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Showing content from https://www.cplusplus.com/reference/cstdio/fflush/ below:

function

fflush

int fflush ( FILE * stream );

Flush stream

If the given stream was open for writing (or if it was open for updating and the last i/o operation was an output operation) any unwritten data in its output buffer is written to the file.

If stream is a null pointer, all such streams are flushed.

In all other cases, the behavior depends on the specific library implementation. In some implementations, flushing a stream open for reading causes its input buffer to be cleared (but this is not portable expected behavior).

The stream remains open after this call.

When a file is closed, either because of a call to fclose or because the program terminates, all the buffers associated with it are automatically flushed.

Parameters

stream: Pointer to a FILE object that specifies a buffered stream.

Return Value A zero value indicates success.
If an error occurs, EOF is returned and the error indicator is set (see ferror).

Example In files open for update (i.e., open for both reading and writing), the stream shall be flushed after an output operation before performing an input operation. This can be done either by repositioning (fseek, fsetpos, rewind) or by calling explicitly fflush, like in this example:

/* fflush example */
#include <stdio.h>
char mybuffer[80];
int main()
{
   FILE * pFile;
   pFile = fopen ("example.txt","r+");
   if (pFile == NULL) perror ("Error opening file");
   else {
     fputs ("test",pFile);
     fflush (pFile);    // flushing or repositioning required
     fgets (mybuffer,80,pFile);
     puts (mybuffer);
     fclose (pFile);
     return 0;
  }
}