> For the complete documentation index, see [llms.txt](https://42-cursus.gitbook.io/guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://42-cursus.gitbook.io/guide/1-rank-01/ft_printf.md). # ft\_printf In a nutshell, the printf function is a command to display a formatted string on the screen. {% hint style="info" %} The word "format" means that format specifiers, which begin with the % character, indicate the location and method of converting a data element (such as a number) into characters. {% endhint %} Let's take an example. First, to use the printf function on C, you must include the "stdio" library (standard input/output). {% code lineNumbers="true" %} ```c #include int main() { printf("hello my name is Laura and i'm 23 years old\n"); printf("hello my name is %s and i'm %d years old\n", "Laura", 22); } ``` {% endcode %} On line 5 I did a printf of a simple string. On the next line, on line 6, I used the format specifiers so you can understand it better. **Both functions will return the same thing but it's in the construction that they differ.**

At the beginning of the sentence, each character is copied/written literally into the function's output. Then, when the function **finds a format specifier** (which starts with a % character), it will retrieve the argument that is in the same position and write it in a very specific way. For example when it finds the first %, it looks at what type it is. In our example, the first one is "s" so it will treat the first argument as a string, and use a specific method to copy this element into the function output. It will do the same thing when it finds the next %. Etc. ### Format specifiers The character after a '%' has different meanings. Let's just look at the ones we need to realize this project for school 42.

Character	Description
%	Prints a % character.
d, i	Print an int as a signed integer. %d and %i are synonymous for output, but are different when used with `scanf` for input (where using %i will interpret a number as hexadecimal if it's preceded by 0x, and octal if it's preceded by 0.)
u	Print decimal unsigned int.
x, X	Print an unsigned int as a hexadecimal number. x uses lower-case letters and X uses upper-case.
s	Print a null-terminated (\0) string.
c	Print a single character (char).
p	Print the address of a pointer or any other variable. The output is displayed in hexadecimal value. It's a format specifier which is used if we want to print data of type (void *).

### Return value (int) The prototype of the printf function is constructed like this: ```c int ft_printf(const char *format, ...); ``` As you can see, the `printf` function returns a value of type int. But why ? In short, it is mainly about **checking for errors**. You can make sure that the operation was successful or not using the return value. Checking the return value of `printf` allows you to detect failure *early* so you don't waste time attempting to write millions of lines when the first line already failed. * If a positive value is returned (indicating the number of characters written) it means the operation was successful. * If a negative number is returned there was some error. If you want to check what value the function returns, you can do a printf of your printf.

int main()
{
    int result = printf("Sentence to know how many %s\n", "characters were written");
    
    printf("%d characters were written", result);
}