What is perror() in C++?

The perror() function displays the description of the error that corresponds to an error code stored in the system variable errno.

errno is a system variable which holds the error code that describes the error condition. This error condition is produced by a call to a library function.

Syntax

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Input

  • str: The pointer to a null-terminated string. This string should contain an explanatory message about the error.

Code

The perror() function outputs the error description by combining the following in the given order:

  • The contents of the string pointed to by str.
  • The character “:”.
  • The error message corresponding to the error code in the variable errno.
#include <stdio.h>
int main () {
  FILE *fp;
  /* If we try to open a file that does not exist
  the variable errno will be set accordingly. */
  fp = fopen("file.txt", "r");
  if( fp == NULL ) {
    // The following string is prepended to the error description.
    perror("The error is ");
    return(-1);
  }
  fclose(fp); 
  return(0);
}

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