C language—perror function

In C language, perror is a library function used to display the error description associated with the previous system call. It is used to process and display  errno error messages from variables.

When a system call fails, such as  open,  read,  write etc., errno it will be set to an error code indicating what error occurred. Use  perror to display a descriptive error message related to the error code to the user.

perror The prototype is as follows:

#include <stdio.h>

void perror(const char *s);

where s is a string usually used to describe where or why the error occurred. When  perror called, it prints the argument  s, followed by a colon, a space, and  errno an error message related to the current value.

Example:

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

int main() {
    FILE *fp;

    fp = fopen("non_existent_file.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        exit(EXIT_FAILURE);
    }

    // ...其他代码...

    fclose(fp);
    return 0;
}

If the file "non_existent_file.txt" does not exist, the above code will output an error message similar to the following:

Error opening file: No such file or directory

Here "Error opening file" is  perror the description we provided, and "No such file or directory" is  errno the system error message associated with the value.