Unix C files , scanf is ignoring whitespace - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <fcntl.h>
void main (int argc, char* arcv[]) {
int fd, quest_num, i, j;
char* str;
char temp[2], buffer[20];
fd = open(arcv[1], O_WRONLY | O_CREAT, 0664);
printf("Insert number of questions\n");
scanf("%s", str);
write(fd, str, sizeof(str));
write(fd, "\n", 1);
quest_num = atoi(str);
for (i = 1; i <= quest_num; i++) {
printf("Insert question %d\n", i);
scanf("%[^\n]s", buffer);
printf("\n %s \n", buffer);
write(fd, "Question ", );
sprintf(temp, "%d", i);
write(fd, temp, sizeof(temp));
write(fd, "\n", 1);
write(fd, str, sizeof(temp));
write(fd, "\n", 1);
}
close(fd);
}
I want to make inputs like this:
Insert Number of Question:
2
Insert Question 1:
X+1=0 x=?
Insert Question 2:
X+y=0
and inside the file content i want it to look like this:
Question 1: X+1=0 x=?
1. 5
2. 2
3. 0
4. 1 Question 2: X+y=0
1. X=y
2. X= y
3. X=1
4. Y=1
but I get this in the terminal:
Insert number of questions
2
Insert question 1
x+y x=?
Insert question 2 (input is ignored here)
and inside the file:
2
Question 1
x+
Question 2
x=
Question 3
2
So in summary, scanf is ignoring the spaces input and there's an extra loop in the file content.

Answer 1

The basic problem is that when you scanf something that doesn't match a newline (either %s or %[^\n]) it won't read the newline at the end of the line and will leave it for the next stdio input rountine to read. You need to actually read and discard those newlines somehow.
The easiest way to do this is to use fgets to read input lines instead of scanf.
printf("Insert number of questions\n");
fgets(buffer, sizeof(buffer), stdin);
write(fd, buffer, strlen(buffer));
quest_num = atoi(buffer);
for (i = 1; i <= quest_num; i++) {
printf("Insert question %d\n", i);
fgets(buffer, sizeof(buffer), stdin);
Note that fgets will read the line including the newline, and the newline will be in the buffer, so if you don't want it, you'll need to strip it out explicitly.
If you really must use scanf, you can use a space in the format to explicitly skip over leading whitespace (including any lingering newline(s)), which might actually be desirable.
printf("Insert number of questions\n");
scanf("%19s", buffer);
write(fd, str, strlen(buffer));
write(fd, "\n", 1);
quest_num = atoi(buffer);
for (i = 1; i <= quest_num; i++) {
printf("Insert question %d\n", i);
scanf("% 19[^\n]", buffer);
printf("\n %s \n", buffer);
Note that when using scanf with %s or %[ and a fixed-size buffer, you should use an explicit size in the format to avoid buffer overruns