The perror function in the stdio.h library of the C programming language is used to print a descriptive error message to the standard error stream (stderr) based on the value of the errno variable. The syntax of the perror function is as follows:

void perror(const char *str);

Here, the argument str is a string to be printed before the error message. If str is NULL, only the error message is printed.

The perror function prints a descriptive error message to the standard error stream, based on the value of the errno variable. The errno variable is a global variable defined in the errno.h library that contains the last error code generated by a system or library call. The perror function prints a message that describes the error represented by the errno value, along with the string str, if provided. If errno is zero, no message is printed.

For example, the following code attempts to open a file that does not exist, and uses the perror function to print the error message:

#include <stdio.h>
#include <errno.h>

int main() {
   FILE *fptr;
   fptr = fopen("nonexistent.txt", "r");
   if (fptr == NULL) {
      perror("Error");
      return 1;
   }
   fclose(fptr);
   return 0;
}

In this example, the fopen function attempts to open the file "nonexistent.txt" for reading. Since the file does not exist, the function returns NULL, indicating an error. The code in the if statement is executed, and the perror function is called to print the error message "Error: No such file or directory", along with the string "Error". The program then returns with an error code of 1.