what does using typedef mean when used without an existing definition

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I was viewing a header file with the following statement:

typedef struct my_server_t my_server_t;

As I understand about typedef, they are used to simplify the definitions of data structures and provides an alias for another data type. So in the following statements:

typedef struct {
   int port;
} my_server;

my_server my_server_t;

my_server_t is of type my_server.

But with no definition of my_server_t present in the header file, what do I interpret from it?

If the header (or a translation unit generally) uses struct my_server_t but does not contain a definition of it, then struct my_server_t is an incomplete type. The compiler knows it is a structure, but it does not know what is in it or how big it is.

You cannot define an object of this type: struct my_server_t x; would fail because the compiler does not know how much space to use for x.

However, you can have a pointer to an incomplete type. The pointers for all structures have the same size and representation, so the compiler has enough information to make a pointer for struct my_server_t even though it does not know the size of struct my_server_t. This means you can call a routine defined in another translation unit that does have a complete definition for struct my_server_t. That routine can create a struct my_server_t (by using malloc or by defining a static object or by other means) and return a pointer to it. You can then use the structure by calling other routines that know what to do with it.

Thus, structures without definitions give a library of routines a way to let its users request and use information without revealing to the users the internal details of how the information is maintained.

The definition of my_server_t as a type name with typedef struct my_server_t my_server_t; merely provides a convenient name to use for the type. It does not affect the meaning or definition of the structure at all.

typedef, typedef is a reserved keyword in the programming languages C and C++. It is used to create an Above, intptr cliff, allen; means defining 2 variables with int* type for both. This is because a type defined Without the typedef, it is generally not possible to use definition syntax and cast syntax interchangeably. For example: typedef is a reserved keyword in the programming languages C and C++.It is used to create an additional name (alias) for another data type, but does not create a new type.As such, it is often used to simplify the syntax of declaring complex data structures consisting of struct and union types, but is just as common in providing specific descriptive type names for integer data types of varying

You can forward declare a structure with an unknown definition in C and use it, as long as the size or the members are not needed (you can for example use a pointer to it or similar).

The typedef does not change anything, it just says, that my_server_t is of type struct my_server. The definition of the struct does not have to be known.

But as soon as you need the definition (i.e. use explicitly or implicitly the size or a member of this struct), you will get an error (incomplete type) if it is not defined yet.

typedef in C Language, typedef is a keyword used in C language to assign alternative names to existing types. Its mostly used with user defined data types, when names of data types� typedef in C. typedef is a keyword used in C language to assign alternative names to existing datatypes. Its mostly used with user defined datatypes, when names of the datatypes become slightly complicated to use in programs. Following is the general syntax for using typedef, typedef <existing_name> <alias_name>

If you see something like:

typedef struct my_server_t my_server_t;

Then, struct my_server_t may be defined somewhere, and my_server_t would be an alias for it.

If you can't find it in the header file you're viewing, then, most likely it will be defined in some other header file that is included in the one you're viewing. Or even, it can be defined in some other header file that is included in one of the header files included in the one you're viewing, etc (consider any level of header files nesting).

How to use the typedef struct in C, How to use the typedef struct in C names for the primitive (e.g., int) and user- defined (e.g struct) data types. adds a new name for some existing data type but does not create a new type. Using typedef struct results in a cleaner, more readable code, and saves the Variable declaration without using typedef : 14. 1 . Definition of typedef in the Definitions.net dictionary. Meaning of typedef. What does typedef mean? Information and translations of typedef in the most comprehensive dictionary definitions resource on the web.

typedef statement in C, The typedef is an advance feature in C language which allows us to create an alias or new name for an existing type or user defined type. Here typedef definition is inside main() function so we can use alias uchar only inside semantic meaning so the compiler doesn't just replace as preprocessor does. The typedef keyword allows the programmer to create new names for types such as int or, more commonly in C++, templated types--it literally stands for "type definition". Typedefs can be used both to provide more clarity to your code and to make it easier to make changes to the underlying data types that you use.

C typedef example program - Complete C tutorial, Typedef is a keyword that is used to give a new symbolic name for the existing name in a C program. When we use “typedef” keyword before struct < tag_name> like above, after that we can simply use type definition “status” in the C program� The typedef is an advance feature in C language which allows us to create an alias or new name for an existing type or user defined type. The syntax of typedef is as follows: Syntax: typedef data_type new_name; typedef: It is a keyword. data_type: It is the name of any existing type or user defined type created using structure/union.

Typedef Declarations, You can use typedef declarations to construct shorter or more meaningful names for types already defined by C or for types that you have declared. It creates synonyms for existing types, or names for types that could be specified exactly the same type, the first without making use of any typedef names. When to use typedef in C language? typedef provides an alias name to the existing complex type definition. With typedef you can simply create alias for any type. Whether it is a simple integer to complex function pointer or structure declaration, typedef will shorten your code. You will use typedef most of the cases for creating alias for

Comments
  • Are you sure it hadn't been defined in some other header file?
  • forward declaration on cppreference
  • If there is no (type) definition of my_server_t in a header, this is simply a variable declaration. If there is a typedef for my_server_t visible from a header, you will probably get a compilation error.
  • When can I define my_server_t? Rather, how would I define my_server_t? Could you please give an example? Let us say in the structure, I want to have a port number and a reference to some function. What would be the way to do this with the forward definition already given in the header file?
  • note that "forward declare" is not terminology used by the standard, which just says "declare" (since adding "forward" would be redundant)
  • @Amanda you could write later on struct my_server_t { int x; }; for example
  • "struct my_server_t should be defined somewhere" That is not true, it is not required.
  • There is something called forward declaration in C
  • Thanks, I've updated the answer to be more accurate.