How to use for loop to count number - c

Question

I would like to write a program that can continuously + 1 to ID once user registered a user. I use for loop to design it, but I don't know how to break the loop after registered a user. Please help me.
Expected Data in idtest.txt:
SID001:USER1
SID002:USER2
SID003:USER3
.
.
and so on
Actual Data in idtest.txt:
SID001:USER1
SID002:USER1
SID003:USER1
.
until SID00999:USER1
My Code:
int main()
{
FILE* f;
char data[1024], user[50];
int num = 0, c, buf;
printf("Enter User: ");
scanf("%s", user);
f = fopen("idtest.txt", "a+");
while (fgets(data, sizeof data, f));
for (buf = 1; buf <= 999; buf++)
{
c = fgetc(f);
if (c == EOF)
{
fprintf(f, "SID00%d:%s\n", num, user);
}
else if (num >= 1)
{
fprintf(f, "SID00%d:%s\n", num, user);
}
else if (num >= 9)
{
fprintf(f, "SID0%d:%s\n", num, user);
}
else if (num >= 99)
{
fprintf(f, "SID%d:%s\n", num, user);
}
break;
}
return 0;
}

Answer 1

There are multiple problems in your code:
you do not test for errors
as coded, num is always 0: the program cannot produce the file contents you posted.
it makes no sense to try an read the next byte with fgetc(f) since the previous loop reached the end of the file already. This call returns EOF, which explains why you always write 2 leading zeroes.
you cannot mix reading and writing operations without an intervening seek operation, it has undefined behavior. Your code manages to write to the file, but it might behave differently with a different compiler / library / operating system...
you can simplify the output code using %03d to format numbers with leading zeroes.
naming an int variable buf is very confusing.
If you intend for the program to register one user at a time, you should read the file and count the number of lines, then write a single entry at the end.
Here is a modified version:
#include <stdio.h>
int main() {
char user[50];
char data[1024];
FILE *f;
int num;
printf("Enter User: ");
if (scanf("%49s", user) != 1) {
fprintf(stderr, "invalid or missing input\n");
return 1;
}
f = fopen("idtest.txt", "a+");
if (f == NULL) {
fprintf(stderr, "cannot open file idtest.txt\n");
return 1;
}
/* count the number of users */
num = 1;
rewind(f);
while (fgets(data, sizeof data, f)) {
num++;
}
/* switch to writing mode */
// No need to seek because the above loop hit the end of file.
// which is a special case where the next call can be a write operation
//fseek(f, 0L, SEEK_CUR);
fprintf(f, "SID%03d:%s\n", num, user);
fclose(f);
return 0;
}