How do I use multiple fgets without them interfering with each other? - c

Question

Here's the basic skeleton of my code:
int getSentence(SearchResults* input){
printf("Enter sentence:");
fgets(input->sentence, 100, stdin);
printf("Enter message ID:");
fgets(input->messageID, 20, stdin);
return 1;
}
As is, this has a couple of problems. One is that when I print the string later, they have a newline character at the end, which I don't want. The other is that if I enter more than the 100 or 20 characters, it will overflow those characters into the next fgets call.
I've found "solutions" online, but they all either introduce new problems, or are recommended against using. Most of what I found is summarized in this SO post: How to properly flush stdin in fgets loop.
fflush works for the second problem, but is apparently "problematic".
I came up with *(strstr(input->sentence, "\n")) = '\0'; for the first issue, but it doesn't help with the second
while ((getchar()) != '\n')(and variations) is a commonly recommended solution for both problems, but it introduces a new issue: my next input requires two enters before the program will continue running. I tried adding fputc('\n', stdin); after the while loop and it solves all three problems, but only when the input exceeds the character limit. If I write a sentence less than 100 chars, I have to hit enter twice for the program to continue.
setbuf(stdin, NULL) is one of the solutions given, but one of the comments says "mucking around with setbuf is even worse [than fflush]"

Answer 1

I recommend that you create your own function that calls fgets, verifies that the entire line was read in and removes the newline character. If the function fails due to the input being too long, you can print an error message, discard the remainder of the line and reprompt the user. Here is a function that I have written myself for this purpose some time ago:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//This function will read exactly one line of input from the
//user. It will remove the newline character, if it exists. If
//the line is too long to fit in the buffer, then the function
//will automatically reprompt the user for input. On failure,
//the function will never return, but will print an error
//message and call "exit" instead.
void get_line_from_user( const char prompt[], char buffer[], int buffer_size )
{
for (;;) //infinite loop, equivalent to while(1)
{
char *p;
//prompt user for input
fputs( prompt, stdout );
//attempt to read one line of input
if ( fgets( buffer, buffer_size, stdin ) == NULL )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
//attempt to find newline character
p = strchr( buffer, '\n' );
//make sure that entire line was read in (i.e. that
//the buffer was not too small to store the entire line)
if ( p == NULL )
{
int c;
//a missing newline character is ok if the next
//character is a newline character or if we have
//reached end-of-file (for example if the input is
//being piped from a file or if the user enters
//end-of-file in the terminal itself)
if ( !feof(stdin) && (c=getchar()) != '\n' )
{
printf( "Input was too long to fit in buffer!\n" );
//discard remainder of line
do
{
if ( c == EOF )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
c = getchar();
} while ( c != '\n' );
//reprompt user for input by restarting loop
continue;
}
}
else
{
//remove newline character by overwriting it with
//null character
*p = '\0';
}
//input was ok, so break out of loop
break;
}
}
Here is a demonstration program in which I apply my function to your code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//forward declaration
void get_line_from_user( const char prompt[], char buffer[], int buffer_size );
int main( void )
{
char sentence[100];
char messageID[20];
get_line_from_user( "Enter sentence: ", sentence, sizeof sentence );
get_line_from_user( "Enter message ID: ", messageID, sizeof messageID );
printf(
"\n"
"The following input has been successfully read:\n"
"sentence: %s\n"
"message ID: %s\n",
sentence, messageID
);
}
//This function will read exactly one line of input from the
//user. It will remove the newline character, if it exists. If
//the line is too long to fit in the buffer, then the function
//will automatically reprompt the user for input. On failure,
//the function will never return, but will print an error
//message and call "exit" instead.
void get_line_from_user( const char prompt[], char buffer[], int buffer_size )
{
for (;;) //infinite loop, equivalent to while(1)
{
char *p;
//prompt user for input
fputs( prompt, stdout );
//attempt to read one line of input
if ( fgets( buffer, buffer_size, stdin ) == NULL )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
//attempt to find newline character
p = strchr( buffer, '\n' );
//make sure that entire line was read in (i.e. that
//the buffer was not too small to store the entire line)
if ( p == NULL )
{
int c;
//a missing newline character is ok if the next
//character is a newline character or if we have
//reached end-of-file (for example if the input is
//being piped from a file or if the user enters
//end-of-file in the terminal itself)
if ( !feof(stdin) && (c=getchar()) != '\n' )
{
printf( "Input was too long to fit in buffer!\n" );
//discard remainder of line
do
{
if ( c == EOF )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
c = getchar();
} while ( c != '\n' );
//reprompt user for input by restarting loop
continue;
}
}
else
{
//remove newline character by overwriting it with
//null character
*p = '\0';
}
//input was ok, so break out of loop
break;
}
}
This program has the following behavior:
Enter sentence: This is a test sentence.
Enter message ID: This is another test sentence that is longer than 20 characters and therefore too long.
Input was too long to fit in buffer!
Enter message ID: This is shorter.
The following input has been successfully read:
sentence: This is a test sentence.
message ID: This is shorter.

Answer 2

Here's what I'm using to solve my issue, thanks to Steve Summit and Andreas Wenzel for their comments.
int getSentence(SearchResults* input){
printf("Enter sentence:");
fgets(input->sentence, 100, stdin);
int temp = strcspn(input->sentence, "\n");
if(temp < 100 - 1) input->sentence[temp] = '\0';
else while ((temp = getchar()) != '\n' && temp != EOF);
printf("Enter message ID:");
fgets(input->messageID, 20, stdin);
temp = strcspn(input->messageID, "\n");
if(temp < 20 - 1) input->messageID[temp] = '\0';
else while ((temp = getchar()) != '\n' && temp != EOF);
return 1;
}
EDIT: For anyone else who has the same issue as me, see comments below for additional problems you may need to account for if you use this solution, such as fgets returning NULL.