C Language

Typedef

Introduction#

The typedef mechanism allows the creation of aliases for other types. It does not create new types. People often use typedef to improve the portability of code, to give aliases to structure or union types, or to create aliases for function (or function pointer) types.

In the C standard, typedef is classified as a ‘storage class’ for convenience; it occurs syntactically where storage classes such as static or extern could appear.

Syntax#

  • typedef existing_name alias_name;

Remarks#

Disadvantages of Typedef

typedef could lead to the pollution of namespace in large C programs.

Disadvantages of Typedef Structs

Also, typedef‘d structs without a tag name are a major cause of needless imposition of ordering relationships among header files.

Consider:

#ifndef FOO_H
#define FOO_H 1

#define FOO_DEF (0xDEADBABE)

struct bar; /* forward declaration, defined in bar.h*/

struct foo {
    struct bar *bar;
};

#endif

With such a definition, not using typedefs, it is possible for a compilation unit to include foo.h to get at the FOO_DEF definition. If it doesn’t attempt to dereference the bar member of the foo struct then there will be no need to include the bar.h file.

Typedef vs #define

#define is a C pre-processor directive which is also used to define the aliases for various data types similar to typedef but with the following differences:

  • typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well.

  • typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.

  • Note that #define cptr char * followed by cptr a, b; does not do the same as typedef char *cptr; followed by cptr a, b;. With the #define, b is a plain char variable, but it is also a pointer with the typedef.

Typedef for Structures and Unions

You can give alias names to a struct:

typedef struct Person {
    char name[32];
    int age;
} Person;

Person person;

Compared to the traditional way of declaring structs, programmers wouldn’t need to have struct every time they declare an instance of that struct.

Note that the name Person (as opposed to struct Person) is not defined until the final semicolon. Thus for linked lists and tree structures which need to contain a pointer to the same structure type, you must use either:

typedef struct Person {
    char name[32];
    int age;
    struct Person *next;
} Person;

or:

typedef struct Person Person;

struct Person {
    char name[32];
    int age;
    Person *next;
};

The use of a typedef for a union type is very similar.

typedef union Float Float;

union Float
{
    float f;
    char  b[sizeof(float)];
};

A structure similar to this can be used to analyze the bytes that make up a float value.

Simple Uses of Typedef

For giving short names to a data type

Instead of:

long long int foo;
struct mystructure object;

one can use

/* write once */
typedef long long ll;
typedef struct mystructure mystruct;

/* use whenever needed */
ll foo;
mystruct object;

This reduces the amount of typing needed if the type is used many times in the program.

Improving portability

The attributes of data types vary across different architectures. For example, an int may be a 2-byte type in one implementation and an 4-byte type in another. Suppose a program needs to use a 4-byte type to run correctly.

In one implementation, let the size of int be 2 bytes and that of long be 4 bytes. In another, let the size of int be 4 bytes and that of long be 8 bytes. If the program is written using the second implementation,

/* program expecting a 4 byte integer */
int foo; /* need to hold 4 bytes to work */
/* some code involving many more ints */

For the program to run in the first implementation, all the int declarations will have to be changed to long.

/* program now needs long */
long foo; /*need to hold 4 bytes to work */
/* some code involving many more longs - lot to be changed */

To avoid this, one can use typedef

/* program expecting a 4 byte integer */
typedef int myint; /* need to declare once - only one line to modify if needed */
myint foo; /* need to hold 4 bytes to work */
/* some code involving many more myints */

Then, only the typedef statement would need to be changed each time, instead of examining the whole program.

The <stdint.h> header and the related <inttypes.h> header define standard type names (using typedef) for integers of various sizes, and these names are often the best choice in modern code that needs fixed size integers. For example, uint8_t is an unsigned 8-bit integer type; int64_t is a signed 64-bit integer type. The type uintptr_t is an unsigned integer type big enough to hold any pointer to object. These types are theoretically optional — but it is rare for them not to be available. There are variants like uint_least16_t (the smallest unsigned integer type with at least 16 bits) and int_fast32_t (the fastest signed integer type with at least 32 bits). Also, intmax_t and uintmax_t are the largest integer types supported by the implementation. These types are mandatory.

To specify a usage or to improve readability

If a set of data has a particular purpose, one can use typedef to give it a meaningful name. Moreover, if the property of the data changes such that the base type must change, only the typedef statement would have to be changed, instead of examining the whole program.

Typedef for Function Pointers

We can use typedef to simplify the usage of function pointers. Imagine we have some functions, all having the same signature, that use their argument to print out something in different ways:

#include<stdio.h>

void print_to_n(int n)
{
    for (int i = 1; i <= n; ++i)
        printf("%d\n", i);
}

void print_n(int n)
{
    printf("%d\n, n);
}

Now we can use a typedef to create a named function pointer type called printer:

typedef void (*printer_t)(int);

This creates a type, named printer_t for a pointer to a function that takes a single int argument and returns nothing, which matches the signature of the functions we have above. To use it we create a variable of the created type and assign it a pointer to one of the functions in question:

printer_t p = &print_to_n;
void (*p)(int) = &print_to_n; // This would be required without the type

Then to call the function pointed to by the function pointer variable:

p(5);           // Prints 1 2 3 4 5 on separate lines
(*p)(5);        // So does this

Thus the typedef allows a simpler syntax when dealing with function pointers. This becomes more apparent when function pointers are used in more complex situations, such as arguments to functions.

If you are using a function that takes a function pointer as a parameter without a function pointer type defined the function definition would be,

void foo (void (*printer)(int), int y){
    //code
    printer(y);
    //code
}

However, with the typedef it is:

void foo (printer_t printer, int y){
    //code
    printer(y);
    //code
}

Likewise functions can return function pointers and again, the use of a typedef can make the syntax simpler when doing so.

A classic example is the signal function from <signal.h>. The declaration for it (from the C standard) is:

void (*signal(int sig, void (*func)(int)))(int);

That’s a function that takes two arguments — an int and a pointer to a function which takes an int as an argument and returns nothing — and which returns a pointer to function like its second argument.

If we defined a type SigCatcher as an alias for the pointer to function type:

typedef void (*SigCatcher)(int);

then we could declare signal() using:

SigCatcher signal(int sig, SigCatcher func);

On the whole, this is easier to understand (even though the C standard did not elect to define a type to do the job). The signal function takes two arguments, an int and a SigCatcher, and it returns a SigCatcher — where a SigCatcher is a pointer to a function that takes an int argument and returns nothing.

Although using typedef names for pointer to function types makes life easier, it can also lead to confusion for others who will maintain your code later on, so use with caution and proper documentation. See also https://stackoverflow.com/documentation/c/250/function-pointers.

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