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C clearerr() function

C library function - clearerr()

The clearerr() function is used to clear the end-of-file and error indicators for the stream to which stream points.

Syntax:

void clearerr(FILE *stream)

clearerr() Parameters:

Name Description Required /Optional
stream Identifies an address for a file descriptor, which is an area of memory associated with an input or output stream. Required

Return value from clearerr()

  • The clearerr() function shall not return a value.

Example: clearerr() function

The following program opens an existing file called test.txt for reading. As the file opens in reading mode so it will not allow to write anything in it, so when fputc() function try to write some text ferror() function will display an error message. This error, however, is cleared using the clearerr() function. Therefore, the next time the ferror() function performs an error check, it displays that no errors have been detected.

#include <stdio.h>

int main ()
{
  FILE * fp;
  fp = fopen("test.txt","r");

  if (fp == NULL) perror ("Error opening file.");
  else {
    fputc('abcd', fp);

    if(ferror(fp)) 
    {
      printf("Error writing to test.txt.\n");
      clearerr(fp);
    }

    fgetc(fp);

    if (!ferror(fp))
      printf("No error reading test.txt.\n"); 

    fclose(fp);
  }

  return 0;
}

Output:

Error writing to test.txt.
No error reading test.txt.

Assuming we have a text file file.txt, which is an empty file, let us compile and run the above program, this will produce the following result because we try to read a file which we opened in write only mode.

C Programming Code Editor:

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C Programming: Tips of the Day

Reading a string with scanf :

An array "decays" into a pointer to its first element, so scanf("%s", string) is equivalent to scanf("%s", &string[0]). On the other hand, scanf("%s", &string) passes a pointer-to-char[256], but it points to the same place.

Then scanf, when processing the tail of its argument list, will try to pull out a char *. That's the Right Thing when you've passed in string or &string[0], but when you've passed in &string you're depending on something that the language standard doesn't guarantee, namely that the pointers &string and &string[0] -- pointers to objects of different types and sizes that start at the same place -- are represented the same way.

Ref : https://bit.ly/3pdEk6f