1

I would able to read all my struct bytes by bytes (block of bytes are large "sizeof(Today)" so it's 8 bytes large). I attempt more and more times, but I can't! Give me help please!

Bin file: "temp.bin"

My struct:

typedef struct
{
    int year;
    int month; 
} Today;

And this is part of code that should be read 8 bytes each cycle:

 Today *d;
 d = malloc(sizeof(Today));

 fp = fopen("temp.bin", "rb"); 

 while(!feof(fp))
 { 
     fread(d, sizeof(Today), 1, fp);

     printf("Year = %i\n", d->year);
     printf("Month = %i\n", d->month);     
 } 

 fclose(fp);

More probably is wrong the while condition and fread() function, but I tried all possible combination but the output is never the right one! Thank you all

user3649959
  • 35
  • 7
  • 3
    Possible duplicate of [Why is “while ( !feof (file) )” always wrong?](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) – cadaniluk Oct 21 '15 at 10:05
  • Call `free` after `malloc`. – cadaniluk Oct 21 '15 at 10:05
  • I call free, but never happens! And Yes, I have 2 duplicated value printed! – user3649959 Oct 21 '15 at 10:20
  • Sidenote: Nothing wrong with `malloc`, but if you are allocating 1 `Today` temporarily, it would be more straightforward to just define it locally as `Today d;`. Then you don't have to deal with `malloc/free` hassle. Your structure is really small in size, so size shouldn't really be an issue. – user694733 Oct 21 '15 at 11:31

2 Answers2

0

You may want to allocate space for all your blocks first e.g.

fp = fopen( "temp.bin", "rb" );
if ( fp != NULL )
{
   fseek( fp, 0L, SEEK_END );
   size_t size = ftell(fp);
   rewind(fp);

   Today *d;
   d = malloc(size);
   fread( d, sizeof(Today), size/sizeof(Today), fp );
   fclose(fp);
}

then you got the structs in an array

e.g. printf( "month: %d", d[5].month);

then do what you need to do on each element in the array then later write it back. Depends on how large your file is though

AndersK
  • 35,813
  • 6
  • 60
  • 86
  • Thank you! But casually, I insert between fwrite() and fread() this line: fsee(pF, 0L, SEEK_SET); and I am able to read the whole FILE :D However I get your tips :D Now unique problem is that while(!feof(f)) do last cycle 2 times and print 2 times the same value! – user3649959 Oct 21 '15 at 10:47
  • ...which is why your post is marked up as a dup. 'read/check/act' is fine. 'check/read/act' is wrong. – Martin James Oct 21 '15 at 11:00
0

there is nothing about your code, i guess your file format is not as you think.

i also wander why you have to allocate memory for struct.

the code here is similar to that you provided. it just demonstrate the writing of the data in the format you described and then retrieving it without any lose.

#include <stdio.h>
#include <string.h>
typedef struct
{
    int year;
    int month; 
} Today;

#define DataFile    "temp.bin"
int main(){
    Today d;
    printf("sizeof(Today)=%d\n",sizeof(Today));

    FILE  *fp = fopen(DataFile, "wb");
    for(int i=1;i<13;i++){
        d.month=i;
        d.year=2000+i;
         fwrite(&d, sizeof(Today), 1, fp);
    } 
    fclose(fp);

    printf("write complete, press a key to read\n");
    _getch();

    fp = fopen(DataFile, "rb"); 

    while(fread(&d, sizeof(Today), 1, fp)){ 
         printf("Year = %i\t", d.year);
         printf("Month = %i\n", d.month);     
    } 
    fclose(fp);
    return 0;
}
milevyo
  • 2,165
  • 1
  • 13
  • 18