How to get struct field in C

Question

I have the code:

main() 
{
   typedef struct
   {
      int data;
   } Information;

   typedef Information *PtrInformation;

   typedef struct InformationListStruct *PtrInformationListStruct;

   typedef struct InformationListStruct
   {
      PtrInformationListStruct ptrNext;
      PtrInformation ptrInf;
   } PtrInformationListStructElement;

   //==============================

   PtrInformationListStruct list;
   list = (PtrInformationListStruct)malloc(sizeof(InformationListStruct));
   PtrInformation ptr = (*list).ptrInf;  // error !!!

}

The compiler throw the error:

"ptrInf" is not a member of InformationListStruct, because the type is not yet defined in function main()

If I put this line:

typedef struct InformationListStruct *PtrInformationListStruct;

after this lines:

   typedef struct InformationListStruct
   {
      PtrInformationListStruct ptrNext;
      PtrInformation ptrInf;
   } PtrInformationListStructElement;

then other error appears:

Type name expected in function main()
Declaration missing ; in function main()

How to get "ptrInf" correctly?

Just a small remark: putting type definition in function body is not a really good pratice to me. — Synxis, Jun 13 '12 at 09:05
Do you get other errors? When you have a question about errors, it's always best to add _all_ the errors to the question, and preferably verbatim (i.e. copy-paste). — Some programmer dude, Jun 13 '12 at 09:07
I am using Unix GCC and I see below errors t.c: In function ‘main’: t.c:22:51: error: ‘InformationListStruct’ undeclared (first use in this function) t.c:22:51: note: each undeclared identifier is reported only once for each function it appears in — Viswesn, Jun 13 '12 at 09:43

score 5 · Accepted Answer · edited May 23 '17 at 11:49

5

You shouldn't cast malloc()'s return in C. Also, use sizeof, don't repeat the type name:

list = malloc(sizeof *list);

edited May 23 '17 at 11:49

Community

1
1

answered Jun 13 '12 at 09:22

unwind

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@LuchianGrigore It can work around the error, your solution does the same by adding the missing `struct` keyword to make the type name legal. My view is that *having* the type name there is almost always a bad idea; the above works as long as the definition of `list` works which it did in the question. – unwind Jun 13 '12 at 09:32
still the same error. I tried the code in gcc (under Linux) and it compiled without errors. It seems, that my Borland C++ 5.5.1 for Win32 works bad. – amigo Jun 13 '12 at 09:47

Luchian Grigore · Answer 2 · 2012-06-13T09:08:52.793

3

You need

 list = (PtrInformationListStruct)malloc(sizeof(struct InformationListStruct));
 //                                               |
 //                                          note struct keyword

or

 list = (PtrInformationListStruct)malloc(sizeof(PtrInformationListStructElement));

since you have a typedef for it.

edited Jun 13 '12 at 09:08

answered Jun 13 '12 at 09:03

Luchian Grigore

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OP seems to use C, not C++, so `malloc` is the good way to create a new memory object. – Cédric Julien Jun 13 '12 at 09:05
@Luchian Grigore, try the both code snippets. But still see the first error. – amigo Jun 13 '12 at 09:16
@Luchian Grigore, I didn`t use ideone before. I post the code without registration. The link is http://ideone.com/0GX33 – amigo Jun 13 '12 at 09:33
@hello_amigo the code you posted doesn't give a compiler error, as you see. – Luchian Grigore Jun 13 '12 at 09:43

score 1 · Answer 3 · answered Jun 13 '12 at 09:05

1

Which Compiler are you using, in visual studio , your code is compiling successfully. Please avoid type definations inside the body of a function.

answered Jun 13 '12 at 09:05

Tejendra

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I use Borland Compiler (BCC55). I have to try visual studio – amigo Jun 13 '12 at 09:07
This should be a comment. Visual studio compiles C++ if you don't name your files `*.c`, and the code is C. – Luchian Grigore Jun 13 '12 at 09:09
1

Well i dont have dat much reputation to add a comment so i posted it as an answer – Tejendra Jun 13 '12 at 09:11

How to get struct field in C

3 Answers3