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After Mark Lakata pointed out that the garbage isn't properly defined in my question I came up with this. I'll keep this updated to avoid confusions.

I am trying to get a function that I can call before a prompt for user input like printf("Enter your choice:); followed a scanf and be sure that only the things entered after the prompt would be scanned in by scanf as valid input.

As far as I can understand the function that is needed is something that flushes standard input completely. That is what I want. So for the purpose of this function the "garbage" is everything in user input i.e. the whole user input before that user prompt.

While using scanf() in C there is always the problem of extra input lying in the input buffer. So I was looking for a function that I call after every scanf call to remedy this problem. I used this, this, this and this to get these answers

//First approach scanf("%*[^\n]\n"); //2ndapproach scanf("%*[^\n]%*c"); //3rd approach int c; while((c = getchar()) != EOF) if (c == '\n') break;

All three are working as far as I could find by hit-and-trial and going by the references. But before using any of these in all of my codes I wanted to know whether any of these have any bugs?

EDIT:

Thanks to Mark Lakata for one bug in 3rd. I corrected it in the question.

EDIT2:

After Jerry Coffin answered I tested the 1st 2 approaches using this program in code:blocks IDE 12.11 using GNU GCC Compiler(Version not stated in the compiler settings).

#include<stdio.h> int main() { int x = 3; //Some arbitrary value //1st one scanf("%*[^\n]\n"); scanf("%d", &x); printf("%d\n", x); x = 3; //2nd one scanf("%*[^\n]%*c"); scanf("%d", &x); printf("%d", x); }

I used the following 2 inputs

First Test Input (2 Newlines but no spaces in the middle of garbage input)

abhabdjasxd 23 bbhvdahdbkajdnalkalkd 46

For the first I got the following output by the printf statements

23 46

i.e. both codes worked properly.

Second Test input: (2 Newlines with spaces in the middle of garbage input)

hahasjbas asasadlk 23 manbdjas sadjadja a 46

For the second I got the following output by the printf statements

23 3

Hence I found that the second one won't be taking care of extra garbage input whitespaces. Hence, it isn't foolproof against garbage input.

I decided to try out a 3rd test case (garbage includes newline before and after the non-whitespace character)

`` hahasjbas asasadlk 23 manbdjas sadjadja a 46

The answer was

3 3

i.e. both failed in this test case.

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  • Why is it tagged with optimization ? Commented Jun 22, 2013 at 7:38
  • @0x90 Because I wanted to know whether they are optimal and if not possible optimization. Commented Jun 22, 2013 at 8:06
  • ok so read this stackoverflow.com/questions/5638999/… Commented Jun 22, 2013 at 8:09
  • @0x90 The accepted answer is good when I am reading large inputs from files but when I am reading very small inputs like 1 or 2 integers wouldn't the approach in the linked question be less optimal due to calls to malloc and then parsing the strings into integers? Commented Jun 22, 2013 at 8:22
  • 1
    Aseem, I think you are going about this problem incorrectly. What is "garbage" input? What are you trying to read? Are you just trying to skip over everything that is not numeric? Commented Jul 3, 2013 at 16:40

5 Answers 5

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+50

The first two are subtly different: they both read and ignore all the characters up to a new-line. Then the first skips all consecutive white space so after it executes, the next character you read will be non-whitespace.

The second reads and ignores characters until it encounters a new-line then reads (and discards) exactly one more character.

The difference will show up if you have (for example) double-spaced text, like:

line 1 line 2

Let's assume you read to somewhere in the middle of line 1. If you then execute the first one, the next character you read in will be the 'l' on line 2. If you execute the second, the next character you read in will be the new-line between line 1 and line 2.

As for the third, if I were going to do this at all, I'd do something like:

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

...and yes, this does work correctly -- && forces a sequence point, so its left operand is evaluated first. Then there's a sequence point. Then, if and only if the left operand evaluated to true, it evaluates its right operand.

As for performance differences: since you're dealing with I/O to start with, there's little reasonable question that all of these will always be I/O bound. Despite its apparent complexity, scanf (and company) are usually code that's been used and carefully optimized over years of use. In this case, the hand-rolled loop may be quite a bit slower (e.g., if the code for getchar doesn't get expanded inline) or it may be about the same speed. The only way it stands any chance of being significantly faster is if the person who wrote your standard library was incompetent.

As far maintainability: IMO, anybody who claims to know C should know the scan set conversion for scanf. This is neither new nor rocket science. Anybody who doesn't know it really isn't a competent C programmer.

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4 Comments

Your explanation helped. I couldn't understand either of them before this. I tried both of them. But for some reason your explanation didn't hold and both worked in test case 1 added into the question. Is that due to scanf's internal working? Any advice regarding the 3rd test case?
@AseemBansal: your tests are kind of broken because %d will skip leading white space, so with it, it makes no difference whether the leading whitespace was skipped already. Try using %c afterwards instead.
@JerryCoffin you may want to reconsider your final paragraph; your first two paragraphs overlook the fact that if the garbage consisted of just a newline then scanf("%*[^\n]%*c"); and scanf("%*[^\n]\n"); stop after the first specifier because it failed to match anything. So neither solves the problem. (A fix would be to break each into two scanf calls).
@MattMcNabb: It looks to me like you're simply asking for something the question didn't ask (at least at the time--it's undergone substantial editing since). Ultimately, it's a question of what he really wants. It's certainly true that scanf will stop scanning as soon as any conversion fails. Whether he wants that to happen or wants to break it into two calls to read a character even if there's nothing preceding the new-line seems open to question.
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The first 2 examples use a feature of scanf that I didn't even know existed, and I'm sure a lot of other people didn't know. Being able to support a feature in the future is important. Even if it was a well known feature, it will be less efficient and harder to read the format string than your 3rd example.

The third example looks fine.

(edit history: I made a mistake saying that ANSI-C did not guarantee left-to-right evaluation of && and proposed a change. However, ANSI-C does guarantee left-to-right evaluation of &&. I'm not sure about K&R C, but I can't find any reference to it and no one uses it anyways...)

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7 Comments

+1 for the bug you pointed out in the 3rd. Didn't see that. About 1st and 2nd, they don't use any additional variable while the 3rd does have repeated function calls. So shouldn't one of those be more efficient?
Look at the internals of scanf and you will see that it is much less efficient. getchar() (depending on implementation) on the other hand is very simple and in some implementations could even be inlined in your code, making it super fast.
What do you mean by inlined? I don't know the term. Is this question and Inline expansion wikipedia page refer to the same thing that you are referring to?
Is this referring to the same inlining that you are refering to? This had simpler explanation.
inline means it is written as a function call, but it is more like a #define macro. There is no overhead of calling the function, and the optimizer can also optimize register usage. C++ has the inline keyword to "force" inlining, but many C compilers will do it with heavy optimization automatically. The number one rule of programming though is to get a program that works first, then worry about optimization later.
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Many other solutions have the problem that they cause the program to hang and wait for input when there is nothing left to flush. Waiting for EOF is wrong because you don't get that until the user closes the input completely!

On Linux, the following will do a non-blocking flush:

// flush any data from the internal buffers fflush (stdin); // read any data from the kernel buffers char buffer[100]; while (-1 != recv (0, buffer, 100, MSG_DONTWAIT)) { }

The Linux man page says that fflush on stdin is non-standard, but "Most other implementations behave the same as Linux."

The MSG_DONTWAIT flag is also non-standard (it causes recv to return immediately if there is no data to be delivered).

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1 Comment

The answers should use only the C standard functions
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You should use getline/getchar:

#include <stdio.h> int main() { int bytes_read; int nbytes = 100; char *my_string; puts ("Please enter a line of text."); /* These 2 lines are the heart of the program. */ my_string = (char *) malloc (nbytes + 1); bytes_read = getline (&my_string, &nbytes, stdin); if (bytes_read == -1) { puts ("ERROR!"); } else { puts ("You typed:"); puts (my_string); } return 0;
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3 Comments

getline() is in Standard C++ not standard C. getchar() is in standard C. I have one of the methods using getchar() but don't understand how getchar() can be used to read int's float's etc.
You should use sscanf read to a locally buffer and use sscanf, strtol, strtof.
The code is using, more or less, the POSIX getline() function. The 'or less' is because the buffer size variable (nbytes) should be of type size_t rather than int. In general, you should free the buffer too. Here, it doesn't matter much, but if this was a function rather than the main() program, it would matter.
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I think if you see carefully at right hand side of this page you will see many questions similar to yours. You can use fflush() on windows.

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3 Comments

That isn't necessary. It depends on the compiler that I use on windows. I use Code:blocks IDE with gcc compiler. It doesn't allow flushing stdin by using fflush(). Also the question isn't specific to C on windows it is about standard C.
What you might be referring to i.e. fflush() working for windows would be specific to Microsoft products if it is documented on MSDN i.e on Visual Studio or other Microsoft products.
fflush is for clearing out file handle buffers (ie at the driver level or at the stdio buffering level), not for reading to the end of line of text.

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