Check new lines before format element scanf - c

Question

So, I want to read some input in two integer variables with fscanf, the input must be
INTEGER ONE SPACE INTEGER \n
So for example a file could be like that:
235 190\n
And what I want to do is checking if there is any new line before the first integer or the second, then throw an exception if that's the case:
So basically what you do with fscanf for that case would be just to do like:
FILE *somefile;
[...]
int w, h;
if (fscanf(somefile, "%d%d") == EOF)
return (1);
But this method does not provide checking if there is a new line between the integers
(for example if there is INTEGER NEW_LINE INTEGER it will still read the second)
as it skips all spaces before seeing a digit, so what I managed to do is more close but still wrong:
FILE *somefile;
int w, h;
char check[2];
if (fscanf(somefile, "%d%c%d%c", &w, check, &h, check + 1) == EOF)
return (-1);
if (check[0] != ' ' || check[1] != '\n')
return (-1);
So after I did that I realized that the %c before the %d just reads one character, so if there is a space then a new line between the two, it will continue reading the file wrongly and won't detect an error.
Now I wonder if there is a way to skip all spaces like when using " %c", while knowing if we skipped a \n. Thanks for your help.
PS: I know we can use fgets and strtol but it would be too easy, in this project I can only use fscanf to parse the file.

Answer 1

scanf happily discards leading whitespace to match a %d conversion specifier, which makes this slightly difficult to do. But you can certainly use fgetc (or getchar) to validate a character and then push it back onto the stream for scanf to consume. Something like:
$ cat a.c
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
int c;
int d[2];
c = getchar();
if( 0
||!( c == '+' || c == '-' || isdigit(c))
|| ( c != ungetc(c, stdin) )
|| ( scanf("%d", d) != 1 )
|| ( getchar() != ' ' )
|| ( (c = getchar()) == EOF )
||!( c == '+' || c == '-' || isdigit(c))
|| ( c != ungetc(c, stdin) )
|| ( scanf("%d", d + 1) != 1 )
|| ( getchar() != '\n' )
){
fprintf(stderr, "invalid input!\n");
return EXIT_FAILURE;
}
printf("Read %d %d\n", d[0], d[1]);
return EXIT_SUCCESS;
}
$ gcc a.c
$ printf '5 28\n' | ./a.out
Read 5 28
$ printf '5 28\n' | ./a.out
invalid input!
$ printf ' 5 28\n' | ./a.out
invalid input!
$ printf ' 5 28' | ./a.out
invalid input!

Answer 2

if there is a way to skip all spaces
I can only use fscanf to parse the file.
Let us assume ungetc() is not allowed - which makes things harder.
"%d" happily consumes all (0 or more) leading white-spaces including '\n' without notice.
Use fscanf("%[...]") to find leading white-spaces first. The usual suspects are " \t\n\r\f\v".
// Consume leading whitespaces
// Return
// 1: \n or EOF found
// 0: not found
int EatLeadingWS(FILE *f) {
int count;
char space[2];
while ((count = fscanf("%1[ \t\n\r\f\v]", space)) == 1) {
if (space[0] == '\n') return 1;
}
return count == EOF;
}
Alternative: use "%c" and isspace(), but then we need a way to put non white-spaces back.
Then look for white-spaces before "%d"
int some_int;
if (EatLeadingWS(f) == 0) {
int count;
if ((count = fscanf("%d", &some_int)) == 1) {
printf("int %d found before a \\n\n", some_int);
}
}
Another alternative is to use char buf[100]; fscanf("%99[^\n]", buf) to read most of the line and then parse the string with sscanf(buf, "%d%1[ ]%d" ....