C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  reinterpret_cast<>          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Aman Guest Posted: Mon Feb 23, 2004 7:46 pm    Post subject: reinterpret_cast<> Hi, wrote this piece of code on SunOS 5.9 , compiler g++ 2.95.3 trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < short int* spt = reinterpret_cast cout << "spt : " << hex << spt < } regards, Aman. -- Posted via Mailgate.ORG Server - http://www.Mailgate.ORG [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Williscroft Guest Posted: Tue Feb 24, 2004 3:25 am    Post subject: Re: reinterpret_cast<> Aman wrote in news:4069be0f43c5d3619e70e2c68cc331c1.26421 @mygate.mailgate.org: Quote: Hi, wrote this piece of code on SunOS 5.9 , compiler g++ 2.95.3 trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- The above should output the same as if you wrote: cout << &x << endl; i.e. the address of x, however that is printed. Quote: char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < cout << "cpt : " << hex << unsigned(*cpt) << endl; Quote: short int* spt = reinterpret_cast cout << "spt : " << hex << spt < As for ipt above. Quote: } #include #include int main() { using namespace std; char *cpt = "test-string"; cout << cpt << endl; } should output test-string. i.e. the streams output a "string" for char *'s and a memory address for other pointers. HTH. Rob. -- http://www.victim-prime.dsl.pipex.com/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jack Applin Guest Posted: Tue Feb 24, 2004 1:31 pm    Post subject: Re: reinterpret_cast<> In comp.lang.c++.moderated Aman <aman (AT) techie (DOT) com> wrote: Quote: trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < A char * is different from an int * in a crucial way: when you send a char * to an output stream, C++ doesn't print the pointer, C++ prints the CHARACTERS that the pointer points to! cout << "Hello theren"; That was a const char *, but you wouldn't expect hex, would you? For this non-portable program, cast your char * to another pointer type: cout << "cpt : " << hex << (void *) cpt < [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Samuel Klatchko Guest Posted: Tue Feb 24, 2004 1:32 pm    Post subject: Re: reinterpret_cast<> "Aman" <aman (AT) techie (DOT) com> wrote Quote: int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < } The iostream code assumes that a char* is a pointer to a string that is '' terminated. So iostreams is dereferencing the pointer and printing out each character until it finds a ''. Most likely, when that bit pattern is turned into a pointer and dereferenced, there is a '' byte in that place in memory. You're lucky the code ran and did nothing instead of crashing when it tried to dereference a random piece of memory. samuel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Balog Pal Guest Posted: Tue Feb 24, 2004 1:36 pm    Post subject: Re: reinterpret_cast<> "Aman" <aman (AT) techie (DOT) com> wrote Quote: trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < short int* spt = reinterpret_cast cout << "spt : " << hex << spt < } Better try output wia printf You're most likely tricked by the streams, char* is not threated as a pointer but as a string, and is output as such. Or redirect the output to a file and hexdump it. Or change your x init to 0x40414243 Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Aman Angrish Guest Posted: Tue Feb 24, 2004 7:29 pm    Post subject: Re: reinterpret_cast<> Quote: Technically, you're right, in that a union would involve undefined behavior, where you are allowed to cast an arbitrary pointer to char* or to unsigned char* and access the memory as an array of bytes. But a union will work equally well in practice. that is correct . I initially tried with the union too , but got no output because of the same reason (char* interpretation by cout) . thanks !! Aman. Quote: And of course, given his code, "equally well" means not at all. Which was the purpose of his posting. Whether you use a union, or a char* which you dereference, you must cast to int to avoid treating the char as a character. -- Posted via Mailgate.ORG Server - http://www.Mailgate.ORG [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Wed Feb 25, 2004 2:53 pm    Post subject: Re: reinterpret_cast<> In message <4069be0f43c5d3619e70e2c68cc331c1.26421 (AT) mygate (DOT) mailgate.org>, Aman <aman (AT) techie (DOT) com> writes Quote: Hi, wrote this piece of code on SunOS 5.9 , compiler g++ 2.95.3 trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < When you use 'operator <<' applied to an ostream and a char * the result is an attempt to output a null terminated C-style string. There is no reason to expect the bytes constituting the binary code for x to be such a string. Indeed there are excellent reasons to suppose that it isn't. You might also note that none of the bytes encoding x represent printable characters in either of the common character encodings (ASCII, EBCDIC). -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Wed Feb 25, 2004 3:03 pm    Post subject: Re: reinterpret_cast<> Aman wrote: Quote: Hi, wrote this piece of code on SunOS 5.9 , compiler g++ 2.95.3 trying to see the byte order of an int or short int by converting to char* . doesn't work . the char* cpt doesn't seem to be initialized !!. why would that be ? There is nothing wrong with the initialisation of cpt. Quote: int main() { int x=0x01020304 ; int* ipt = &x ; cout << "ipt : "<< hex << ipt << endl ; // ok --- char* cpt = reinterpret_cast cout << "cpt : " << hex << cpt < If you write a char-pointer to an ostream, it is treated as a pointer to a null-terminated string. But cpt doesn't point to a null-terminated string; it points to the bytes {1, 2, 3, 4} or {4, 3, 2, 1} (depending on which architecture you are running SunOS on). which don't correspond to printable characters. Note that since these don't include a null byte the formatting function continues to search for a null byte in the memory after x. This has undefined behaviour, so don't tell it to do this! So you need to convert cpt to void * before printing it: cout << "cpt : " << hex << static_cast Quote: short int* spt = reinterpret_cast cout << "spt : " << hex << spt < I'm not convinced that this is generally safe, because the result of the reinterpret_cast is unspecified and so might not be convertible to void *. Quote: } I wonder whether this program will do what you intend, even with the output corrected. There is no guarantee that you can use *(short *)&x to access the least significant 16 bits (or however many there are in a short) of x, so you cannot use the value of (short *)&x to determine where the least significant 16 bits are stored. The result of the cast is likely to be the same address as &x, regardless of whether the machine is big- or little-endian. The safe way to determine byte order is to look at documentation. Failing that, print out the bytes cpt[0] to cpt[sizeof(x)-1], but first convert them to unsigned int so that they are treated as numbers and not character codes. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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