# Basic I/O concepts in C

> Learn the basics of input and output in C with the stdio.h library, the printf() and scanf() functions, and the stdin, stdout, and stderr streams.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2020-02-14 | Topics: [C](https://flaviocopes.com/tags/clang/) | Canonical: https://flaviocopes.com/c-input-output/

C is a small language, and the "core" of [C](https://flaviocopes.com/c-introduction/) does not include any Input/Output (I/O) functionality.

This is not something unique to C, of course. It's common for the language core to be agnostic of I/O.

In the case of C, Input/Output is provided to us by the C Standard Library via a set of functions defined in the `stdio.h` header file.

You can import this library using:

```c
#include <stdio.h>
```

on top of your C file.

This library provides us, among many other functions:

- `printf()`
- `scanf()`
- `sscanf()`
- `fgets()`
- `fprintf()`

Before describing what those functions do, I want to take a minute to talk about **I/O streams**.

We have 3 kinds of I/O streams in C:

- `stdin` (standard input)
- `stdout` (standard output)
- `stderr` (standard error)

With I/O functions we always work with streams. A stream is a high level interface that can represent a device or a file. From the C standpoint, we don't have any difference in reading from a file or reading from the command line: it's an I/O stream in any case.

That's one thing to keep in mind.

Some functions are designed to work with a specific stream, like `printf()`, which we use to print characters to `stdout`. Using its more general counterpart `fprintf()`, we can specify the stream to write to.

Since I started talking about `printf()`, let's introduce it now.

## printf()

`printf()` is one of the first functions you'll use when learning C programming.

In its simplest usage form, you pass it a string literal:

```c
printf("hey!");
```

and the program will print the content of the string to the screen.

You can print the value of a variable, and it's a bit tricky because you need to add a special character, a placeholder, which changes depending on the type of the variable. For example we use `%d` for a signed decimal integer digit:

```c
int age = 37;

printf("My age is %d", age);
```

We can print more than one variable by using commas:

```c
int age_yesterday = 36;
int age_today = 37;

printf("Yesterday my age was %d and today is %d", age_yesterday, age_today);
```

There are other format specifiers like `%d`:

- `%c` for a char
- `%s` for a string
- `%f` for floating point numbers
- `%p` for pointers

and many more.

We can use escape characters in `printf()`, like `\n` which we can use to make the output create a new line.

## scanf()

`printf()` is used as an output function. I want to introduce an input function now, so we can say we can do all the I/O thing: `scanf()`.

This function is used to get a value from the user running the program, from the command line.

We must first define a variable that will hold the value we get from the input:

```c
int age;
```

Then we call `scanf()` with 2 arguments: the format (type) of the variable, and the address of the variable:

```c
scanf("%d", &age);
```

If we want to get a string as input, remember that a string name is a pointer to the first character, so you don't need the `&` character before it:

```c
char name[20];
scanf("%s", name);
```

Here's a little program that uses both `printf()` and `scanf()`:

```c
#include <stdio.h>

int main(void) {
  char name[20];
  printf("Enter your name: ");
  scanf("%s", name);
  printf("you entered %s", name);
}
```