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Showing content from https://www.tutorialspoint.com/c_standard_library/c_function_ferror.htm below:

C Library - ferror() function

The C library ferror(FILE *stream) function tests the error indicator for the given stream.When working with file operations, it is important to be aware of any errors that may occur, as they can lead to corrupted data, crashes, or undefined behavior. The ferror function helps in detecting such errors.

Following is the C library syntax of the ferror() function −

int ferror(FILE *stream);

• FILE *stream: This is a pointer to the FILE object that identifies the stream to be checked for errors. This stream is usually obtained from functions like fopen.

The ferror function returns a non-zero value if an error has occurred with the specified stream.It returns 0 if no error has occurred.

This example attempts to write some text to a file. After writing, it checks if any error occurred using ferror.

Below is the illustration of C library ferror() function.

#include <stdio.h>

int main() {
   FILE *file = fopen("example1.txt", "w");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   fputs("Hello, World!", file);

   if (ferror(file)) {
       printf("An error occurred while writing to the file.\n");
   } else {
       printf("Writing to the file was successful.\n");
   }

   fclose(file);
   return 0;
}

The above code produces following result−

Writing to the file was successful.

In this example we force an error by closing the file and then attempting to write to it. It checks for the error using ferror.

#include <stdio.h>

int main() {
   FILE *file = fopen("example3.txt", "w");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   fputs("This is a test.", file);

   fclose(file);

   // Attempt to write to the closed file
   fputs("This will cause an error.", file);

   if (ferror(file)) {
       printf("An error occurred because the file was already closed.\n");
   } else {
       printf("Writing to the file was successful.\n");
   }

   return 0;
}

After execution of above code, we get the following result

An error occurred because the file was already closed.

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