I'm developing a code where the user will type several paragraphs and it will stop reading when the user begin a paragraph with "END". The code will manipulate the string by counting each letter and showing a graph and blah blah blah, but this is irrelevant to the question.
The thing is: which paragraph must have no more than 1000 characters.
A smaller version of the code is the following (considering I just want to storage 5-char-string - even though I'll expand that).
#include <stdio.h>
#include <string.h>
int main()
{
char paragraph[5];
for ( ; ; )
{
fgets(paragraph, 5, stdin);
if (paragraph[0]=='E' && paragraph[1]=='N' && paragraph[2]=='D')
{ return 0; }
printf("%s", paragraph);
}
return 0;
My problem is: if I type more than 5 characters, the printf function still prints more than 5 characters, I don't know why. I've already checked everything I could possible check.
Help a beginner like me, please.
fgets() reads in at most one less than size characters from stream and
stores them into the buffer pointed to by s. Reading stops after an
EOF or a newline. If a newline is read, it is stored into the buffer.
A terminating null byte ('\0') is stored after the last character in
the buffer.
So when entering more than 4 characters (newline included) only 4 is read and the rest stays in the buffer ready to be read next fgets.
Your printf will not print any newline in this case and will be called multiple times, making it look like printing more than 4 characters.
As suggested in comments, try printf("[%s]", paragraph); to see the individual printf calls.
You should use strstr in string.h because it's cleaner.
if (strstr(paragraph, "END"))
return 0;
instead of
if (paragraph[0]=='E' && paragraph[1]=='N' && paragraph[2]=='D')
return 0;
Try modifying your code in the following way and you'll immediately see what actually happens with the fgets() function when you enter more characters than the size of your buffer. It doesn't read from the keyboard, but from the stdinbuffer. These SO posts may also be interesting for you to read:(1), (2). Enjoy the demo and be sure to thoroughly read the man pages.
#include <stdio.h>
int main()
{
char paragraph[5];
for ( ; ; )
{
printf("Enter the string: \n\t");
if(fgets(paragraph, 5, stdin) != NULL)
printf("%s\n", paragraph);
if (paragraph[0]=='E' && paragraph[1]=='N' && paragraph[2]=='D')
return 0;
}
return 0;
}
Related
So far I have been using if statements to check the size of the user-inputted strings. However, they don't see to be very useful: no matter the size of the input, the while loop ends and it returns the input to the main function, which then just outputs it.
I don't want the user to enter anything greater than 10, but when they do, the additional characters just overflow and are outputted on a newline. The whole point of these if statements is to stop that from happening, but I haven't been having much luck.
#include <stdio.h>
#include <string.h>
#define SIZE 10
char *readLine(char *buf, size_t sz) {
int true = 1;
while(true == 1) {
printf("> ");
fgets(buf, sz, stdin);
buf[strcspn(buf, "\n")] = 0;
if(strlen(buf) < 2 || strlen(buf) > sz) {
printf("Invalid string size\n");
continue;
}
if(strlen(buf) > 2 && strlen(buf) < sz) {
true = 0;
}
}
return buf;
}
int main(int argc, char **argv) {
char buffer[SIZE];
while(1) {
char *input = readLine(buffer, SIZE);
printf("%s\n", input);
}
}
Any help towards preventing buffer overflow would be much appreciated.
When the user enters in a string longer than sz, your program processes the first sz characters, but then when it gets back to the fgets call again, stdin already has input (the rest of the characters from the user's first input). Your program then grabs another up to sz characters to process and so on.
The call to strcspn is also deceiving because if the "\n" is not in the sz chars you grab than it'll just return sz-1, even though there's no newline.
After you've taken input from stdin, you can do a check to see if the last character is a '\n' character. If it's not, it means that the input goes past your allowed size and the rest of stdin needs to be flushed. One way to do that is below. To be clear, you'd do this only when there's been more characters than allowed entered in, or it could cause an infinite loop.
while((c = getchar()) != '\n' && c != EOF)
{}
However, trying not to restructure your code too much how it is, we'll need to know if your buffer contains the newline before you set it to 0. It will be at the end if it exists, so you can use the following to check.
int containsNewline = buf[strlen(buf)-1] == '\n'
Also be careful with your size checks, you currently don't handle the case for a strlen of 2 or sz. I would also never use identifier names like "true", which would be a possible value for a bool variable. It makes things very confusing.
In case that string inside the file is longer that 10 chars, your fgets() reads only the first 10 chars into buf. And, because these chars doesn't contain the trailing \n, function strcspn(buf, "\n") returns 10 - it means, you are trying to set to 0 an buf[10], so it is over buf[] boundaries (max index is 9).
Additionally, never use true or false as the name of variable - it totally diminishes the code. Use something like 'ok' instead.
Finally: please clarify, what output is expected in case the file contains string longer than 10 characters. It should be truncated?
I have written a small script to detect the full value from the user input with the getchar() function in C. As getchar() only returns the first character i tried to loop through it... The code I have tried myself is:
#include <stdio.h>
int main()
{
char a = getchar();
int b = strlen(a);
for(i=0; i<b; i++) {
printf("%c", a[i]);
}
return 0;
}
But this code does not give me the full value of the user input.
You can do looping part this way
int c;
while((c = getchar()) != '\n' && c != EOF)
{
printf("%c", c);
}
getchar() returns int, not char. And it only returns one char per iteration. It returns, however EOF once input terminates.
You do not check for EOF (you actually cannot detect that instantly when getchar() to char).
a is a char, not an array, neither a string, you cannot apply strlen() to it.
strlen() returns size_t, which is unsigned.
Enable most warnings, your compiler wants to help you.
Sidenote: char can be signed or unsigned.
Read a C book! Your code is soo broken and you confused multiple basic concepts. - no offense!
For a starter, try this one:
#include <stdio.h>
int main(void)
{
int ch;
while ( 1 ) {
ch = getchar();
x: if ( ch == EOF ) // done if input terminated
break;
printf("%c", ch); // %c takes an int-argument!
}
return 0;
}
If you want to terminate on other strings, too, #include <string.h> and replace line x: by:
if ( ch == EOF || strchr("\n\r\33", ch) )
That will terminate if ch is one of the chars listed in the string literal (here: newline, return, ESCape). However, it will also match ther terminating '\0' (not sure if you can enter that anyway).
Storing that into an array is shown in good C books (at least you will learn how to do it yourself).
Point 1: In your code, a is not of array type. you cannot use array subscript operator on that.
Point 2: In your code, strlen(a); is wrong. strlen() calculates the length of a string, i.e, a null terminated char array. You need to pass a pointer to a string to strlen().
Point 3: getchar() does not loop for itself. You need to put getchar() inside a loop to keep on reading the input.
Point 4: getchar() retruns an int. You should change the variable type accordingly.
Point 5: The recommended signature of main() is int main(void).
Keeping the above points in mind,we can write a pesudo-code, which will look something like
#include <stdio.h>
#define MAX 10
int main(void) // nice signature. :-)
{
char arr[MAX] = {0}; //to store the input
int ret = 0;
for(int i=0; i<MAX; i++) //don't want to overrrun array
{
if ( (ret = getchar())!= EOF) //yes, getchar() returns int
{
arr[i] = ret;
printf("%c", arr[i]);
}
else
;//error handling
}
return 0;
}
See here LIVE DEMO
getchar() : get a char (one character) not a string like you want
use fgets() : get a string or gets()(Not recommended) or scanf() (Not recommended)
but first you need to allocate the size of the string : char S[50]
or use a malloc ( #include<stdlib.h> ) :
char *S;
S=(char*)malloc(50);
It looks like you want to read a line (your question mentions a "full value" but you don't explain what that means).
You might simply use fgets for that purpose, with the limitation that you have to provide a fixed size line buffer (and handle - or ignore - the case when a line is larger than the buffer). So you would code
char linebuf[80];
memset (linebuf, 0, sizeof(linbuf)); // clear the buffer
char* lp = fgets(linebuf, sizeof(linebuf), stdin);
if (!lp) {
// handle end-of-file or error
}
else if (!strchr(lp, '\n')) {
/// too short linebuf
}
If you are on a POSIX system (e.g. Linux or MacOSX), you could use getline (which dynamically allocates a buffer). If you want some line edition facility on Linux, consider also readline(3)
Avoid as a plague the obsolete gets
Once you have read a line into some buffer, you can parse it (e.g. using manual parsing, or sscanf -notice the useful %n conversion specification, and test the result count of sscanf-, or strtol(3) -notice that it can give you the ending pointer- etc...).
This seems like it should be a simple thing but after hours of searching I've found nothing...
I've got a function that reads an input string from stdin and sanitizes it. The problem is that when I hit enter without typing anything in, it apparently just reads in some junk from the input buffer.
In the following examples, the prompt is "input?" and everything that occurs after it on the same line is what I type. The line following the prompt echoes what the function has read.
First, here is what happens when I type something in both times. In this case, the function works exactly as intended.
input? abcd
abcd
input? efgh
efgh
Second, here is what happens when I type something in the first time, but just hit enter the second time:
input? abcd
abcd
input?
cd
And here is what happens when I just hit enter both times:
input?
y
input?
y
It happens to return either 'y' or '#' every time when I run it anew. 'y' is particularly dangerous for obvious reasons.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#define STRLEN 128
int main() {
char str[STRLEN];
promptString("input?", str);
printf("%s\n", str);
promptString("input?", str);
printf("%s\n", str);
return EXIT_SUCCESS;
}
void promptString(const char* _prompt, char* _writeTo) {
printf("%s ", _prompt);
fgets(_writeTo, STRLEN, stdin);
cleanString(_writeTo);
return;
}
void cleanString(char* _str) {
char temp[STRLEN];
int i = 0;
int j = 0;
while (_str[i] < 32 || _str[i] > 126)
i++;
while (_str[i] > 31 && _str[i] < 127) {
temp[j] = _str[i];
i++;
j++;
}
i = 0;
while (i < j) {
_str[i] = temp[i];
i++;
}
_str[i] = '\0';
return;
}
I've tried various methods (even the unsafe ones) of flushing the input buffer (fseek, rewind, fflush). None of it has fixed this.
How can I detect an empty input so that I can re-prompt, instead of this annoying and potentially dangerous behavior?
This part of cleanString
while (_str[i] < 32 || _str[i] > 126)
i++;
jumps over \0 when the string is empty.
You should add _str[i] != '\0' into the loop's condition.
To detect an empty string, simply check it's length just after the input:
do {
printf("%s ", _prompt);
fgets(_writeTo, STRLEN, stdin);
} while (strlen(_writeTo) < 2);
(comparing with two because of '\n' which fgets puts into the end of buffer)
Why do you have a bunch of variable names with leading underscores? That's nasty.
Anyway, the first thing you must do is check the return value of fgets. If it returns NULL, you didn't get any input. (You can then test feof or ferror to find out why you didn't get input.)
Moving on to cleanString, you have a while loop that consumes a sequence of non-printable characters (and you could use isprint for that instead of magic numbers), followed by a while loop that consumes a sequence of printable characters. If the input string doesn't consist of a sequence of non-printables followed by a sequence of printables, you will either consume too much or not enough. Why not use a single loop?
while(str[i]) {
if(isprint(str[i]))
temp[j++] = str[i];
++i;
}
This is guaranteed to consume the whole string until the \0 terminator, and it can't keep going past the terminator, and it copies the "good" characters to temp. I assume that's what you wanted.
You don't even really need to use a temp buffer, you could just copy from str[i] to str[j], since j can never get ahead of i you'll never be overwriting anything that you haven't already processed.
While I could use strings, I would like to understand why this small example I'm working on behaves in this way, and how can I fix it ?
int ReadInput() {
char buffer [5];
printf("Number: ");
fgets(buffer,5,stdin);
return atoi(buffer);
}
void RunClient() {
int number;
int i = 5;
while (i != 0) {
number = ReadInput();
printf("Number is: %d\n",number);
i--;
}
}
This should, in theory or at least in my head, let me read 5 numbers from input (albeit overwriting them).
However this is not the case, it reads 0, no matter what.
I understand printf puts a \0 null terminator ... but I still think I should be able to either read the first number, not just have it by default 0. And I don't understand why the rest of the numbers are OK (not all 0).
CLARIFICATION: I can only read 4/5 numbers, first is always 0.
EDIT:
I've tested and it seems that this was causing the problem:
main.cpp
scanf("%s",&cmd);
if (strcmp(cmd, "client") == 0 || strcmp(cmd, "Client") == 0)
RunClient();
somehow.
EDIT:
Here is the code if someone wishes to compile. I still don't know how to fix
http://pastebin.com/8t8j63vj
FINAL EDIT:
Could not get rid of the error. Decided to simply add #ReadInput
int ReadInput(BOOL check) {
...
if (check)
printf ("Number: ");
...
# RunClient()
void RunClient() {
...
ReadInput(FALSE); // a pseudo - buffer flush. Not really but I ignore
while (...) { // line with garbage data
number = ReadInput(TRUE);
...
}
And call it a day.
fgets reads the input as well as the newline character. So when you input a number, it's like: 123\n.
atoi doesn't report errors when the conversion fails.
Remove the newline character from the buffer:
buf[5];
size_t length = strlen(buffer);
buffer[length - 1]=0;
Then use strtol to convert the string into number which provides better error detection when the conversion fails.
char * fgets ( char * str, int num, FILE * stream );
Get string from stream.
Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first.
A newline character makes fgets stop reading, but it is considered a valid character by the function and included in the string copied to str. (This means that you carry \n)
A terminating null character is automatically appended after the characters copied to str.
Notice that fgets is quite different from gets: not only fgets accepts a stream argument, but also allows to specify the maximum size of str and includes in the string any ending newline character.
PD: Try to have a larger buffer.
Hello i'm practising C and i have a little problem with the following code. First of all, my program just reads the input from the user and if there is memory availble, it stores it, otherwise it makes nothing.
I have an array of pointers to char called "lines" and an array of chars for the temporary storage of the input called "line".
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXWIDTH 81
#define MAXLINES 100
int main ()
{
char* lines [MAXLINES];
char line[MAXWIDTH];
int i ;
int n ;
Then i will check if my array of pointers has space and the input is non-zero. I do it in a for-loop to fill the array and normally the for-loop should stop when i type in nothing and just press enter or when the array is full.
If there is space i will check if there is enough memory in the space where the pointer is pointing to. If that's ok (!= NULL), the program copies the input from gets(line) in the memory.
for (n = 0; n < MAXLINES && gets(line) != NULL; n++)
{
if ((lines[n] = malloc(strlen(line) + 1)) == NULL)
exit (1);
strcpy(lines[n], line);
}
The rest of the code is just for the output and freeing of the memory.
for (i = 0; i < n; i++)
{
puts(lines[n-i-1]);
free(lines[n-i-1]);
}
return 0;
}
Now the problemis , that the program runs without any errors, but it's not working as i want.
It is just performing a infinte loop where i can type in as long as i want, what i want without any reaction.
gets doesn't return NULL when you type an empty line, if that's what you tried to check for. It will still be an empty string. You'll need to check if the first character is a \0 if you want to look for empty lines.
On a side note, gets is extremely unsafe since it will overrun your buffer if your line is too long and cause evil bugs. Use fgets instead, it lets you specify the size of your buffer. (Note that fgets will add a \n to the end of your string, even if it's an empty line.)
Well for a start, I suggest reading the following about why you should avoid using gets(). I suggest using scanf() or fgets() instead...
http://www.gidnetwork.com/b-56.html
then note that you're doing a loop to 100 taking input, and only after all 100 are you outputting. So you'll need to enter 100 lines of input currently before you see anything...
You're not checking for an empty line.
You need something like:
if('\0' == line[0])
{
break;
}
And use fgets() not gets(). It's safer. However you then need to do:
if('\n' == line[0] || '\r' == line[0])
{
break;
}
Well, how do you terminate your input? Entering an empty line won't help because line will be "" not NULL. Have you tried pressing Ctrl+Z in the console (if it's windows)?