get 3 (max) digit integer from txt file - c

Question

First of all I am new to files in c, so it may be a simple question,
however I still didn't find a solution:
let's say that's the content of my file:
99
blah blah
...
...
I want to scan only the number from the beginning (it is always in a separate line)
My question is how to make it take the number (99) as one number and stop scanning.
int main(){
FILE* fp = fopen(file_name, "r");
int integer;
...
fclose(fp);
printf("%d", integer);
}
output for the file example:
99
-the nuber can be between 1 and 100-

Answer 1

I want to scan only the number from the beginning (it is always in a separate line).
That's a good hint, suggesting a line by line parsing of the input. You can use a combination of fgets(1) and sscan(2) to read that number.
fgets will read up to a certain number of character from a stream and store those character into a buffer. If it finds a newline, it stops reading, store the newline into the buffer followed by the null-terminator. Otherwise it only adds the terminator. If it fails, it returs a NULL pointer.
sscanf works basically like scanf or fscanf, but it reads from a character array, not from a stream.
It's also better to always check the return value of those library function.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUF_SIZE 1024
int main(void)
{
char const *file_name = "data.txt";
FILE *in_file = fopen(file_name, "r");
if (!in_file) {
fprintf(stderr, "Error while reading \"%s\": %s", file_name, strerror(errno));
return EXIT_FAILURE;
}
char buffer[BUF_SIZE];
int number = 0;
while( fgets(buffer, BUF_SIZE, in_file) ) {
if ( sscanf(buffer, "%d", &number) == 1 ) {
if ( 0 < number && number < 100 ) {
printf("%d", number);
break;
}
}
}
fclose(in_file);
return EXIT_SUCCESS;
}
Example.
Some references of the functions used in the previous snippet
1) fgets: man-pages or cppreference.
2) sscanf: man-pages or cppreference

Answer 2

Why not use scanf? (fscanf to be more precise):
On success, the function returns the number of items of the argument
list successfully filled.
(source: cppreference)
So just check how many values did you read, if 0 that means it's not a number so you can just skip that string, for that you can use "%*" prefix to ignore the data.
You also said:
I want to scan only the number from the beginning (it is always in a
separate line)
so after you read the number just skip the whole line with "%*[^\n]" (reads
data until a new line symbol is encountered)
int num;
int scanReturn;
FILE* f = fopen("file.txt", "r");
...
do {
scanReturn = fscanf(f, "%d", &num);
if(scanReturn == 0)
{
scanReturn = fscanf(f, "%*s");
}
else if(scanReturn != EOF)
{
fscanf(f, "%*[^\n]");
printf("%d, ", num);
}
} while(scanReturn != EOF);
fclose(f);