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I'm trying to get multiple consecutive user inputs, which will later be printed. It works fine as long as you don't input a string longer than the boundary, but when you do input something longer it overflows into the next input. For example, this code:

printf("Enter student's first name:\t"); char nameF[20]; scanf("%20s", nameF); printf("Enter student's last name:\t"); char nameL[20]; scanf("%20s", nameL); printf("Enter student's ID Number:\t"); char id[9]; scanf("%9s", id); printf("Enter student's e-mail:\t"); char mail[26]; scanf("%26s", mail);

Results in this:

Enter student's first name: 01234567890123456789012345 Enter student's last name: Enter student's ID Number: 0123456789012345 Enter student's e-mail: First: 01234567890123456789 Last: ID Number: 012345678 E-mail: 9012345

I skipped over the print function for the sake of not having a wall of code. If you want to see that as well, let me know and I'll add it.

It should be noted that I tried fgets(), with very similar results. If I replace each scanf() line with fgets(var, sizeof(var), stdin);, I get this: 

Enter student's first name: 01234567890123456789012345 Enter student's last name: Enter student's ID Number: 0123456789012345 Enter student's e-mail: First: 0123456789012345678 Last: 9012345 ID Number: 01234567 E-mail: 89012345

When I inserted getchar() after the scanf() statements, it ignored the overflow input, but still skipped over the next scan. I have tried throwaway input variables, and I've looked into other input methods, but couldn't seem to find anything to help. I'm sure this is a pretty simple fix for someone with experience, but I'm only a couple of weeks into learning C, so I don't know much beyond pointers and structs.

I can almost guarantee somebody will find a duplicate of this - they always do - but I did search around, for about a solid hour, and I didn't find anything that worked.

Thanks in advance for any help you can give me.

EDIT: I understand why the input is overflowing into the next one. It saves the first x characters, and the rest remains in the buffer and is entered into the next input scan.

I guess the more narrowed down question would be: How can I clear or divert the input buffer so that if the user inputs extra characters they won't remain in the buffer?

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    Apparently you're allowing free-form input. Therefore you should use a large generic input buffer. Only after you have validated the input string should the input be copied to the appropriate variable. Commented May 8, 2014 at 19:33
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    Although part of the problem here is covered by the proposed duplicate, the overlong %20s format specifications part of the problem is not covered by the proposed duplicate, so it is not appropriate to close this question as a duplicate of the other. Commented May 8, 2014 at 20:14
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    The %*[^\n] part of the format is a scan-set with assignment suppression. It will read an arbitrary number of non-newlines. The final newline is a mistake. All white space in formats (blanks, tabs, newlines) maps to an optional sequence of white space followed by a non-white space. In this case, it reads characters and doesn't stop reading characters until it got something other than white space. This is diabolical for interactive input. The correct notation would be %*[n], looking for a newline character in a scan-set and not assigning it. This is what I mean about scanf() being tricky! Commented May 9, 2014 at 6:12
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    No. In printf(), you can use notations such as %*.*s with each * representing a number read from the argument list for a length. In scanf(), you can't do that; the * is for assignment suppression. If you read The Practice of Programming (an excellent book), Kernighan & Pike recommend creating the format string with the length you need on the fly (using snprintf() and a fair amount of care). Commented May 9, 2014 at 17:41
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    It's a problem. Having code to generate the format strings for scanf() is probably as good as it gets. It depends on how widespread the format strings are and how often you think you might change lengths. The extra flexibility from format generation is useful, but makes the code harder to understand. You can try playing games with string concatenation and preprocessor tricks, but there are things that the preprocessor can't handle the C can. But this might make another question; we're running long on comments here. Commented May 9, 2014 at 17:54

3 Answers 3

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I believe scanf is using buffered input. It allows you to enter characters until you press enter. At that point, it takes the 20 that you specify, and the remaining buffer is then used as the input for the next scanf call. fgets's results are more predictable, with the next string being populated with the remnants of the previous buffer.

I cannot, however, explain why last name is blank, yet email is not in the first example.

Take a look here for an example of how to flush the type-ahead input. You can write a function like so:

void flushInput(void) { int c; while((c = getchar()) != '\n' && c != EOF) /* discard */ ; }

...and then call flushInput(); after each scanf().

As an additional note, I believe you should be specifying one character less in your scanf calls. It appears that....

char nameF[20]; scanf("%20s", nameF);

...is putting 20 characters into nameF and the '\0' terminator is going beyond the bounds of the array. fgets appears to properly place the terminator within the specified size.

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Your last paragraph covers a very important point (see my answer), but I missed it when scanning your answer before writing my own. I think you should emphasize it more.
Thank you , the flushInput() was what I was looking for. Also, I fixed the scan range to allow for the \0.
@JonathanLeffler I posted a new question that I think you could figure out pretty easily. stackoverflow.com/questions/23572319/…
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There's an off-by-one problem:

char nameF[20]; scanf("%20s", nameF);

This is incorrect; the size in the format string doesn't count the trailing null, so you have to use 19 in the format with 20 in the variable definition, or 20 in the format and 21 in the variable definition:

char nameF[21]; scanf("%20s", nameF);

The original code means that you can easily have one variable clobbering another with its trailing null.

This is different from most other parts of C where you specify the whole length and the function reserves the last byte for the terminating null byte (for example: fgets()). In an ideal world, the inconsistency would be fixed, but it was there since the Standard I/O library was created in the late 70s and there was too much code that would be broken if C standard had changed the behaviour. So, you have to be aware of this quirk and work around it accordingly.

Also note that when it reaches the end of the format specified, scanf() leaves any left-over data on the line for the next input. If you want line-by-line input, read a line of data (fgets() or POSIX getline()), then scan with sscanf(). This is very often a better way of doing business.

char line[4096]; if (fgets(line, sizeof(line), stdin) == 0) ...handle EOF... if (sscanf(line, "%19s", nameF) != 1) ...handle format error...

Note that you should error check every raw input functions (fgets()) for EOF, and sscanf() or scanf() for the correct number of conversions.

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Thanks, I fixed the off-by-one bug. Unfortunately, the sscanf() stuff is currently a bit beyond the scope of my C know-how. I just need to flush the rest of the input buffer, for which RJP gave me a simple solution.
I'm not sure why you can't use sscanf() if you can use scanf(); they're very, very similar. Using fgets() avoids needing to flush the input buffer; it reads everything on the line before you use sscanf() on it (and can lead to better error reporting). However, what was proposed by RJP should work too. On some systems (Microsoft), you could use fflush(stdin) and probably get the extra data flushed. However, standard C says that operation is undefined, and the getchar() loop is good (it was done correctly).
fflush(stdin) was among the things I tried. I am working on Linux, so that didn't work. I'll look up sscanf(). I'm sure I can figure it out, it just seemed complex from my perspective. I think I'm largely just unused to using input functions directly as expressions. As I said, I'm still rather newb-ish. Thanks for the advice!
The Linux documentation seems to suggest fflush(stdin) should work: For input streams, fflush() discards any buffered data that has been fetched from the underlying file, but has not been consumed by the application. Empirically, I've not managed to make it work; you found the same problem. This has been mentioned before; I don't know that there's a definitive question/answer to point you at. And I've not gone looking at the glibc source code to see why it does not behave as documented.
fgets() handles lines longer than 4095 characters/bytes (when given 4096 bytes for the buffer) by terminating the input. You can tell this happened because the last character before the null is not a newline. So, you'd get slightly odd results, but it really serves them right for trying to break your program.
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It's not programs fault but I will tell you what is happening when range is crossed. . . Actually the characters which comes in range are stored in ID and others are covered to characters and stored to it's next variable which is E-mail. . Program is not telling to enter email because the value is overflowed from id and our is stored in email so email already contains value overflowed by ID . I hope it will help you

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