The Confusing Differences Between struct and typedef struct

Overview

It’s rather unfortunate that the syntax around defining a struct in the C programming language can be confusing, especially the behaviour differences between using struct and typedef struct. This page will hopefully clear up the confusion!

Just Using a Tag

If you define a struct with just a tag, i.e. struct <tag>, you get no created variables, and you have to type struct <tag> everytime you want to make one. For example, if we type:

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struct Person {
  uint8_t age;
  char name[50];
};

You then create a variable of this type with:

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struct Person my_person;

Tag and Variable

What happens if we add a word after the } but before the ;? This creates a variable of the struct type that we can use:

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struct Person {
  uint8_t age;
  char name[50];
} my_person;

We can now use this struct variable:

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// Now we get one struct already created for us
my_person.age = 4;
// We can still now make more variables of this type
struct Person my_person2;

Typedef

Now most of the confusion comes from when you add typedef into the mix. If you add typedef to the front of the struct definition like this:

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typedef struct Person {
  uint8_t age;
  char name[50];
} Person;

This changes the meaning of the word after the }, it now becomes an alias for the struct type, and not the name of a variable as it did above. What this does is save you having to type struct Person all the time, now you can just type Person. This is why most people use typedef with struct.

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Person my_person;

Note that I have used the same word Person as the typedef alias as I did for the struct tag. This is fine, there will be no naming collisions, and I could still create variables using struct Person if I wanted to.

You can also leave out the last Person when defining a struct using typedef, but there is not much point in this!

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typedef struct Person {
  uint8_t age;
  char name[50];
} Person;

What you will see often though is people using typedef struct and then skipping the tag, and only providing an alias at the end:

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typedef struct {
  uint8_t age;
  char name[50];
} Person;

Which then allows you to use the word Person as a type, but not struct Person (which is generally acceptable):

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Person my_person; // This works
struct Person my_person; // This won't (but this is normally fine!)