typedef struct vs struct declaration error in code

**shaswat** · 07-22-2016

Hi All,

I was wondering that why this snippet of code gives error

error: conflicting types for 'fun'|

Code:

struct{
  u8 Address1;
  u8 Address2;
}st_varib;






struct st_varib gInstantReturn; 	/* global variable */


struct st_varib fun(); 		/*Prototype of function fun*/


struct st_varib fun()			/* fun function definition*/
{
    printf("Hi");
	gInstantReturn.Address1 = 5;    /*some action inside this function and final value would be save in some global variable or return*/
	gInstantReturn.Address2 = 5;
    return gInstantReturn;
}


int main ()
{
	gInstantReturn = fun();
	return 3;
}

while another one doesn't. I just add typedef keyword before the data type declaration

Code:

typedef struct{
  u8 Address1;
  u8 Address2;
}st_varib;






st_varib gInstantReturn; 	/* global variable */


st_varib fun(); 		/*Prototype of function fun*/


st_varib fun()			/* fun function definition*/
{
    printf("Hi");
	gInstantReturn.Address1 = 5;    /*some action inside this function and final value would be save in some global variable or return*/
	gInstantReturn.Address2 = 5;
    return gInstantReturn;
}


int main ()
{
	gInstantReturn = fun();
	return 3;
}

Any idea would be appreciated.

Thanks

**GReaper** · 07-22-2016

Because the first creates an unnamed struct and then declares a variable named "st_varib", while the second creates an unnamed struct and then gives it a new type named "st_varib".

TL;DR:
In the first one, "st_varib" is a variable.
In the second one, "st_varib" is a type.

**c99tutorial** · 07-23-2016

Originally Posted by shaswat

Hi All,

I was wondering that why this snippet of code gives error

Code:

struct{
  u8 Address1;
  u8 Address2;
}st_varib;
...

If you're not using typedef, you probably want to be using this form instead, judging from the rest of your code:

Code:

struct st_varib {
    u8 Address1;
    u8 Address2;
};

This declares struct st_varib as a type. Remember that struct tags have a separate name space, so the following is a valid C statement which declares a symbol called st_varib whose type is struct st_varib:

Code:

struct st_varib st_varib;

Some programmers find that confusing, so they declare a typedef like this:

Code:

struct st_varib {
    u8 Address1;
    u8 Address2;
};
typedef struct st_varib st_varib;

After writing that, you can now say 'st_varib s;' to declare that s has type 'st_varib' (which is the same as type 'struct st_varib').

Some programmers find the above confusing because 'st_varib' appears twice with different meaning, so they write this equivalent definition instead:

Code:

struct _st_varib {
    u8 Address1;
    u8 Address2;
};
typedef struct _st_varib st_varib;

This version creates a throwaway symbol '_st_varib' in the struct tag name space which the programmer intends to use exactly one time (in the typedef line). From then on he uses 'st_varib' as the type name. Finally the above can be simplified to the following (which you used in your example):

Code:

typedef struct {
    u8 Address1;
    u8 Address2;
} st_varib;

This version declares the struct as an anonymous struct but then typedef's it to the name st_varib. So again you can write 'st_varib s;' to declare a symbol called s which has the type 'st_varib' which is now the same as the struct type. It is conceptually equivalent to writing

Code:

struct { u8 Address1; u8 Address2; } s;

except that writing the above will probably not work very well, because each time you write struct with a member definitions list, it introduces a new type, even if the struct member definitions are exactly the same as a previous definition.

**shaswat** · 07-25-2016

Thanks for elaborating and such a great answer. Your answer leads me to understand more about typedef's.

Thread: typedef struct vs struct declaration error in code

Thread Tools

Search Thread

Display

typedef struct vs struct declaration error in code

Similar Threads

Struct declaration failed. (very few lines of code)

differences between struct and typedef struct

syntax error: enum and typedef struct

Diff btw typedef struct and struct?

typedef struct vs. struct

Tags for this Thread