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Hi
say i have a char array of size 19 (meaning 20 characters can be input)
now i got a problem, if i input more then 20 characters it will store them still and be used for my other variables (overflow)
Is there a way to find how many characters a char array is holding?
If you are using it as a string and the size is 19, only 18 characters can be input, since the last char would be the terminating null character.Quote:
say i have a char array of size 19 (meaning 20 characters can be input)
Unless the array has decayed to a pointer (e.g., you passed it as an argument), sizeof() could be used to find the size.
oh ok so like this:
char name[20] = abc
if (sizeof(name) > 20)
output error
or is there anyway to say if more than 20 characters have been inputted dont store the rest? how do i prevent the overflow
that will always return 20. you want to use strlen.Quote:
Originally Posted by taurus
Or just don't ask for more than 20?
Code:char name[20];
fgets(name, sizeof(name), stdin);
By the time strlen is applied, it's too late. The key is stopping before you've read too much, as zacs shows.Quote:
Originally Posted by robwhit
yes and thats what i dont know how to do?
use fgets. see zacs7's post.
Hey taurus
have you finished the psuedocode yet???
oh alright so let me see if i understand this:
the user can enter a name, now his statement checks if its below 20 characters and then if it is stores
OR
does it work by only storing 20 characters and ignore the rest inputed there?
fgets(str, 20, stdin); reads 19 and leaves the rest in the stdin buffer.
unless a newline is read first.
but say i have more variables after that, it wont write the excess in the buffer to those variables?
only if you read it in with an input function.
and yea theres the problem, i am going to be inputting other things, problem?
See the FAQ for flusing the input buffer then, http://faq.cprogramming.com/cgi-bin/...&id=1043284392
Taurus your class mate is asking you somenthing LOLQuote:
Originally Posted by juststartedC
ssharish
You have to understand that user input is kept in a buffer called stdin. It is dynamic and resizes itself so you don't have to worry about making things fit into it. When you store 20 bytes (characters) of data into an array you are using a function to read 20 bytes from stdin and store them into the array. If you don't tell it to read 20 bytes (if you use a function like scanf() where the array size is not passed to the function, it will read bytes from stdin until it encounters a NULL or a byte represented in hex as 00 meaning the end of the string). If you understood nothing I just said, just remember to use fgets and specify the length of your array and you will never run into problems concerning overflow.
1) The data is read from a stream, not a buffer. A buffer may be in there, but it is irrelevant to the concept of reading from a stream.
2) You can tell scanf() the buffer size:
It even works for dynamically sized buffers:Code:scanf("%20s", buffer);
3) The limit character for fgets() is the newline, '\n', not the NUL. A text stream will in fact never emit a NUL; at best it will emit an EOF.Code:scanf("%*s", size, buffer);
Hmm? In the scanf family, the * is for assignment suppression.Quote:
Originally Posted by CornedBee
Oh, sorry. In that case, you have to create a format string.
Stupid omission, IMO.Code:sprintf(buffer1, "%%%is", size);
scanf(buffer1, buffer2);
So how big do you make buffer1? [/nitpick]
ahhhhhhh i see, so instead of using all that buffer thing i can use fgets?Quote:
Originally Posted by keira
ceil(log10(UINT_MAX)) + 3Quote:
Originally Posted by Dave_Sinkula
So how ugly does the scanf version get if you want its fgets equivalent?Quote:
Originally Posted by CornedBee
Very ugly. I never said it was nice, only that it was possible.
jus a quick question. Is there a command to erase the buffer in C. say i enter a string. now i can say if string>20 then erase buffer and reinput?
You mean like this FAQ question:
http://faq.cprogramming.com/cgi-bin/...&id=1043284392
--
Mats
ya i was doing the buffer thing: scanf("%20s", buffer);
but once i enter more than 20 characters i get a segmentation fault. How can i create an error message instead?
Quote:
Originally Posted by taurus
Yes, becasue scanf is "stupid" and can't deal with things going wrong very well - there is A REASON we suggest using fgets() instead - it is more fault-tolerant and unless you do things wrong, you can't make it overwrite other variables and such.
But unfortunately, you are in a class where you can't use that [unless I'm confused], so you have to live with the application not being particularly fault-tolerant.
--
Mats
If you have
char buffer[20];
The max you can have is
scanf( "%19s", buffer );
Unlike fgets(), making scanf() respect the current size of the buffer is damn hard work.
Hi im getting an error for the following. Can someone tell me whats wrong:
Code:char name[20]="User\0";
bool nameLength = false;
bool nameCase = false;
do
{
fprintf(output,"%s please enter your name (20 max): ",name); fflush(output);
fscanf(input,"%s",name);
if (sizeof(name)<19)
{
nameCase = true;
nameLength = true;
}
else
{
nameLength = false;
}
}
while(nameLength == false);
ok fixed.
edit: actually not, see when i enter more than 20 character it says segmentation fault and doesnt perform my do-while loop?
> see when i enter more than 20 character it says segmentation fault
Haven't we explained this enough times already?
Also, sizeof() is not the same as strlen()
For any given variable or type, sizeof() will always give you the same answer, so using it to find out how much input there was is just broken.
And the segfault is all down to buffer overflow.
Stop messing about with fscanf to try and read a potentially long string into a small space and wondering why it goes pear shaped when it trashes memory.
Use fgets(), breath in, breath out and relax.
yea but i tried replacing my fscanf with fgets and i get errors. Is there something i got to do else like importing a library?
There is one thing that you have to understand.Quote:
Originally Posted by taurus
If you have a buffer with a certain size and you input more data that this buffer can hold then the damage is already done. There is no way to correct that problem aferwards.
You have to make shure before doing the input that no bufferoverflow will happen.
fgets will do just that. If you tell it the right size of the buffer it will not overwrite its bounds.
But still you will have to take special care for the case that the user inputs more data then the buffer can hold ( e.g. by checking if the last char in the returned string was '\n' ), otherwise the access input will stay in the inputbuffer and will screw up your following inputs.
The same is true when you use fscanf with a size argument in the format string. It's just that it is a lot harder to detect the case where not all the data was read from the input buffer.
Kurt
hmmm ok i see, but isint \n meaning 'enter'? so how would i do that co check
Guess there is plenty of that in the faq's.
You might want to try something like this
KurtCode:#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main() {
char buffer[10];
int done = 0;
char * p;
while ( !done ) {
fgets( buffer, 10, stdin );
p = strchr(buffer,'\n');
if ( p != 0 ) { /* was all input read ? */
*p= 0; /* replace '\n' with string terminator */
printf("%s\n",buffer);
done = 1;
}
else { /* just print 9 chars, fgets appends '\0' */
printf("%s",buffer);
}
}
}
also got a problem when entering something in the char name field. if i enter 'bob the builder' it wont display anything after the bob, so after the space nothing?
Another reason to use fgets.
Kurt
so do i basically replace fscanf with fgets? simply change the word
No, of course not. fscanf requires a parse string, fgets does not. fscanf takes the FILE* as the first parameter, fgets as the last. Look in your C reference for fgets, the correct way to call it is given there.