fgets() doesn't function as expected [duplicate] - c

Question

This question already has answers here:
fgets doesn't work after scanf [duplicate]
(7 answers)
scanf() leaves the newline character in the buffer
(7 answers)
Closed 5 years ago.
This is my code
int main(){
int N,i,radius,diameter,count =0;
char str[20];
char color[N][20];
printf("Get the num : ");
scanf("%d",&N);
printf("Enter the mesage\n");
for(i=0;i<N;i++){
fgets(color[i],20,stdin);
}
for(i=0;i<N;i++){
printf("%s",color[i]);
}
return 0;
}
Given inputs are:
N = 3
red 50,
50 green,
blue 50
Here the problem is fgets inside for loop gets executed only twice if N is 3. This problem doesn't arise if I comment the scanf statement. Can somebody explain me what is causing this problem and how it can be solved?

Answer 1

After a few hours of scratching my head, I realized the following:
Avoid using scanf. Managing buffer overflows is not easy.
Always try to use fgets to get user inputs.
Try this code here:
#include<stdio.h>
#include<stdlib.h>
int main(){
int N,i,radius,diameter,count =0;
char str[20];
printf("Get the num : ");
char buffer[64];
fgets(buffer, 64, stdin);
N = strtoul(buffer,NULL,10);
char color[N][20];
printf("%d\n",sizeof(color));
printf("Enter the mesage\n");
for(i=0;i<N;i++){
fgets(color[i],20,stdin);
if(color[i][strlen(color[i])-1]=='\n'){
color[i][strlen(color[i])-1]='\0';
}
else{
while((getchar())!='\n');//this is just to prevent an overflow for the size of char arrays
}
}
for(i=0;i<N;i++){
printf("%s\n",color[i]);
}
return 0;
}
Notice that I first input a number inside a char array. Convert that into a number using strtoul(string to unsigned long). Now inside the for loop I again use fgets to take inputs. The problem was, if you enter a string greater than 19 chars, the remaining part will be left inside the input buffer and shall be assigned to the subsequent input. To manage that I used getchar inside a while loop, which consumes all the unnecessary characters and the newline character from the input stream. Avoid using fflush as it may result in undefined behavior as answered here
-fflush(stdin) function does not work
-http://www.geeksforgeeks.org/clearing-the-input-buffer-in-cc/
Also note that you are using Variable Length Arrays which may not always be a good choice. Older versions of c compiler prohibit them. You had declared color[N][20] first before initializing N. That was wrong.
I suggest you to read this too
-C - scanf() vs gets() vs fgets()

Answer 2

After using scanf you'd need to clean the buffer. I suggest to never use scanf, just use fgets and then convert the output to a number:
int main(){
int N,i,radius,diameter,count =0;
char str[20];
char color[N][20];
printf("Get the num : ");
char buffer[64];
fgets(buffer, 64, stdin);
N = atoi(buffer); // you need to include stdlib.h
printf("Enter the mesage\n");
for(i=0;i<N;i++){
fgets(color[i],20,stdin);
}
for(i=0;i<N;i++){
printf("%s",color[i]);
}
return 0;
}

Answer 3

Due to this reference:
https://www.tutorialspoint.com/c_standard_library/c_function_fgets.htm
this will be correct:
int _tmain(int argc, _TCHAR* argv[])
{
int N, i, radius, diameter, count = 0;
char str[10];
char color[20];
printf("Get the num : ");
scanf_s("%d", &N);
printf("Enter the mesage\n");
//for (i = 0; i<N; i++){
fgets(color, 20, stdin);
//}
//for (i = 0; i<N; i++){
printf("%s", color);
//}
return 0;
}
I changed scanf to scanf_s for VC++.