fread() 在 C 中 fseek() 和 fwrite() 后不起作用

Question

In my program i make a binary file contains structs (each struct contains one integer)...and i put 3 structs in the file...i want first to create the file...then close it..then reopen it as "rb+" mode...and i want to read the structs from the file and change its value (member data) and rewrite it at the same file as this way:

#include <stdio.h>
main()
     {
         int i;

         struct
         {
             int data;
         }x;  

         FILE* myfile=fopen("d:\\text.bin","wb");

         for(i=1;i<4;i++)
         {
              x.data=i;
              fwrite(&x,sizeof(x),1,myfile);
         }

         fclose(myfile);

         myfile=fopen("d:\\text.bin","rb+");

         for(i=0;i<3;i++)
         {
              fread(&x,sizeof(x),1,myfile);
              printf("%d\n",x.data);
              fseek(myfile,-sizeof(x),SEEK_CUR);
              x.data=2*x.data;
              fwrite(&x,sizeof(x),1,myfile);
         }

         fclose(myfile);

     }`

but...my output in stdout file was: 1 2 2

it should be 1 2 3

BUT...when i added fseek(myfile,0,SEEK_CUR); after fwrite(&x,sizeof(x),1,myfile);....it is run correctly and output : 1 2 3

can any one help me ???

Answer 1

You just need to tell the program to switch the write mode to the read mode so

fseek(myfile,0,SEEK_CUR); 

It is necessary if you don't, you just get the same result with no change at all, or you might get an infinite loop.