fgets from stdin with unpredictable input size

Question

I'm trying to read a line from stdin but I don't know to properly handle the cases when input size is at least equal to the limit. Example code:

void myfun() { char buf[5]; char somethingElse; printf("\nInsert string (max 4 characters): "); fgets (buf, 5, stdin); ... printf("\nInsert char: "); somethingElse = getchar(); }

Now, the user can do three things:

Input less than 4 characters (plus newline): in this case there's nothing left in stdin and the subsequent getchar() correctly waits for user input;
Input exactly 4 characters (plus newline): in this case there's a newline left in stdin and the subsequent getchar() reads it;
Input more than 4 characters (plus newline): in this case there's at least another character left in stdin and the subsequent getchar() reads it, leaving at least a newline in.

Cases 2 and 3 would require emptying stdin using something like while(getchar() != '\n'), whereas case 1 doesn't require any additional action. As I understand from reading answers to similar questions and c-faq, there's no standard/portable way to know whether the actual scenario is the one described in 1 or not.

Did I get it well? Or there actually is a portable way to do it? Or maybe a totally different approach?

It's maximum 3 chars, not 4, because the \n goes also into the buffer. — Jabberwocky
– Jabberwocky, Commented Dec 8, 2017 at 16:59
You're right @MichaelWalz, the user should be warned about that. — pimple
– pimple, Commented Dec 8, 2017 at 17:09

dbush · Accepted Answer · 2017-12-08 16:56:32Z

The fgets function will store the newline in the buffer if there is room for it. So if the last character in the string is not a newline, you know you need to flush the buffer.

fgets (buf, 5, stdin); if (strrchr(buf, '\n') == NULL) { // flush buffer int c; while ((c = getchar()) != '\n') && (c != EOF)); }

That's actually so simple, I feel ashamed that I haven't thought about it.

chux · Accepted Answer · 2017-12-08 18:44:36Z

If ones assumes that a null character '\0' is never read, then @dbush answer will work.

If a null character is read, then strrchr(buf, '\n') does not find any '\n' that may have been read.

Code could pre-set the buffer to see if a '\n' was read in the end.

buf[sizeof buf - 2] = '\n'; if (fgets (buf, sizeof buf, stdin)) { if (strrchr(buf, '\n') == NULL) { // incomplete line read. (the usual detection) } else if (buf[sizeof buf - 2] != '\n') { // incomplete line read with prior null character (see below note). } }

Yet the C standard does not specify that data past what was read in buf[] is unchanged, pre-filling the buffer with some pattern is not sufficient to detect if a null character '\0' was read.

is a portable way to do it?

The most portable way is to use repeated calls to fgetc() or the like instead of fgets().

maybe a totally different approach?

I recommend fgetc() or the common but not C standard getline()

Another alternative: Use scanf("%4[^\n]%n", buf, &n): It is very cumbersome, yet a portable way is possible. It keeps track of the number of characters read before the '\n' even if some are null characters.

 int n = 0; cnt = scanf("%4[^\n]%n", buf, &n); // Cumbersome to get that 4 here. // Lots of TBD code now needed: // Handle if cnt != 1 (\n to be read or EOF condition) // Handle if n == sizeof buf - 1, (0 or more to read) // Handle if n < sizeof buf - 1, (done, now get \n) // A \n may still need to be consumed // Handle EOF conditions

I discarded the "repeated fgetch() option, but I din't think about null characters being input by user. It's not that likely to happen, but you're never too safe, I guess. The fact that the C standard does not guarantee data after the \0 to stay unchanged, actually brings the fgetc() option back to life. Good answer!
@pimple "It's not that likely to happen." I agree - unless that user is a hacker. Much of (early) C is predicated on a trusted user. That model is risky going forward. Code as needed.

Collectives™ on Stack Overflow

fgets from stdin with unpredictable input size

2 Answers 2

1 Comment

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Related