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Dear all,
I developed the following program: void parsebytes(unsigned char* data); struct info { unsigned char day; unsigned char month; short year; }; struct info info1; struct info info2; int main(int argc, char *argv[]) { info1.day=12; info1.month=8; info1.year=2007; parsebytes((unsigned char*)&info1); system("PAUSE"); return EXIT_SUCCESS; } void parsebytes(unsigned char* data) { printf("day is %d\n", data[0]); printf("month is %d\n", data[1]); printf("year is %d\n", ((data[2] <<  | data[3]));} The above program gives proper value of 12,8 for day and month.But year value I always get junk.What should be done to correct this and where have I gone wrong? Looking farward for your replies and advanced thanks, Regards, s.subbarayan |
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ssubbarayan <> wrote:
> I developed the following program: > void parsebytes(unsigned char* data); > struct info > { > unsigned char day; > unsigned char month; > short year; > }; > struct info info1; > struct info info2; > int > main(int argc, char *argv[]) > { > info1.day=12; > info1.month=8; > info1.year=2007; > parsebytes((unsigned char*)&info1); > system("PAUSE"); > return EXIT_SUCCESS; > } > void parsebytes(unsigned char* data) > { > printf("day is %d\n", data[0]); > printf("month is %d\n", data[1]); > printf("year is %d\n", ((data[2] << | data[3]));> } > The above program gives proper value of 12,8 for day and month.But > year value I always get junk.What should be done to correct this and > where have I gone wrong? There are at least two aspects that lead to problems. First of all you can't assume that the members of a structure are all following each other directly without any "spacing" in between. A compiler is allowed to put as many "padding bytes" as it want's between the members of a structure. This normally happens due to alignement issues - some types of variables can't start at arbitrary addresses and the compiler must make sure that those members start at allowed addresses. The second problem is that you make some assumptions about the way a short int is stored in memory which might be wrong. You assume that a short int only consists of two bytes and that the most-significant byte is stored at a lower address than the least-significant byte. Both assumptions can be correct on your machine but they don't are generally correct. While a short int requires at least two 8-bit bytes (but there are also machines with more bits in a byte, e.g. 16 bits, so a short int may be stored in a single byte) it can be longer. And the assumption about the ordering of the two bytes is, assuming that two 8-bit bytes are used, only true on big-endian machines, on many (low- endian) machines the least-significant byte is stored at the lower address. Regards, Jens -- \ Jens Thoms Toerring ___ \__________________________ http://toerring.de |
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On 11 Aug, 13:48, ssubbarayan <ssu...@gmail.com> wrote:
> I developed the following program: > > void parsebytes(unsigned char* data); > > struct info > { > *unsigned char day; > *unsigned char month; > *short year; > > }; > > struct info info1; > struct info info2; > > int > main(int argc, char *argv[]) > { > * * * * * * * * info1.day=12; > * * * * info1.month=8; > * * * * info1.year=2007; > > * * * * * * * *parsebytes((unsigned char*)&info1); > * * * * * * * * system("PAUSE"); > * * * * * * * *return EXIT_SUCCESS; > > } > > void parsebytes(unsigned char* data) > { you missed #include <stdlib.h> #include <stdio.h> at the beginning of your program > printf("day is %d\n", data[0]); > *printf("month is %d\n", data[1]); > *printf("year *is %d\n", ((data[2] << | data[3]));> > } > > The above program gives proper value of 12,8 for day and month.But > year value I always get junk. junk? what is junk? Was it 55,047? > What should be done to correct this you have a struct you cast it to an array of unsigned char and then you print the array of unsigned char. Why? A lot of casts simply result in Undefined Behaviour when you try and access one thing as another. The exception is casting to unsigned char. You can cast any pointer to an object to a pointer to unsigned char and print the unsigned chars. But what you see is highly implementation dependent. short is probably 2 bytes (but it doesn't have to be) and the implementation is permitted to have them in either order (so if I guessed the output of your program correctly, that is how I did it). > and where have I gone wrong? I don't know. What are you trying to do? If you wanted to know the internal representation of that particular struct on your particular implementation then perhaps you did nothing wrong. -- Nick Keighley |
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On Aug 11, 6:25*am, Richard Heathfield <r...@see.sig.invalid> wrote:
> ssubbarayan said: > > > > > > > Dear all, > > I developed the following program: > > > void parsebytes(unsigned char* data); > > > struct info > > { > > *unsigned char day; > > *unsigned char month; > > *short year; > > }; > > > struct info info1; > > struct info info2; > > > int > > main(int argc, char *argv[]) > > { > > * * * * * * * * info1.day=12; > > *info1.month=8; > > *info1.year=2007; > > > * * * * * * * *parsebytes((unsigned char*)&info1); > > * * * * * * * * system("PAUSE"); > > * * * * * * * *return EXIT_SUCCESS; > > } > > > void parsebytes(unsigned char* data) > > { > > printf("day is %d\n", data[0]); > > *printf("month is %d\n", data[1]); > > *printf("year *is %d\n", ((data[2] << | data[3]));> > } > > > The above program gives proper value of 12,8 for day and month.But > > year value I always get junk. > > Let me guess: 55047? > > > What should be done to correct this and > > where have I gone wrong? > > Well, the best way to correct it is to have parsebytes accept a pointer to > an info1 structure, rather than a pointer to unsigned char. > > You've gone wrong in assuming that the significance of the bytes going to > make up a short int is in decreasing order - i.e. that the most > significant byte comes first. On some systems, that's true. On others, it > isn't. Look up "endianness", "big-endian", "little-endian", and - would > you believe - "middle-endian". > > -- How did you know that it was 55047? |
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On Aug 11, 6:14*pm, Nick Keighley <nick_keighley_nos...@hotmail.com>
wrote: > On 11 Aug, 13:48, ssubbarayan <ssu...@gmail.com> wrote: > > > > > > > I developed the following program: > > > void parsebytes(unsigned char* data); > > > struct info > > { > > *unsigned char day; > > *unsigned char month; > > *short year; > > > }; > > > struct info info1; > > struct info info2; > > > int > > main(int argc, char *argv[]) > > { > > * * * * * * * * info1.day=12; > > * * * * info1.month=8; > > * * * * info1.year=2007; > > > * * * * * * * *parsebytes((unsigned char*)&info1); > > * * * * * * * * system("PAUSE"); > > * * * * * * * *return EXIT_SUCCESS; > > > } > > > void parsebytes(unsigned char* data) > > { > > you missed > * * #include <stdlib.h> > * * #include <stdio.h> > at the beginning of your program > > > printf("day is %d\n", data[0]); > > *printf("month is %d\n", data[1]); > > *printf("year *is %d\n", ((data[2] << | data[3]));> > > } > > > The above program gives proper value of 12,8 for day and month.But > > year value I always get junk. > > junk? what is junk? Was it 55,047? > > > What should be done to correct this > > you have a struct you cast it to an array of unsigned char > and then you print the array of unsigned char. Why? > > A lot of casts simply result in Undefined Behaviour > when you try and access one thing as another. The exception > is casting to unsigned char. You can cast any pointer to > an object to a pointer to unsigned char and print the unsigned chars. > But what you see is highly implementation dependent. > short is probably 2 bytes (but it doesn't have to be) > and the implementation is permitted to have them > in either order (so if I guessed the output of your > program correctly, that is how I did it). > > > and where have I gone wrong? > > I don't know. What are you trying to do? If you wanted > to know the internal representation of that particular struct > on your particular implementation then perhaps you did > nothing wrong. > > -- > Nick Keighley- Hide quoted text - > > - Show quoted text - Hi, Yes it prints 55047. Regards, s.subbarayan |
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On 12 Aug, 05:39, ssubbarayan <ssu...@gmail.com> wrote:
> On Aug 11, 6:14*pm,Nick Keighley<nick_keighley_nos...@hotmail.com> > > On 11 Aug, 13:48, ssubbarayan <ssu...@gmail.com> wrote: > > > > I developed the following program: > > > > void parsebytes(unsigned char* data); > > > > struct info > > > { > > > *unsigned char day; > > > *unsigned char month; > > > *short year; > > > }; > > > > struct info info1; > > > struct info info2; > > > > int > > > main(int argc, char *argv[]) > > > { > > > * * * * * * * * info1.day=12; > > > * * * * info1.month=8; > > > * * * * info1.year=2007; > > > > * * * * * * * *parsebytes((unsigned char*)&info1); > > > * * * * * * * * system("PAUSE"); > > > * * * * * * * *return EXIT_SUCCESS; > > > > } > > > > void parsebytes(unsigned char* data) > > > { > > > you missed > > * * #include <stdlib.h> > > * * #include <stdio.h> > > at the beginning of your program > > > > printf("day is %d\n", data[0]); > > > *printf("month is %d\n", data[1]); > > > *printf("year *is %d\n", ((data[2] << | data[3]));> > > > } > > > > The above program gives proper value of 12,8 for day and month.But > > > year value I always get junk. > > > junk? what is junk? Was it 55,047? > > > > What should be done to correct this > > > you have a struct you cast it to an array of unsigned char > > and then you print the array of unsigned char. Why? > > > A lot of casts simply result in Undefined Behaviour > > when you try and access one thing as another. The exception > > is casting to unsigned char. You can cast any pointer to > > an object to a pointer to unsigned char and print the unsigned chars. > > But what you see is highly implementation dependent. > > short is probably 2 bytes (but it doesn't have to be) > > and the implementation is permitted to have them > > in either order (so if I guessed the output of your > > program correctly, that is how I did it). > > > > and where have I gone wrong? > > > I don't know. What are you trying to do? If you wanted > > to know the internal representation of that particular struct > > on your particular implementation then perhaps you did > > nothing wrong. > > > -- > >Nick Keighley> don't quote sigs (the bit after "-- ") > Hi, > Yes it prints 55047. and so? Is that not what you wanted? I repeat, what are you trying to do? What output do you want? -- Nick Keighley |
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