Updating and deleting contents of a file - c

Question

I have a file named as Empdata.txt having fields as "(%s)id,(%s)name,(%d)salary". Firstly, i am inserting some values to it. Now i want to update record having the same id as given by the user. I am using this code:
struct employee
{
char id[100];
char name[100];
int sal;
};
int main()
{
FILE *ptr;
struct employee e;
ptr=fopen("Empdata.txt","w+");
printf("Enter Employee id");
scanf("%s",id);
while(!feof(ptr))
{
fread(&e,sizeof(e),1,ptr);
if(strcmp(id,e.id)==0)
{
printf("Enter name");
scanf("%s",e.name);
printf("Enter basic salary");
scanf("%d",&e.sal);
fwrite(&e,sizeof(e),1,ptr);
}
};
fclose(ptr);
}
But it is not matching the ids. So it is not updating even. Further i have to perform delete operation also. I am not getting how to delete record having a particular id.
Please assume all variables are initialized properly.

Answer 1

If you plan to update a record, you will have to both read, and write records. Hence the "w" file mode is insufficient (ie:fopen("Empdata.txt","w");). Perhaps the "r+" mode would be more appropriate?
First thing in your while() loop, you need to read a record from your file.
At the bottom of the loop, you need to note your file position (ie:ftell()), so that if the next record is a match, you will know where to fseek() back to in order to write the updated record back to the file.
long position=0;
...
while(!feof(ptr))
{
fread(&e,sizeof(e),1,ptr);
if(strcmp(id,e.id)==0)
{
...
}
position=ftell(ptr);
};
If you find a matching record (ie:if(strcmp(id,e.id)==0)), the very last thing you should do is re-position to the beginning of the matching record (ie:fseek()) and then write the updated record. And since you have accomplished your task, you should break out of the while loop and return.
{
...
fseek(ptr, position, SEEK_SET);
fwrite(&e,sizeof(e),1,ptr);
}