The **`perror()` function** prints the description of the error to the standard error output stream that corresponds to an error code saved in the system variable `errno`.
> The `errno` variable is of type `int` -- its value describes the error condition that occurs in the code.

No special library needs to be added in the code to use the `perror` function.

# Parameters

The function takes in one parameter, the pointer to a string. If the parameter is not a `null` pointer, then the custom message is followed by a colon and space; else, only the #key# error message: corresponds to the error code in the erno variable #key# is printed.

# Return value

The `perror` function has a return type of `void`, so it has no return value.


# Example



The code below tries to open a file with the name "demo.txt". Since no such file exists in the directory, an error is raised. In line 9, The custom message added in the `perror` function is "Error". When the program runs, "Error" is printed, followed by a colon and a space before the error message.



#include <iostream>
using namespace std;

int main() {
  
   // Code tries to open a non existant file "demo.txt"
   FILE * file = fopen("demo.txt", "r");
   // The following string is prepended to the error description.
   perror("Error");
   return(-1);


}

How to use perror() in C++

The perror() function prints error descriptions from errno to standard error with an optional custom message.