Notes on typedef and struct

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Here are my notes on using C's struct and typedef keywords.

struct

To create a C structure, first create a new structure type:

struct my_struct_tag
{
    int member1;
    float member2;
};

Note that in the example above, the struct keyword is used only to create a new data type but it has not defined any variables yet.

To define new varaibles of this structure type, do this:

struct my_struct_tag mystruct1;
struct my_struct_tag mystruct2;

You can optionally combine these two steps and create the new data type and define variables:

struct my_struct_tag
{
    int member1;
    float member2;
} mystruct1, mystruct2;

In this case the structure tag my_struct_tag is optional.

typedef

From the K&R book, the typedef keyword is used to create new data type names. For example:

typedef int Length;

The name Length is a synonym for int and can be used as follows:

Length len, maxlen;

The typedef keyword can also be used to rename struct data types. Using the example above:

typedef struct my_struct_tag MyStructType;
MyStructType mystruct1;
MyStructType mystruct2;

This creates a new data type name, MyStructType which is synonymous with the struct my_struct_tag data type, and then defines two variables, mystruct1 and mystruct2.

You could combine the creation of the type name with the creation of the struct data type:

typedef struct my_struct_tag
{
    int member1;
    float member2;
} MyStructType;

Like the previous example, this creates a new structure data type and creates a new type name, MyStructType which is synonymous with struct my_struct_tag. Like the combo example in the struct section above, the structure tag, my_struct_tag is optional. However, unlike that example, the identifier following the last curly brace (MyStructType) is the new type name and not a new variable. To define the variables, use:

MyStructType mystruct1;
MyStructType mystruct2;

See also How to share non-global C data structures
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