So the problem with simply reading input from the user and echoing it back out is that a malevolent actor can simply insert a function pointer or executable code and voila, you are no longer in control. Ideally, you would just write a script that automatically tries increasing string sizes and reads the exit code of the program.
Once it detects an error, you simply work your way back to a close estimate of the stack. This also makes our program more robustbecause as mentioned in the source, simply adjust the code to read from the command line, a file, or stdin, without much trouble.
Crucially, the fgets function also allows us to specify a specific buffer lengththereby disallowing any buffer overflow attacks. As expected, we dynamically allocate a buffer of predetermined size.
Dynamically allocating it as opposed to stack-allocating it gives us another layer of protection against stack smashing. Note: We are explicitly zeroing out the memory in the inputBuffer. C does nothing for you, and malloc is no exception. The call to malloc returns a pointer to the first memory cell of the requested memory, but the values in every single one of those cells are unchanged from before the call.
For all intents and purposes, they are garbage, so we zero them out. Note also that by zeroing out, we are automatically giving ourselves the null terminator, although the fgets function does actually append a null to the end of the input string, provided there is enough room in the buffer. If that is true, this means that the user passed in an input string that was too long.
Otherwise, the program will simply read from the old input, which has not yet been used, rather than prompting the user for additional input. To handle this, I supply two additional functions.
Hence, the above-described method is used. Note: if fgets returns an erroryou should call ferror to figure out what went wrong. Become industry ready at a student-friendly price. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.
Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. OK, I got this. This code worked actually. But I cannot explain myself how it works? Because in the while statement, we use getchar!
The behavior you see at line 2 is correct, but that's not quite the correct explanation. Your getchar! Also, I feel obligated to discourage you from using scanf entirely: Why does everyone say not to use scanf? What should I use instead? It also checks against EOF in case the input stream gets closed before the end of the line. The type of c must be int or larger in order to be able to hold the value EOF. There is no portable way to find out if there are any more lines after the current line if there aren't, then getchar will block for input.
It reads all the characters in the stream and discards them up to and including the next linefeed or EOF is encountered. For the test to be true, it has to read the linefeed first; so when the loop stops, the linefeed was the last character read, but it has been read.
As for why it reads a linefeed instead of a carriage return, that's because the system has translated the return to a linefeed. When enter is pressed, that signals the end of the line That might be platform dependent. Also, using fflush on an input stream doesn't work on all platforms; for example it doesn't generally work on Linux.
Am I misunderstanding something?? The thing you may not realize is that the comparison happens after getchar removes the character from the input buffer.Below is its syntax.
Why to use fgets() over scanf() in C?
Don't use e. By now it should be clear what a buffer means. Logic to remove word from file in C programming. Then check the first character. The statement fflush stdin has been inserted into the program in order to clear the buffer before getchar is called stdin represents the standard input device, that is, the keyboard. Your OS might provide a way to clear the keyboard buffer as well.understanding getchar and input buffer in c
Y: On various occasions you may need to clear the unwanted buffer so as to get the next input in the desired container and not in the buffer of previous variable. When it is used after the scanfit flushes the input buffer also.
Clearing input buffer in C/C++
Por isso, usaremos bastante o scanf ao longo de nosso curso de C. The scanf function will not read anything that does not conform to the required input type. The scanf function can do a lot of different things, but can be unreliable because it doesn't handle human errors very well.
Using of clrscr function in C is always optional but it should be place after variable or function declaration only. You can check my other answer here. Similar to the first example, arr refers to the left boundary while arr refers to the right boundary. It returns zero if successful, otherwise returns EOF and feof error indicator is set. I need to clear my standard input buffer before scanf command. It does not support delete operation. Instead of checking for what you could do is read in that character and not use it.
This will clear any pesky characters. Newsgroups: comp. Simplesmente consuma a quebra de linha! I've tried flushing the input with fflush stdin Understanding how printf and scanf works internally is the key to writing advance code. The C way to "clear the keyboard buffer" is to read in and discard characters until there's nothing left with getchar or fgets or whatever. It is a predefined function in "conio. Prev Next. If buffer is a null pointer, the function automatically allocates a buffer using size as a hint on the size to use.
Temporary storage area is called buffer. If a newline is read, it is stored into the buffer. And the best way to clear it is to use rewind stdin.
Using memset, to zero the buffer before using these functions is redundant and inefficient. Above is just for example only.
After I entered "loops", "s1" was assigned a blank line automatically. How does it happen? I'm sure my keyboard works fine. Either do something to consume the newline, or my preferred solution fgets and then sscanf from that string.
To use fgets to read the next line you need to manually remove the rest of the current line:. Geekoaur has answered your question well, I'm just pointing out another "problem" with your code. This is because strlen basicaly finds the index of the existing null-terminator and returns it! Here's a valid, unoptimized implementation of strlen:.
SIG11's are a complete biatch to find because unless you "hook" them with a signal-processor and say otherwise, they cause unix to hard-terminate your program, so you can't log anything after the fact to help figure out where-in-the-hell-did-that-come-from I learned this the hard way FYI: Here's a "safe" unoptimized version of strlen which I'll call strnlen I reckon this should be in stdlib. I know this is very old.
I'm new to c and wanted to check my method, which uses getchar :. Another way to ignore following newline character due to hitting ENTER after scanning the integer in variable label loops is:. The conversion that follows occurs as usual, but no pointer is used; the result of the conversion is simply discarded. For example in your code, loops is a local un-initialized variable, containing garbage. If scanf fails to fill the desired value, following while loop may run arbitrarily.
To slove this we have to do something to consume the newline. Let's see the problem. This will effectively clear the input buffer and allow you to use fgets afterwards.
EDIT: I have recently learned that there is an easier solution than the one above. If you say getchar after scanfit will allow you to use fgets without issues. Learn more. Asked 9 years, 2 months ago.
Active 3 months ago. Viewed 15k times. Vayn Vayn 1, 3 3 gold badges 18 18 silver badges 32 32 bronze badges. Some variant of this question seems to be asked here every day or so. The function delspace is definitely wrong!
Rule of thumb: for interactive input always read lines. Related: stackoverflow. Active Oldest Votes. If I want to use scanfhow I can consume the new line? Yeah, that'll work too, but I personally just use fgets and then sscanf as suggested by geekosaur because it works "uniformly" for all sorts of dataBut I cannot explain myself how it works?
Because in the while statement, we use getchar! Am I misunderstanding something?? The thing you may not realize is that the comparison happens after getchar removes the character from the input buffer. OK, I got this. This code worked actually. The behavior you see at line 2 is correct, but that's not quite the correct explanation. Your getchar! Also, I feel obligated to discourage you from using scanf entirely: Why does everyone say not to use scanf?
What should I use instead? It reads all the characters in the stream and discards them up to and including the next linefeed or EOF is encountered. For the test to be true, it has to read the linefeed first; so when the loop stops, the linefeed was the last character read, but it has been read. As for why it reads a linefeed instead of a carriage return, that's because the system has translated the return to a linefeed.
When enter is pressed, that signals the end of the line That might be platform dependent. Also, using fflush on an input stream doesn't work on all platforms; for example it doesn't generally work on Linux. As for the proposed solutions, see again from the comp. Will fflush stdin work? If fflush won't work, what can I use to flush input? Another solution not mentioned yet is to use: rewind stdin. How do you set, clear, and toggle a single bit?In a way, you could argue that scanf is the input version of the printf function.
Because of that, scanf is quite particular about how text is input. In this version, placeholder is a conversion character, and variable is a type of variable that matches the conversion character. The scanf function is prototyped in the stdio. The preceding statement reads an integer value into the variable highscore.
This assumes that highscore is an int variable. The preceding scanf statement waits for a floating-point value to be input, which is then stored in the temperature variable. In the preceding line, scanf accepts the first character input and stores it in the key variable.
So a space or a tab or the Enter key terminates the string. That sucks. One of the most common ways to put the scanf function to use is to read in a chunk of text from standard input. Exercise 1: Type the source code from scanf Swallows a String into a new project, ex, in Code::Blocks.
Build and run. Line 5 declares a char array — a string variable — named firstname. The number in the brackets indicates the size of the array, or the total number of characters that can be stored there. Basically, the statement at Line 5 sets aside storage for up to 15 characters. The scanf function in Line 8 reads a string from standard input and stores it in the firstname array.
Prompt the user for their last name as well, and then display both names by using a single printf function. The number in the brackets refer to Line 5 gives the size of the char array, or the length of the string, plus one. When you create a char array, or string variable, ensure that you create it of a size large enough to hold the text.
That size should be the maximum number of characters plus one. The reason for increasing the char array size by one is that all strings in C end with a specific termination character. You must remember to add room for that character when you set aside storage for string input. The scanf function can do more than read strings. It can read in any value specified by a conversion character.
In scanf Eats an Integer, the scanf function reads in an integer value. That character directs scanf to look for an int value for variable fav. Exercise 3 : Create a project, ex, using the source code shown in scanf Eats an Integer. Test the program by typing various integer values, positive and negative. The character is a C operator — specifically, the memory address operator. An ampersand must prefix any variable specified in the scanf function. The exception is an array, such as the firstname char array in scanf Eats an Integer.
Try running the program again, but specify a decimal value, such as The reason you see incorrect output is that scanf is very specific. It fetches only the variable type specified by the conversion character.Remember Me? Why do I have to clear the input buffer after reading a string with scanf? Why do I have to do this to keep may console screen from disappearing: Code:.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. If at first you don't succeed, try writing your phone number on the exam paper. You shoul use Code:. That way you know if you got the entire file one of the shortcomings of it fgets. It only leaves it if there isn't room in the buffer you provide.
Try it - nice simple program with a buffer size of say 10 bytes, then type in a nice long line and see how many fgets calls you have to make before one of them has a newline in it. Yes i'm aware of how fgets works, your statement did not make sense though. I think there's just a miscommunication going on. One person is talking about leaving the newline in the buffer passed to fgets instead of "adding" it there and another person is talking about leaving it in the internal input buffer.
Just to make sure it's clear, scanf leaves the newline in the internal input buffer.
If the buffer you pass to fgets is too small to accomodate the entire line, it will only store as much as it can and leave the rest including the newline in the internal input buffer. If you understand what you're doing, you're not learning anything. Replies: 8 Last Post:AM. Replies: Last Post:AM.
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