Thread: Use of fseek() function

hey people
i have written a program to demonstrate the use of fseek() function on a file.
the program creates a file student.txt in writing mode and the user enters the name and roll number of four students which are stored on the file.
after this the file is opened in reading mode and the user enters the record number which he needs to see.
then using the fseek() function the program displays only that record of the file.
the program is working fine but when i open the text file created, its contents are pretty messed up.
here is my code

Code:

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>

struct student
{
char name[30];
char roll[12];
}st;

void main()
{
FILE *fp;
int i=0;
clrscr();
fp=fopen("D:\student.txt","w");
if(fp==NULL)
{
printf("\nError opening file...");
exit(0);
}
for(i=0;i<=3;i++)
{
printf("\nenter name : ");
scanf("%s",&st.name);
printf("\nenter roll : ");
scanf("%s",&st.roll);
fwrite(&st,sizeof(struct student),1,fp);
}
fclose(fp);

int x;
printf("\nEnter record number : ");
scanf("%d",&x);
fp=fopen("D:\student.txt","r");
if(fp==NULL)
{
printf("\nerror opening file...");
exit(0);
}
fseek(fp,(x-1)*sizeof(struct student),SEEK_SET);
if(fread(&st,sizeof(struct student),1,fp)==1)
{
printf("\nName = ");
printf("%s\n",st.name);
printf("Roll = ");
printf("%s",st.roll);
}
fclose(fp);
}

My output is....

Code:

enter name : abhilash
enter roll : 123
enter name : arati
enter roll : 456
enter name : jayajeet
enter roll : 789
enter name : hyder
enter roll : 135

Enter record number : 3

name= jayajeet
roll= 789

the program works fine with any number entered between 1 and 4.
but when i open the file created in notepad its contents are all messed up.
here is the content of the file student.txt

Code:

abhilash                      123         arati sh                      456         jayajeet                      789         hyder et                      135

the entries made in the file have random letters in a weird pattern.
i want to give it a traditional database look like the following one.

Code:

abhilash 123
arati 456
jayajeet 789
hyder 135

this i can do easily with fprintf() function but then the fseek() function does not work.
this program is gonna be part of my project. any suggestion ???

You cannot have both a nice legible datafile and the possibility to fseek in that file.
The closest you can get is to initialize the stringfields with e.g ' ' characters before reading input using scanf().
Kurt

thank you so much for this info.
helped a lot.

Originally Posted by hk_mp5kpdw

If you are writing the null characters stored when entering the name/roll fields (which you are doing with the fwrite call), then all that is "messed" up will be the file when you view it with notepad. When you read the data back to output to the screen, you read in the stored null as well. The printf calls know to stop output of string data when it reaches the null so your program's display would still look correct even if the file itself did not look correct.

I was wondering if i can use fseek() function to delete a particular entry from the database. so far I've only been able to display a particular entry and in some other operations I need to delete a particular entry while keeping others intact.
Any suggestion how i can do this??

Originally Posted by phoneix

I was wondering if i can use fseek() function to delete a particular entry from the database. so far I've only been able to display a particular entry and in some other operations I need to delete a particular entry while keeping others intact.
Any suggestion how i can do this??

There are two ways to do this:

1) Copy the file - record by record - and skip the one you want to delete.

2) "fseek" to the record and mark it as "deleted", i.e. overwrite the name with some special value that seems appropriate. For example: "*****"

i think that process would be long what about using fseek to go to the location of the unwanted data and then cutting to a str and emptying the string with str = null may be i think it may be better...

Originally Posted by Nyah Check

i think that process would be long what about using fseek to go to the location of the unwanted data and then " and emptying the string with str = null may be i think it may be better...

In that case, the "null"-string would be what I called "some special value that seems appropriate". But keep in mind that the record size must not change. Otherwise your file structure is broken and "fseek()"ing will no longer work as you expect it. So you cannot really "cut" the record, e.g. make it occupy less memory by replacing it by a single '\0' value. You can overwrite it with up to sizeof(struct student) bytes instead.