want to call fgets() multiple times based on users need - c

Question

User enters '1' or '0' choice to continue getting string using fgets(). So when the user enters the choice, fgets reads it from console. I am storing it in another variable. But fgets gets the choice and stores it in messages. I have tried using fflush(stdin) after receiving the choice. Please help me out.
int main() {
int choice=1;
char *message;
int i=0;
while (choice == 1) {
fflush(stdout);
printf("Enter the message: ");
fflush(stdout);
message = fgets(message,200,stdin);
while (message[i]!='\n') {
i++;
}
message[i] = '\0';
send_message(message);
printf("\nType '1' to continue or '0' to quit: ");
scanf("%d",&choice);
fflush(stdin);
}
}

Answer 1

It looks like you're trying to scanf() to read the user's input -- this is inherently dangerous. (See https://www.reddit.com/r/learnprogramming/comments/1d0w4x/c_scanf_d_but_error_if_user_enters_a_character/).
I'd recommend either using %s for your format string, or better yet, build a subroutine to do safe input and parse it the old-fashioned way, such as something along these lines:
/* getsafe() - Generic input using the preferred input method rather than gets() */
#include <stdio.h>
#include <string.h>
char *getsafe(char *inpstr,int inpsiz) {
char *seachr; /* Result of search via strchr() */
if (inpstr==NULL) {
return(NULL);
}
if (fgets(inpstr,inpsiz,stdin)==NULL) {
return(NULL);
}
seachr=strchr(inpstr,'\n');
if (seachr!=NULL) *seachr=0;
return(inpstr);
}
That way you can specify the buffer length and provide a string (array of characters) of sufficient length as to prevent buffer overruns (security issue), and then parse the [0] position in that array for your answer.
#define ANSSIZ 80 /* Maximum allowed size of user answer */
char usrans[ANSSIZ]; /* User Answer */
printf("Enter 'y' or 'n': ");
getsafe(usrans, ANSSIZ-1);

Answer 2

There's a lot of problems with this - It probably belongs on Code Review
However, here is a critique on some of the major problems
int main() {
int choice=1;
char *message; // This is a pointer, but is not malloc'ed. You might want "char message[200]" instead?
int i=0; // This is the only time "i" is set to 0. It needs to be reset at the start of the loop
while (choice == 1) {
fflush(stdout); // No need for this
printf("Enter the message: ");
fflush(stdout);
message = fgets(message,200,stdin);
while (message[i]!='\n') { // Why not use strlen?
i++; // "i" can keep growing forever if there is no newline (if someone entered 199 characters before pressing enter)
}
message[i] = '\0'; // fgets does this for you - The past loop was pointless
send_message(message);
printf("\nType 'y' to continue or 'n' to quit: "); // You forgot to flush here!
scanf("%d",&choice); // I don't think this will result in a 0 or 1 output... %d is for a digit, and you're asking the user for y or n.
fflush(stdin); // This is invalid and unneeded - You can't flush stdin
}
}