While writing into a file , Where intermediate data stored in between fopen() and fclose()?

问题

Below is a small program which takes information from user and write it into a file teacher.txt. I am using only one array q2[30] for taking input and writing into a file using fprintf(). But when i want to enter more teacher then again loop will execute but at this time fclose() will not appear so data will not be write/save(don't know) into file also previous value of q2 get erased/overwrite with new input. So in this case where data is stored/write by fprintf().Because when i manually open teacher.txt before fclose() there is no any new data.

#include <conio.h>
#include <iostream.h> 

int main()
{
 system("cls");
 int yoe;
 char cond[]="yes";char q2[30];

 FILE *p;
 p = fopen("teacher.txt","a+"); //opening file in reading + appending mode
 printf("\nDo you want to add more Teacher ? (yes/no)\n");
 gets(q2);
 fflush(stdin);
 if(!strcmp(q2,cond))
 {
     do
     {
         printf("\nEnter Teacher's Name\n");
         gets(q2);
         fprintf(p,"\n!%s!",q2);
         printf("Enter Teacher's Qualifications\n");
         fflush(stdin);
         gets(q2);
         fprintf(p,"%s!",q2);
         printf("Enter Teacher's year of experience (0-30)\n");
         fflush(stdin);
         scanf("%d",&yoe);
         fprintf(p,"%d!",yoe);
         printf("Enter Teacher's Mobile number(id) [Should be of 10 digits]\n");
         fflush(stdin);
         gets(q2);
         fprintf(p,"%s!",q2);
         printf("\nDo you want to add more Teacher ? (yes/no)\n");
         fflush(stdin);
         gets(q2);
     }while(!strcmp(q2,cond));  // condition to check , if user want to add more Teacher , if yes then loop will execute again.
     fclose(p);  // when user enter 'no' then only fclose will appear.
 }
 fclose(p);printf("\nPress any key to return to Admin menu\n");
 getch();
 system("pause");
}

回答1:

When you open a file with fopen, the output you write is buffered, meaning it's not actually send to the lower layers until the buffer is either full or you explicitly flush it with fflush.

Also note that the layers under fopen/fprintf/fclose, all the way down to the actual hardware, may also have some buffering that can delay the actual update of the on-disk data.

来源：https://stackoverflow.com/questions/31072636/while-writing-into-a-file-where-intermediate-data-stored-in-between-fopen-an