What is uintptr t data type

Question

What is uintptr t and what can it be used for

User · Accepted Answer

uintptr t is an unsigned integer type that is capable of storing a data pointer   Which typically means that it s the same size as a pointer  It is optionally defined in C  11 and later standards  A common reason to want an integer type that can hold an architecture s pointer type is to perform integer-specific operations on a pointer  or to obscure the type of a pointer by providing it as an integer  quot handle quot

User · Answer

There are already many good answers to the part  what is uintptr t data type   I will try to address the  what it can be used for   part in this post   Primarily for bitwise operations on pointers  Remember that in C   one cannot perform bitwise operations on pointers  For reasons see Why can t you do bitwise operations on pointer in C  and is there a way around this   Thus in order to do bitwise operations on pointers one would need to cast pointers to type unitpr t and then perform bitwise operations    Here is an example of a function that I just wrote to do bitwise exclusive or of 2 pointers to store in a XOR linked list so that we can traverse in both directions like a doubly linked list but without the penalty of storing 2 pointers in each node    template  lt typename T gt   T  xor ptrs T  t1  T  t2          return reinterpret cast lt T  gt  reinterpret cast lt uintptr t gt  t1  reinterpret cast lt uintptr t gt  t2

User · Answer

It s an unsigned integer type exactly the size of a pointer  Whenever you need to do something unusual with a pointer - like for example invert all bits  don t ask why  you cast it to uintptr t and manipulate it as a usual integer number  then cast back

User · Answer

Running the risk of getting another Necromancer badge  I would like to add one very good use for uintptr t  or even intptr t  and that is writing testable embedded code  I write mostly embedded code targeted at various arm and currently tensilica processors   These have various native bus width and the tensilica is actually a Harvard architecture with separate code and data buses that can be different widths   I use a test driven development style for much of my code which means I do unit tests for all the code units I write   Unit testing on actual target hardware is a hassle so I typically write everything on an Intel based PC either in Windows or Linux using Ceedling and GCC   That being said  a lot of embedded code involves bit twiddling and address manipulations  Most of my Intel machines are 64 bit  So if you are going to test address manipulation code you need a generalized object to do math on  Thus the uintptr t give you a machine independent way of debugging your code before you try deploying to target hardware  Another issue is for the some machines or even memory models on some compilers  function pointers and data pointers are different widths  On those machines the compiler may not even allow casting between the two classes  but uintptr t should be able to hold either  -- Edit -- Was pointed out by  chux  this is not part of the standard and functions are not objects in C  However it usually works and since many people don t even know about these types I usually leave a comment explaining the trickery  Other searches in SO on uintptr t will provide further explanation   Also we do things in unit testing that we would never do in production because breaking things is good

User · Answer

First thing  at the time the question was asked  uintptr t was not in C    It s in C99  in  lt stdint h gt   as an optional type  Many C  03 compilers do provide that file  It s also in C  11  in  lt cstdint gt   where again it is optional  and which refers to C99 for the definition   In C99  it is defined as  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    Take this to mean what it says  It doesn t say anything about size   uintptr t might be the same size as a void   It might be larger  It could conceivably be smaller  although such a C   implementation approaches perverse  For example on some hypothetical platform where void  is 32 bits  but only 24 bits of virtual address space are used  you could have a 24-bit uintptr t which satisfies the requirement  I don t know why an implementation would do that  but the standard permits it

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