# What it does

**`printf_s`** is a function in the C library. Like the `printf` function, the `printf_s` also prints given arguments into the standard output stream, but it has the added functionality of validating these arguments before they are printed. These validation checks take place at run time. If any them fail, the execution ceases.



# Function syntax



# Parameters

The **restrict format** string contains specifications like `%s` and `%n` that determine the output format of the arguments to print.

# Return value

`printf_s` returns a negative value if there is an error in execution or if any run time checks fail. Otherwise, it returns the number of characters transmitted to the output stream for successful execution.

>The `printf_s` function is optional and not all compilers support it. If it is supported, the macro __STDC_WANT_LIB_EXT1__ will be defined to 1.
# Runtime checks

`printf_s` performs the following checks at run time on the argument it receives:


* The **format** is not a null pointer. 
* The **%n** specifier is not present inside the string pointed to by the format argument. 
* The argument corresponding to a **%s** specifier present in the format does not point to null.


# Example

Since `printf_s` function's compiler support is optional, your code must always accommodate both potential options. This is done by only invoking `printf_s` when the macro  __STDC_WANT_LIB_EXT1__  is set to 1.

if __STDC_WANT_LIB_EXT1__ == 1
    printf_s( "%s", "print_s works\n" );
else
    printf( "%s", "print_s not supported\n" );

Output in the case `printf_s` is supported:
```
print_s works
```



Output in the case `printf_s` is not supported:
```
print_s not supported
```

What is printf_s in C?

`printf_s` in C is a safer alternative to `printf`, performing runtime checks on arguments. If checks fail, execution stops and returns a negative value. It's optionally supported.