 bit and uses various tricks to map many more than 16 bits worth of memory.
The in-memory representation of a pointer is the same as an integer of the same bit length as the architecture.
I think this assumption is false because on the 80186, for example, a 32-bit pointer is held in two registers (an offset register an a segment register), and which half-word went in which register matters during access.
Multiplication and division of pointer data types are only forbidden by the compiler.
You can't multiply or divide types. ;P
I'm unsure why you would want to multiply or divide a pointer.
All pointer values can be casted to a single integer. In other words, what architectures still make use of segments and offsets?
The C99 standard allows pointers to be stored in intptr_t, which is an integer type. So, yes.
Incrementing a pointer is equivalent to adding sizeof(the pointed data type) to the memory address stored by the pointer. If p is an int32* then p+1 is equal to the memory address 4 bytes after p.
x + y where x is a T * and y is an integer is equivilent to (T *)((intptr_t)x + y * sizeof(T)) as far as I know. Alignment may be an issue, but padding may be provided in the sizeof. I'm not really sure.
In general, the answer to all of the questions is "yes", and it's because only those machines that implement popular languages directly saw the light of day and persisted into the current century. Although the language standards reserve the right to vary these "invariants", or assertions, it hasn't ever happened in real products, with the possible exception of items 3 and 4 which require some restatement to be universally true.
It's certainly possible to build segmented MMU designs, which correspond roughly with the capability-based architectures that were popular academically in past years, but no such system has typically seen common use with such features enabled. Such a system might have conflicted with the assertions as it would probably have had large pointers.
In addition to segmented/capability MMUs, which often have large pointers, more extreme designs have tried to encode data types in pointers. Few of these were ever built. (This question brings up all of the alternatives to the basic word-oriented, a pointer-is-a-word architectures.)
Specifically:
The in-memory representation of all pointers for a given architecture is the same regardless of the data type pointed to. True except for extremely wacky past designs that tried to implement protection not in strongly-typed languages but in hardware.
The in-memory representation of a pointer is the same as an integer of the same bit length as the architecture. Maybe, certainly some sort of integral type is the same, see LP64 vs LLP64.
Multiplication and division of pointer data types are only forbidden by the compiler. Right.
All pointer values can be casted to a single integer. In other words, what architectures still make use of segments and offsets? Nothing uses segments and offsets today, but a C int is often not big enough, you may need a long or long long to hold a pointer.
Incrementing a pointer is equivalent to adding sizeof(the pointed data type) to the memory address stored by the pointer. If p is an int32* then p+1 is equal to the memory address 4 bytes after p. Yes.
It is interesting to note that every Intel Architecture CPU, i.e., every single PeeCee, contains an elaborate segmentation unit of epic, legendary, complexity. However, it is effectively disabled. Whenever a PC OS boots up, it sets the segment bases to 0 and the segment lengths to ~0, nulling out the segments and giving a flat memory model.
There were lots of "word addressed" architectures in the 1950s, 1960s and 1970s. But I cannot recall any mainstream examples that had a C compiler. I recall the ICL / Three Rivers PERQ machines in the 1980s that was word addressed and had a writable control store (microcode). One of its instantiations had a C compiler and a flavor of Unix called PNX, but the C compiler required special microcode.
The basic problem is that char* types on word addressed machines are awkward, however you implement them. You often up with sizeof(int *) != sizeof(char *) ...
Interestingly, before C there was a language called BCPL in which the basic pointer type was a word address; that is, incrementing a pointer gave you the address of the next word, and ptr!1 gave you the word at ptr + 1. There was a different operator for addressing a byte: ptr%42 if I recall.
EDIT: Don't answer questions when your blood sugar is low. Your brain (certainly, mine) doesn't work as you expect. :-(
Minor nitpick:
p is an int32* then p+1
is wrong, it needs to be unsigned int32, otherwise it will wrap at 2GB.
Interesting oddity - I got this from the author of the C compiler for the Transputer chip - he told me that for that compiler, NULL was defined as -2GB. Why? Because the Transputer had a signed address range: -2GB to +2GB. Can you beleive that? Amazing isn't it?
I've since met various people that have told me that defining NULL like that is broken. I agree, but if you don't you end up NULL pointers being in the middle of your address range.
I think most of us can be glad we're not working on Transputers!
I would like to know architectures which violate the assumptions I've
listed below.
I see that Stephen C mentioned PERQ machines, and MSalters mentioned 68000s and PICs.
I'm disappointed that no one else actually answered the question by naming any of the weird and wonderful architectures that have standards-compliant C compilers that don't fit certain unwarranted assumptions.
sizeof(int *) == sizeof(char *) == sizeof(void *) == sizeof(func_ptr
*) ?
Not necessarily. Some examples:
Most compilers for Harvard-architecture 8-bit processors -- PIC and 8051 and M8C -- make sizeof(int *) == sizeof(char *),
but different from the sizeof(func_ptr *).
Some of the very small chips in those families have 256 bytes of RAM (or less) but several kilobytes of PROGMEM (Flash or ROM), so compilers often make sizeof(int *) == sizeof(char *) equal to 1 (a single 8-bit byte), but sizeof(func_ptr *) equal to 2 (two 8-bit bytes).
Compilers for many of the larger chips in those families with a few kilobytes of RAM and 128 or so kilobytes of PROGMEM make sizeof(int *) == sizeof(char *) equal to 2 (two 8-bit bytes), but sizeof(func_ptr *) equal to 3 (three 8-bit bytes).
A few Harvard-architecture chips can store exactly a full 2^16 ("64KByte") of PROGMEM (Flash or ROM), and another 2^16 ("64KByte") of RAM + memory-mapped I/O.
The compilers for such a chip make sizeof(func_ptr *) always be 2 (two bytes);
but often have a way to make the other kinds of pointers sizeof(int *) == sizeof(char *) == sizeof(void *) into a a "long ptr" 3-byte generic pointer that has the extra magic bit that indicates whether that pointer points into RAM or PROGMEM.
(That's the kind of pointer you need to pass to a "print_text_to_the_LCD()" function when you call that function from many different subroutines, sometimes with the address of a variable string in buffer that could be anywhere in RAM, and other times with one of many constant strings that could be anywhere in PROGMEM).
Such compilers often have special keywords ("short" or "near", "long" or "far") to let programmers specifically indicate three different kinds of char pointers in the same program -- constant strings that only need 2 bytes to indicate where in PROGMEM they are located, non-constant strings that only need 2 bytes to indicate where in RAM they are located, and the kind of 3-byte pointers that "print_text_to_the_LCD()" accepts.
Most computers built in the 1950s and 1960s use a 36-bit word length or an 18-bit word length, with an 18-bit (or less) address bus.
I hear that C compilers for such computers often use 9-bit bytes,
with sizeof(int *) == sizeof(func_ptr *) = 2 which gives 18 bits, since all integers and functions have to be word-aligned; but sizeof(char *) == sizeof(void *) == 4 to take advantage of special PDP-10 instructions that store such pointers in a full 36-bit word.
That full 36-bit word includes a 18-bit word address, and a few more bits in the other 18-bits that (among other things) indicate the bit position of the pointed-to character within that word.
The in-memory representation of all pointers for a given architecture
is the same regardless of the data type pointed to?
Not necessarily. Some examples:
On any one of the architectures I mentioned above, pointers come in different sizes. So how could they possibly have "the same" representation?
Some compilers on some systems use "descriptors" to implement character pointers and other kinds of pointers.
Such a descriptor is different for a pointer pointing to the first "char" in a "char big_array[4000]" than for a pointer pointing to the first "char" in a "char small_array[10]", which are arguably different data types, even when the small array happens to start at exactly the same location in memory previously occupied by the big array.
Descriptors allow such machines to catch and trap the buffer overflows that cause such problems on other machines.
The "Low-Fat Pointers" used in the SAFElite and similar "soft processors" have analogous "extra information" about the size of the buffer that the pointer points into. Low-Fat pointers have the same advantage of catching and trapping buffer overflows.
The in-memory representation of a pointer is the same as an integer of
the same bit length as the architecture?
Not necessarily. Some examples:
In "tagged architecture" machines, each word of memory has some bits that indicate whether that word is an integer, or a pointer, or something else.
With such machines, looking at the tag bits would tell you whether that word was an integer or a pointer.
I hear that Nova minicomputers have an "indirection bit" in each word which inspired "indirect threaded code". It sounds like storing an integer clears that bit, while storing a pointer sets that bit.
Multiplication and division of pointer data types are only forbidden
by the compiler. NOTE: Yes, I know this is nonsensical. What I mean is
- is there hardware support to forbid this incorrect usage?
Yes, some hardware doesn't directly support such operations.
As others have already mentioned, the "multiply" instruction in the 68000 and the 6809 only work with (some) "data registers"; they can't be directly applied to values in "address registers".
(It would be pretty easy for a compiler to work around such restrictions -- to MOV those values from an address register to the appropriate data register, and then use MUL).
All pointer values can be casted to a single data type?
Yes.
In order for memcpy() to work right, the C standard mandates that every pointer value of every kind can be cast to a void pointer ("void *").
The compiler is required to make this work, even for architectures that still use segments and offsets.
All pointer values can be casted to a single integer? In other words,
what architectures still make use of segments and offsets?
I'm not sure.
I suspect that all pointer values can be cast to the "size_t" and "ptrdiff_t" integral data types defined in "<stddef.h>".
Incrementing a pointer is equivalent to adding sizeof(the pointed data
type) to the memory address stored by the pointer. If p is an int32*
then p+1 is equal to the memory address 4 bytes after p.
It is unclear what you are asking here.
Q: If I have an array of some kind of structure or primitive data type (for example, a "#include <stdint.h> ... int32_t example_array[1000]; ..."), and I increment a pointer that points into that array (for example, "int32_t p = &example_array[99]; ... p++; ..."), does the pointer now point to the very next consecutive member of that array, which is sizeof(the pointed data type) bytes further along in memory?
A: Yes, the compiler must make the pointer, after incrementing it once, point at the next independent consecutive int32_t in the array, sizeof(the pointed data type) bytes further along in memory, in order to be standards compliant.
Q: So, if p is an int32* , then p+1 is equal to the memory address 4 bytes after p?
A: When sizeof( int32_t ) is actually equal to 4, yes. Otherwise, such as for certain word-addressable machines including some modern DSPs where sizeof( int32_t ) may equal 2 or even 1, then p+1 is equal to the memory address 2 or even 1 "C bytes" after p.
Q: So if I take the pointer, and cast it into an "int" ...
A: One type of "All the world's a VAX heresy".
Q: ... and then cast that "int" back into a pointer ...
A: Another type of "All the world's a VAX heresy".
Q: So if I take the pointer p which is a pointer to an int32_t, and cast it into some integral type that is plenty big enough to contain the pointer, and then add sizeof( int32_t ) to that integral type, and then later cast that integral type back into a pointer -- when I do all that, the resulting pointer is equal to p+1?
Not necessarily.
Lots of DSPs and a few other modern chips have word-oriented addressing, rather than the byte-oriented processing used by 8-bit chips.
Some of the C compilers for such chips cram 2 characters into each word, but it takes 2 such words to hold a int32_t -- so they report that sizeof( int32_t ) is 4.
(I've heard rumors that there's a C compiler for the 24-bit Motorola 56000 that does this).
The compiler is required to arrange things such that doing "p++" with a pointer to an int32_t increments the pointer to the next int32_t value.
There are several ways for the compiler to do that.
One standards-compliant way is to store each pointer to a int32_t as a "native word address".
Because it takes 2 words to hold a single int32_t value, the C compiler compiles "int32_t * p; ... p++" into some assembly language that increments that pointer value by 2.
On the other hand, if that one does "int32_t * p; ... int x = (int)p; x += sizeof( int32_t ); p = (int32_t *)x;", that C compiler for the 56000 will likely compile it to assembly language that increments the pointer value by 4.
I'm most used to pointers being used in a contiguous, virtual memory
space.
Several PIC and 8086 and other systems have non-contiguous RAM --
a few blocks of RAM at addresses that "made the hardware simpler".
With memory-mapped I/O or nothing at all attached to the gaps in address space between those blocks.
It's even more awkward than it sounds.
In some cases -- such as with the bit-banding hardware used to avoid problems caused by read-modify-write -- the exact same bit in RAM can be read or written using 2 or more different addresses.
Related
In C, can one stuff a -1 value (e.g. 0xFFFFFFFF) into a pointer, using an approach such as this one, and expect that such memory address is never allocated at runtime?
The idea is that the pointer value be used as a memory address, except if it has this "special" -1 value. The pointer should be considered memory address even if it is NULL (in which case, the object to which it points to has not yet been built).
I understand this may be platform dependent, but the program in question is expected to run in Linux, Windows and MacOSX.
The problem at hand is much larger than what is described here, so comments or answers which question this approach are not useful. I know it's a bit hacky, but the alternative is a major refactor :/
Thanks in advance.
It is GRAS (generally recognized as safe). No major OS will allocate memory that would collide with your chosen sentinel. However, there are a few pathological cases where it would be invalid to make this assumption. For instance, a pathological C++ compiler may choose to start the stack at 0xFFFFFFFF, without violating any constraints in the spec.
Within just the scope of sane OS's, it is nearly impossible to have 0xFFFFFFFF (or its 64-bit equivalent) to be a valid memory address. It cannot be a valid memory address of an array (C++ rules forbid it). It could technically be a valid index of a char of an object allocated at the end of space, but there's two things that prevent that.
Most OSs have some padding
Most OSs use high memory values as Kernel memory.
If you have an opportunity to use a global value as a sentinel, it is guaranteed to be safe.
char sentinel;
char* p = "Hello";
char* p2 = 0; // null pointer
char* p3 = &sentinel;
if (p3 == &sentinel)
cout << "p3 was a sentinel" << endl;
One way to define a sentinel value that no other valid address will coincide with is a static variable:
static t sentinel;
t *p = &sentinel;
If you are going to assume a flat address space and that all pointers have the same width, you can minimize the overhead by declaring sentinel of type char instead of t.
To answer your question about (t*)-1:
-1 has type int. I would recommend (t*)(uintptr_t)-1, which is more likely to be the last address even for a 64-bit flat address space.
it is not very clean, but it should work on all commonplace architectures because, as long as the compiler intends to compare pointers using the unsigned comparison assembly instruction (as it usually does), for any object a that the compiler could hope to place at the end of the address space, &a + 1 has to compare greater than &a. In practice, this prevents the last address to be used to store anything.
This is taken from C, and is based on that.
Let's imagine we have a 32 bit pointer
char* charPointer;
It points into some place in memory that contains some data. It knows that increments of this pointer are in 1 byte, etc.
On the other hand,
int* intPointer;
also points into some place in memory and if we increase it it knows that it should go up by 4 bytes if we add 1 to it.
Question is, how are we able to address full 32 bits of addressable space (2^32) - 4 gigabytes with those pointers, if obviously they contain some information in them that allows them to be separated one from another, for example char* or int*, so this leaves us with not 32 bytes, but with less.
When typing this question I came to thinking, maybe it is all syntatic sugar and really for compiler? Maybe raw pointer is just 32 bit and it doesn't care of the type? Is it the case?
You might be confused by compile time versus run time.
During compilation, gcc (or any C compiler) knows the type of a pointer, in particular knows the type of the data pointed by that pointer variable. So gcccan emit the right machine code. So an increment of a int * variable (on a 32 bits machine having 32 bits int) is translated to an increment of 4 (bytes), while an increment of a char* variable is translated to an increment of 1.
During runtime, the compiled executable (it does not care or need gcc) is only dealing with machine pointers, usually addresses of bytes (or of the start of some word).
Types (in C programs) are not known during runtime.
Some other languages (Lisp, Python, Javascript, ....) require the types to be known at runtime. In recent C++ (but not C) some objects (those having virtual functions) may have RTTI.
It is indeed syntactic sugar. Consider the following code fragment:
int t[2];
int a = t[1];
The second line is equivalent to:
int a = *(t + 1); // pointer addition
which itself is equivalent to:
int a = *(int*)((char*)t + 1 * sizeof(int)); // integer addition
After the compiler has checked the types it drops the casts and works only with addresses, lengths and integer addition.
Yes. Raw pointer is 32 bits of data (or 16 or 64 bits, depending on architecture), and does not contain anything else. Whether it's int *, char *, struct sockaddr_in * is just information for compiler, to know what is the number to actually add when incrementing, and for the type it's going to have when you dereference it.
Your hypothesis is correct: to see how different kinds of pointer are handled, try running this program:
int main()
{
char * pc = 0;
int * pi = 0;
printf("%p\n", pc + 1);
printf("%p\n", pi + 1);
return 0;
}
You will note that adding one to a char* increased its numeric value by 1, while doing the same to the int* increased by 4 (which is the size of an int on my machine).
It's exactly as you say in the end - types in C are just a compile-time concept that tells to the compiler how to generate the code for the various operations you can perform on variables.
In the end pointers just boil down to the address they point to, the semantic information doesn't exist anymore once the code is compiled.
Incrementing an int* pointer is different from a incrementing char* solely because the pointer variable is declared as int*. You can cast an int* to char* and then it will increment with 1 byte.
So, yes, it is all just syntactic sugar. It makes some kinds of array processing easier and confuses void* users.
I saw some usage of (void*) in printf().
If I want to print a variable's address, can I do it like this:
int a = 19;
printf("%d", &a);
I think, &a is a's address which is just an integer, right?
Many articles I read use something like this:
printf("%p", (void*)&a);
What does %p stand for? (A pointer?)
Why use (void*)? Can't I use (int)&a instead?
Pointers are not numbers. They are often internally represented that way, but they are conceptually distinct.
void* is designed to be a generic pointer type. Any pointer value (other than a function pointer) may be converted to void* and back again without loss of information. This typically means that void* is at least as big as other pointer types.
printfs "%p" format requires an argument of type void*. That's why an int* should be cast to void* in that context. (There's no implicit conversion because it's a variadic function; there's no declared parameter, so the compiler doesn't know what to convert it to.)
Sloppy practices like printing pointers with "%d", or passing an int* to printf with a "%p" format, are things that you can probably get away with on most current systems, but they render your code non-portable. (Note that it's common on 64-bit systems for void* and int to be different sizes, so printing pointers with %d" is really non-portable, not just theoretically.)
Incidentally, the output format for "%p" is implementation-defined. Hexadecimal is common, (in upper or lower case, with or without a leading "0x" or "0X"), but it's not the only possibility. All you can count on is that, assuming a reasonable implementation, it will be a reasonable way to represent a pointer value in human-readable form (and that scanf will understand the output of printf).
The article you read is entirely correct. The correct way to print an int* value is
printf("%p", (void*)&a);
Don't take the lazy way out; it's not at all difficult to get it right.
Suggested reading: Section 4 of the comp.lang.c FAQ. (Further suggested reading: All the other sections.
EDIT:
In response to Alcott's question:
There is still one thing I don't quite understand. int a = 10; int *p = &a;, so p's value is a's address in mem, right? If right, then p's value will range from 0 to 2^32-1 (if cpu is 32-bit), and an integer is 4-byte on 32-bit OS, right? then What's the difference between the p's value and an integer? Can p's value go out of the range?
The difference is that they're of different types.
Assume a system on which int, int*, void*, and float are all 32 bits (this is typical for current 32-bit systems). Does the fact that float is 32 bits imply that its range is 0 to 232-1? Or -231 to 231-1? Certainly not; the range of float (assuming IEEE representation) is approximately -3.40282e+38 to +3.40282e+38, with widely varying resolution across the range, plus exotic values like negative zero, subnormalized numbers, denormalized numbers, infinities, and NaNs (Not-a-Number). int and float are both 32 bits, and you can take the 32 bits of a float object and treat it as an int representation, but the result won't have any straightforward relationship to the value of the float. The second low-order bit of an int, for example, has a specific meaning; it contributes 0 to the value if it's 0, and 2 to the value if it's 1; the corresponding bit of a float has a meaning, but it's quite different (it contributes a value that depends on the value of the exponent).
The situation with pointers is quite similar. A pointer value has a meaning: it's the address of some object (or any of several other things, but we'll set that aside for now). On most current systems, interpreting the bits of a pointer object as if it were an integer gives you something that makes sense on the machine level. But the language itself does not guarantee, or even hint, that that's the case.
Pointers are not numbers.
A concrete example: some years ago, I ran across some code that tried to compute the difference in bytes between two addresses by casting to integers. It was something like this:
unsigned char *p0;
unsigned char *p1;
long difference = (unsigned long)p1 - (unsigned long)p0;
If you assume that pointers are just numbers, representing addresses in a linear monolithic address space, then this code makes sense. But that assumption is not supported by the language. And in fact, there was a system on which that code was intended to run (the Cray T90) on which it simply would not have worked. The T90 had 64-bit pointers pointing to 64-bit words. Byte pointers were synthesized in software by storing an offset in the 3 high-order bits of a pointer object. Subtracting two pointers in the above manner, if they both had 0 offsets, would give you the number of words, not bytes, between the addresses. And if they had non-0 offsets, it would give you meaningless garbage. (Conversion from a pointer to an integer would just copy the bits; it could have done the work to give you a meaningful byte index, but it didn't.)
The solution was simple: drop the casts and use pointer arithmetic:
long difference = p1 - p0;
Other addressing schemes are possible. For example, an address might consist of a descriptor that (perhaps indirectly) references a block of memory, plus an offset within that block.
You can assume that addresses are just numbers, that the address space is linear and monolithic, that all pointers are the same size and have the same representation, that a pointer can be safely converted to int, or to long, and back again without loss of information. And the code you write based on those assumptions will probably work on most current systems. But it's entirely possible that some future systems will again use a different memory model, and your code will break.
If you avoid making any assumptions beyond what the language actually guarantees, your code will be far more future-proof. And even leaving portability issues aside, it will probably be cleaner.
So much insanity present here...
%p is generally the correct format specifier to use if you just want to print out a representation of the pointer. Never, ever use %d.
The length of an int and the length of a pointer (void* or otherwise) have no relationship. Most data models on i386 just happen to have 32-bit ints AND 32-bit pointers -- other platforms, including x86-64, are not the same! (This is also historically known as "all the world's a VAX syndrome".) http://en.wikipedia.org/wiki/64-bit#64-bit_data_models
If for some reason you want to hold a memory address in an integral variable, use the right types! intptr_t and uintptr_t. They're in stdint.h. See http://en.wikipedia.org/wiki/Stdint.h#Integers_wide_enough_to_hold_pointers
In C void * is an un-typed pointer. void does not mean void... it means anything. Thus casting to void * would be the same as casting to "pointer" in another language.
Using (int *)&a should work too... but the stylistic point of saying (void *) is to say -- I don't care about the type -- just that it is a pointer.
Note: It is possible for an implementation of C to cause this construct to fail and still meet the requirements of the standards. I don't know of any such implementations, but it is possible.
Although it the vast majority of C implementations store pointers to all kinds of objects using the same representation, the C Standard does not require that all implementations do so, nor does it even provide any means by which a program which would exploit commonality of representations could test whether an implementation follows the common practice and refuse to run if an implementation doesn't.
If on some particular platform, an int* held a word address, while both char* and void* combine a word address with a word that identifies a byte within a word, passing an int* to a function that is expecting to retrieve a variadic argument of type char* or void* would result in that function trying to fetch more data from the stack (a word address plus the supplemental word) than had been pushed (just the word address). This could cause the system to malfunction in unpredictable ways.
Many compilers for commonplace platforms that use the same representation for all pointers will process an action which passes a non-void pointer precisely the same way as they would process an action which casts the pointer to void* before passing it. They thus have no reason to care about whether the pointer type that is passed as a variadic argument will precisely match the pointer type expected by the recipient. Although the Standard could have specified that such implementations which would have no reason to care about pointer types should behave as though the pointers were cast to void*, the authors of C89 Standard avoided describing anything which wouldn't be common to all conforming compilers. The Standard's terminology for a construct that 99% of implementations should process identically, but 1% would might process unpredictably, is "Undefined Behavior". Implementations may, and often should, extend the semantics of the language by specifying how they will treat such constructs, but that's a Quality of Implementation issue outside the Standard's jurisdiction.
Hi I'm sure this must be a common question but I can't find the answer when I search for it. My question basically concerns two pointers. I want to compare their addresses and determine if one is bigger than the other. I would expect all addresses to be unsigned during comparison. Is this true, and does it vary between C89, C99 and C++? When I compile with gcc the comparison is unsigned.
If I have two pointers that I'm comparing like this:
char *a = (char *) 0x80000000; //-2147483648 or 2147483648 ?
char *b = (char *) 0x1;
Then a is greater. Is this guaranteed by a standard?
Edit to update on what I am trying to do. I have a situation where I would like to determine that if there's an arithmetic error it will not cause a pointer to go out of bounds. Right now I have the start address of the array and the end address. And if there's an error and the pointer calculation is wrong, and outside of the valid addresses of memory for the array, I would like to make sure no access violation occurs. I believe I can prevent this by comparing the suspect pointer, which has been returned by another function, and determining if it is within the acceptable range of the array. The question of negative and positive addresses has to do with whether I can make the comparisons, as discussed above in my original question.
I appreciate the answers so far. Based on my edit would you say that what I'm doing is undefined behavior in gcc and msvc? This is a program that will run on Microsoft Windows only.
Here's an over simplified example:
char letters[26];
char *do_not_read = &letters[26];
char *suspect = somefunction_i_dont_control(letters,26);
if( (suspect >= letters) && (suspect < do_not_read) )
printf("%c", suspect);
Another edit, after reading AndreyT's answer it appears to be correct. Therefore I will do something like this:
char letters[26];
uintptr_t begin = letters;
uintptr_t toofar = begin + sizeof(letters);
char *suspect = somefunction_i_dont_control(letters,26);
if( ((uintptr_t)suspect >= begin) && ((uintptr_t)suspect < toofar ) )
printf("%c", suspect);
Thanks everyone!
Pointer comparisons cannot be signed or unsigned. Pointers are not integers.
C language (as well as C++) defines relative pointer comparisons only for pointers that point into the same aggregate (struct or array). The ordering is natural: the pointer that points to an element with smaller index in an array is smaller. The pointer that points to a struct member declared earlier is smaller. That's it.
You can't legally compare arbitrary pointers in C/C++. The result of such comparison is not defined. If you are interested in comparing the numerical values of the addresses stored in the pointers, it is your responsibility to manually convert the pointers to integer values first. In that case, you will have to decide whether to use a signed or unsigned integer type (intptr_t or uintptr_t). Depending on which type you choose, the comparison will be "signed" or "unsigned".
The integer-to-pointer conversion is wholly implementation defined, so it depends on the implementation you are using.
That said, you are only allowed to relationally compare pointers that point to parts of the same object (basically, to subobjects of the same struct or elements of the same array). You aren't allowed to compare two pointers to arbitrary, wholly unrelated objects.
From a draft C++ Standard 5.9:
If two pointers p and q of the same type point to different objects
that are not members of the same object or elements of the same array
or to different functions, or if only one of them is null, the results
of p<q, p>q, p<=q, and p>=q are unspecified.
So, if you cast numbers to pointers and compare them, C++ gives you unspecified results. If you take the address of elements you can validly compare, the results of comparison operations are specified independently of the signed-ness of the pointer types.
Note unspecified is not undefined: it's quite possible to compare pointers to different objects of the same type that aren't in the same structure or array, and you can expect some self-consistent result (otherwise it'd be impossible to use such pointers as keys in trees, or to sort a vector of such pointers, binary search the vector etc., where a consistent intuitive overall < ordering is needed).
Note that in very old C++ Standards the behaviour was undefined - like the 2005 WG14/N1124 draft andrewdski links to under James McNellis's answer -
To complement the other answers, comparison between pointers that point to different objects depends on the standard.
In C99 (ISO/IEC 9899:1999 (E)), §6.5.8:
5 [...] In all other cases, the behavior is undefined.
In C++03 (ISO/IEC 14882:2003(E)), §5.9:
-Other pointer comparisons are unspecified.
I know several of the answers here say you cannot compare pointers unless they point to within the same structure, but that's a red herring and I'll try to explain why. One of your pointers points to the start of your array, the other to the end, so they are pointing to the same structure. A language lawyer could say that if your third pointer points outside of the object, the comparison is undefined, so x >= array.start might be true for all x. But this is no issue, since at the point of comparison C++ cannot know if the array isn't embedded in an even bigger structure. Furthermore, if your address space is linear, like it's bound to be these days, your pointer comparison will be implemented as an (un)signed integer comparison, since any other implementation would be slower. Even in the times of segments and offsets, (far) pointer comparison was implemented by first normalising the pointer and then comparing them as integers.
What this all boils down to then, is that if your compiler is okay, comparing the pointers without worrying about the signs should work, if all you care about is that the pointer points within the array, since the compiler should make the pointers signed or unsigned depending on which of the two boundaries a C++ object may straddle.
Different platforms behave differently in this matter, which is why C++ has to leave it up to the platform. There are even platforms in which both addresses near 0 and 80..00h are not mappable or already taken at process start-up. In that case, it doesn't matter, as long as you're consistent about it.
Sometimes this can cause compatibility issues. As an example, in Win32 pointers are unsigned. Now, it used to be the case that of the 4GB address space only the lower half (more precisely 10000h ... 7FFFFFFFh, because of the NULL-Pointer Assignment Partition) was available to applications; high addresses were only available to the kernel. This caused some people to put addresses in signed variables, and their programs would keep working since the high bit was always 0. But then came /3GB switch, which made almost 3 GB available to applications (more precisely 10000h ... BFFFFFFFh) and the application would crash or behave erratically.
You explicitly state your program will be Windows-only, which uses unsigned pointers. However, maybe you'll change your mind in the future, and using intptr_t or uintptr_t is bad for portability. I also wonder if you should be doing this at all... if you're indexing into an array it might be safer to compare indices instead. Suppose for example that you have a 1 GB array at 1500000h ... 41500000h, consisting of 16,384 elements of 64 kB each. Suppose you accidentally look up index 80,000 – clearly out of range. The pointer calculation will yield 39D00000h, so your pointer check will allow it, even though it shouldn't.
With a 32-bit OS, we know that the pointer size is 4 bytes, so sizeof(char*) is 4 and sizeof(int*) is 4, etc. We also know that when you increment a char*, the byte address (offset) changes by sizeof(char); when you increment an int*, the byte address changes by sizeof(int).
My question is:
How does the OS know how much to increment the byte address for sizeof(YourType)?
The compiler only knows how to increment a pointer of type YourType * if it knows the size of YourType, which is the case if and only if the complete definition of YourType is known to the compiler at this point.
For example, if we have:
struct YourType *a;
struct YourOtherType *b;
struct YourType {
int x;
char y;
};
Then you are allowed to do this:
a++;
but you are not allowed to do this:
b++;
..since struct YourType is a complete type, but struct YourOtherType is an incomplete type.
The error given by gcc for the line b++; is:
error: arithmetic on pointer to an incomplete type
The OS doesn't really have anything to do with that - it's the compiler's job (as #zneak mentioned).
The compiler knows because it just compiled that struct or class - the size is, in the struct case, pretty much the sum of the sizes of all the struct's contents.
It is primarily an issue for the C (or C++) compiler, and not primarily an issue for the OS per se.
The compiler knows its alignment rules for the basic types, and applies those rules to any type you create. It can therefore establish the alignment requirement and size of YourType, and it will ensure that it increments any YourType* variable by the correct value. The alignment rules vary by hardware (CPU), and the compiler is responsible for knowing which rules to apply.
One key point is that the size of YourType must be such that when you have an array:
YourType array[20];
then &array[1] == &array[0] + 1. The byte address of &array[1] must be incremented by sizeof(YourType), and (assuming YourType is a structure), each of the elements of array[1] must be properly aligned, just as the elements of array[0] must be properly aligned.
Also remember types are defined in your compiled code to match the hardware you are working on. It is entirely up to the source code that is used to work this out.
So a low end chipset 16 bit targeted C program might have need to define types differently to a 32 bit system.
The programming language and compiler are what govern your types. Not the OS or hardware.
Although of course trying to stick a 32 bit number into a 16 bit register could be a problem!
C pointers are typed, unlike some old languages like PL/1. This not only allows the size of the object to be known, but so widening operations and formatting can be carried out. For example getting the data at *p, is that a float, a double, or a char? The compiler needs to know (think divisions, for example).
Of course we do have a typeless pointer, a void *, which you cannot do any arithmetic with simply because the compiler has no idea how much to add to the address.