Thread: Binary file I/O

Okay, I'll be the first to admit that I'm in a little bit over my head here.
Got this assignment from school where we're supposed to read from/write to a binary file, more precisely an index covering name, age and weight.

We are supposed to use structs and arrays to accomplish this task.

I can not make any sense of this, and have no idea how to get fread to work.
when I printf the values (e.g. personinfo[0].name) it's all nonsense.

Am I totally confused in my coding, or is there some sense in it?

Any help would be very much appreciated, since my teacher is on vacation.

Code:

#include <stdio.h>
#include <string.h>

struct personinfo
{
  char name[30];
  int age[3];
  float weight[5];
};

int main (void) {

int i,j;



printf("How many entries?\n");
scanf("%d",&j);

FILE *fp;
fp=fopen("data.bin", "rb");

struct personinfo person[4];

i=0;
  for (i=0;i<j;i++){
  printf("Name:\n");
  scanf("%s",person[i].name);
  fwrite(person[i].name,sizeof(struct personinfo),sizeof(i),fp);

  printf("Age:\n");
  scanf("%d",person[i].age);
  fwrite(person[i].age,sizeof(struct personinfo),sizeof(i),fp);

  printf("Weight:\n");
  scanf("%f",person[i].weight);
  fwrite(person[i].weight,sizeof(struct personinfo),sizeof(i),fp);
}

i=0;
for (i=0;i<j;i++){
  fread(person[i].name,sizeof(struct personinfo),sizeof(i),fp);
  fread(person[i].age,sizeof(struct personinfo),sizeof(i),fp);
  fread(person[i].weight,sizeof(struct personinfo),sizeof(i),fp);

}


fclose(fp);
}

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

rb means open file for reading but you want to write to it, so...

Maybe your teacher intends you to do something like this:

Code:

struct personinfo person;
for (i=0; i < j; i++){
        printf("Name: ");
        scanf("%s", person.name);
        printf("Age: ");
        scanf("%d", &person.age);
        printf("Name: ");
        scanf("%f", &person.weight);
        fwrite(&person, sizeof(person), 1, fp);
}

// here you need to close & reopen, or rewind, or something. . .
// then read the structs back:
while (!feof(fp)){
        fread(&person, sizeof(person), 1, fp);
}

The beauty of using fread & fwrite is that you can dump an entire struct to a file such that (assuming you have no pointers in your struct) when you read the data back you get exactly what you wrote.

Thanks sugarfree, changed it now.

Kennedy, your method is undoubtedly a whole lot prettier than mine.
Though I can't get it to read anything back to me.

My code now looks as you suggested:

Code:

i=0;
  for (i=0;i<j;i++){
  printf("Name:\n");
  scanf("%s",person[i].name);
  printf("Age:\n");
  scanf("%d",&person[i].age);
  printf("Weight:\n");
  scanf("%f",&person[i].weight);
  fwrite(&person,sizeof(person),1,fp);
}

rewind(fp);

while(!feof(fp)){
  fread(&person, sizeof(person),1,fp);
}

fclose(fp);
}

But when I run the program, after entering the values of my choice, it only displays a blank row, nothing else. Could it be that the write operation failed somehow and that it has nothing to display, or is it because the reading is failing?

Very thankful for any input!

Are you opening the file for reading and writing?

Thanks Adak, good point, I only opened it in wb.
After I changed it to wrb though, it just exits the program as soon as i have entered the values of my choice. Any input?

You need to print out the data, if you want to see it. Just reading it into a struct, won't display it on the screen.

Ah, sorry for me being slow on the uptake! Now that i print it, it prints the values! Thank you very much!

Though it does not seem to write anything to the file, but just use the buffer values.

Post up your whole code, obviously, something is wrong if nothing is being written out to the file.

Here is my code in full as is of now

Code:

#include <stdio.h>
#include <string.h>

struct personinfo
{
  char name[30];
  int age;
  float weight;
};

int main (void) {

int i,j;



printf("How many entries?\n");
scanf("%d",&j);

FILE *fp;
fp=fopen("data.txt", "wrb");

struct personinfo person[4];

i=0;
  for (i=0;i<j;i++){
  printf("Name:\n");
  scanf("%s",person[i].name);
  printf("Age:\n");
  scanf("%d",&person[i].age);
  printf("Weight:\n");
  scanf("%f",&person[i].weight);
  fwrite(&person,sizeof(person),1,fp);
}

rewind(fp);

while(!feof(fp)){
  fread(&person, sizeof(person),1,fp);
  printf("%s\n",person[1].name);

}

fclose(fp);
}

Code:

while(!feof(fp)){
  fread(&person, sizeof(person),1,fp);
  printf("%s\n",person[1].name);

}

This is problematic.

Code:

  struct personinfo p;
 while(  fread(&p,sizeof(p),1,fp) == 1 ) {
  printf("%s\n",p.name);
 }
  // check EOF or read error

c-faq

Bayint:
Tried changing to your suggestion and got a segmentation fault after entering all the data in the program. Am I missing something?

Salem:
I must honestly say that I'm not very up to speed on the fread parameters. Which one is causing this?
As I understand it, I now write the whole person struct with all it's (3) values, with the size of the whole struct, and one element. Am I wrong in that assumption?

Best regards

sizeof(person) should evaluate to the size of the array in bytes, so that is wrong as well.