Thread: Flushing Out fflush(stdin);

Hey folks,

I have first concentrated on writing functional code that does what I want. However, I have read the FAQ and numerous posts about portability issues with fflush so I have now attempted to replace it with some more reliable, if more complex, code in the program I'm working on.

Here's what I had. These 3 lines are simple and effective for my purposes:

Code:

printf ("\nEnter a new name.\n\n>");
      scanf  ("%[^\n]", entry.name);
      fflush(stdin);

And here's the best mess I've come up with to replace it:

Code:

printf ("\nEnter a new name.\n\n>");
     fgets(entry.name, sizeof(entry.name), stdin);
     if (strchr (entry.name, '\n' && EOF) == NULL)          /* if overflow        */
     {
     printf ("\nYour entry is too long, please try again.");
           while (strchr (entry.name, '\n' && EOF) == NULL) /* keep reading until */
           {                                                /* stdin is empty     */
           fgets(entry.name, sizeof(entry.name), stdin);
           }
           input ();                                        /* recursive          */
     }     
           entry.name[strlen(entry.name) - 1] = ' ';        /* replace \n         */

I commented what I think it should be/want it to be doing. However, no matter what I enter, the if statements seem to always == NULL. I think this has something to do with the EOF because when I remove it the behavior changes. When I compile with only the newline in the if statements the first if works as I intend but I'm still getting junk from over-length entries in the name field output and the correct second entry from the "try again" is no where to be found.

Aside from actually fixing the code above, I'm open to any suggestions on how to streamline this as there's likely a much simpler way to do it. Thanks in advance for suggestions and insights. Cheers.

-SH

Okay, I just read about

Code:

while((ch = getchar()) != '\n' && ch != EOF);

In the recent Explanation of a Function thread and some bells are ringing. I'd still appreciate comments on my newline removal or anything else that's looking floppy. Cheers.

-SH

This uses do / while loops to accomplish what I think you are after. Any input longer than the max allowed is eaten and tossed into the bit bucket and the process is repeated until a valid length entry is entered. It also prompts again if no data is entered.

Code:

#include <stdio.h>
#include <string.h> 
  
int main()
{
	char entry [10] ; 
	int flag ;  
	do {
		flag = 1 ;  // assume true (a correct length string is entered) 
		printf ("\nEnter a new name. (max &#37;d characters)\n\n>", sizeof(entry)-2);
		fgets(entry , sizeof(entry), stdin);
		do { 
			if ( strchr(entry, '\n') == NULL ) { 
				entry[0] = '\0' ; // get rid of string if too long 
				flag = 0 ;        // set to false - string was too long 
				fgets(entry , sizeof(entry), stdin); // read any remaining buffer into bit bucket 
			}
		} while (strchr(entry, '\n') == NULL) ;  
		if (strlen(entry) == 1) flag = 0 ;   // empty string - only ENTER was pressed.  
	} while ( !flag )  ;  
    entry[(strchr(entry,'\n')-entry)] = '\0';       // replace \n with null term  	 
	printf("entry = >%s<\n", entry) ; 
	return 0 ; 
}

I'm on a Mac. CTRL+Z does nothing. CTRL+D does nothing. CTRL+C ends up with a SIGINT and kills the app.

How's this? :

Code:

#include <stdio.h>
#include <string.h> 
  
int main()
{
	char entry [10] ; 
	int flag ; 
	char *fgets_ptr ;  
	do {
		flag = 1 ;  // assume true (a correct length string is entered) 
		printf ("\nEnter a new name. (max &#37;d characters)\n\n>", sizeof(entry)-2);
		fgets_ptr = fgets(entry , sizeof(entry), stdin);
		if (fgets_ptr==NULL) return -1 ; 
		do { 
			if ( strchr(entry, '\n') == NULL ) { 
				entry[0] = '\0' ; // get rid of string if too long 
				flag = 0 ;        // set to false - string was too long 
				fgets_ptr = fgets(entry , sizeof(entry), stdin); // read any remaining buffer into bit bucket 
				if (fgets_ptr==NULL) return -1 ; 
			}
		} while (strchr(entry, '\n') == NULL) ;  
		if (strlen(entry) == 1) flag = 0 ;   // empty string - only ENTER was pressed.  
	} while ( !flag )  ;  
    entry[(strchr(entry,'\n')-entry)] = '\0';       // replace \n with null term  	 
	printf("entry = >%s<\n", entry) ; 
	return 0 ; 
}

I've tried CONTROL+D and CMD+D. Neither do it. I'm running under XCode. Could XCode be intercepting it and disabling it?

Wowzers, thanks for the very informative replies. I'm still boggling at how much this code (or something like it) increases both the size and complexity of my once innocent little prog... growing pains.
Cheers.

-SH

Yeah, that's what I was going for, thanks for fleshing it out Todd and matsp. I'm still not quite sure what this line does at the end of the do blocks in Todd's code:

Code:

if (fgets_ptr==NULL) return -1 ;

I removed it and the function still seems to work fine. I'm sure you put it there for something...

Also, I'm puzzled by:

Code:

entry[(strchr(entry,'\n')-entry)] = '\0';

What's the -entry for? Isn't it enough to insert a null char at the location of the newline as found by (strchr(entry,'\n')?
Thanks again.

-SH

I too thank you Matsp! You serve this forum well. Lots to learn from you.

Originally Posted by matsp

I prefer a construction like this:

Code:

size_t len = strlen(entry);
if (entry[len-1] == '\n') 
       entry[len-1] = '\0';

It's cleaner than the strchr() - entry, and also copes with non-newline ending lines without any problem.

But, by the definition of fgets() (it stop at a newline, or EOF), there's no point in testing to see if a newline exists. If a newline does not exist, the flow will never get here. So, isn't testing for a newline at this point in this logic redundant?

Todd