Hidden printable text - c

I build a small program that simply copy the text from an input.txt file to an output.txt file. It works with no apparent problem with the command:
./myCopier.txt < rand.txt > randout.txt
on a GCC89 compiler, used for didactic purposes.
#include<stdio.h>
int main(void){
int c;
while ((c = getchar()) != EOF)
putchar(c);
/*The text in printf() does not appear anywhere*/
printf("My theories and toughts!\n");
}
Later, I add printf() in order to print my thoughts about how the program worked. It compiles without errors or warnings even with restrictive parameters (-Wall), but this text does not appear in any place. By any place, I mean both output.txt and the standard output.
I am thinking that:
The command ./myCopier.exe alone clearly create an endless loop. Without text input there is no EOL character, hence the print() command is never reached.
So, why when the input is provided, the printf() command has no apparent effect? I'd expect the text from printf()to appear on the standard output, just when the loop closes and output.txt is created. Why does it not occur?
My theory is that ./myCopier.txt < rand.txt > randout.txt does not allow it. For some reason, it makes the program only transfer the input to the output, "ignoring" everything does not come from the input. Am I correct? If so, why?
If you are interested in the problem with deeper detail, here is the assembly code:
http://text-share.com/view/79f31f38

Omitting the return statement in main() causes undefined behavior in C89/C90.
Normally, returning from main() forces the output buffer to be flushed. If you omit the return statement, anything can happen.
Try to add return 0 at the end of main() as was required in C90. GCC in c90/gnu90 mode will warn you otherwise: "control reaches end of non-void function".
Try to add fflush(stdout);.

Related

Is adding the newline character to printf() equivalent to flushing the stream?

The following program shows how buffered I/O can cause problems in programs when errors like 'divide by zero' happen:
int main()
{
int a = 1, b = 0, c;
printf("Dividing...");
c = a/b;
printf("Answer is: %d\n", c);
return 0;
}
The output is Floating point exception (core dumped).
Fair enough. But surprisingly, if I changed the first printf to printf("Dividing...\n");, this text actually gets printed before the program crashes (I'm running GCC on Linux, by the way).
Am I to conclude that adding a newline is equivalent to flushing? And if so, if all my printf() strings end in \n, I'm actually depriving myself of the benefits of buffered I/O?
If your standard output is going to a terminal, yes, it's equal to flushing, as printf() will use line-based buffering.
If it's redirected to a file, then no. In this case printf() uses much bigger buffers, usually corresponding to file system block sizes or whatever, to optimize IO.

Flushing stdin after every input - which approach is not buggy?

After Mark Lakata pointed out that the garbage isn't properly defined in my question I came up with this. I'll keep this updated to avoid confusions.
I am trying to get a function that I can call before a prompt for user input like printf("Enter your choice:); followed a scanf and be sure that only the things entered after the prompt would be scanned in by scanf as valid input.
As far as I can understand the function that is needed is something that flushes standard input completely. That is what I want. So for the purpose of this function the "garbage" is everything in user input i.e. the whole user input before that user prompt.
While using scanf() in C there is always the problem of extra input lying in the input buffer. So I was looking for a function that I call after every scanf call to remedy this problem. I used this, this, this and this to get these answers
//First approach
scanf("%*[^\n]\n");
//2ndapproach
scanf("%*[^\n]%*c");
//3rd approach
int c;
while((c = getchar()) != EOF)
if (c == '\n')
break;
All three are working as far as I could find by hit-and-trial and going by the references. But before using any of these in all of my codes I wanted to know whether any of these have any bugs?
EDIT:
Thanks to Mark Lakata for one bug in 3rd. I corrected it in the question.
EDIT2:
After Jerry Coffin answered I tested the 1st 2 approaches using this program in code:blocks IDE 12.11 using GNU GCC Compiler(Version not stated in the compiler settings).
#include<stdio.h>
int main()
{
int x = 3; //Some arbitrary value
//1st one
scanf("%*[^\n]\n");
scanf("%d", &x);
printf("%d\n", x);
x = 3;
//2nd one
scanf("%*[^\n]%*c");
scanf("%d", &x);
printf("%d", x);
}
I used the following 2 inputs
First Test Input (2 Newlines but no spaces in the middle of garbage input)
abhabdjasxd
23
bbhvdahdbkajdnalkalkd
46
For the first I got the following output by the printf statements
23
46
i.e. both codes worked properly.
Second Test input: (2 Newlines with spaces in the middle of garbage input)
hahasjbas asasadlk
23
manbdjas sadjadja a
46
For the second I got the following output by the printf statements
23
3
Hence I found that the second one won't be taking care of extra garbage input whitespaces. Hence, it isn't foolproof against garbage input.
I decided to try out a 3rd test case (garbage includes newline before and after the non-whitespace character)
``
hahasjbas asasadlk
23
manbdjas sadjadja a
46
The answer was
3
3
i.e. both failed in this test case.
The first two are subtly different: they both read and ignore all the characters up to a new-line. Then the first skips all consecutive white space so after it executes, the next character you read will be non-whitespace.
The second reads and ignores characters until it encounters a new-line then reads (and discards) exactly one more character.
The difference will show up if you have (for example) double-spaced text, like:
line 1
line 2
Let's assume you read to somewhere in the middle of line 1. If you then execute the first one, the next character you read in will be the 'l' on line 2. If you execute the second, the next character you read in will be the new-line between line 1 and line 2.
As for the third, if I were going to do this at all, I'd do something like:
int ch;
while ((ch=getchar()) != EOF && ch != '\n')
;
...and yes, this does work correctly -- && forces a sequence point, so its left operand is evaluated first. Then there's a sequence point. Then, if and only if the left operand evaluated to true, it evaluates its right operand.
As for performance differences: since you're dealing with I/O to start with, there's little reasonable question that all of these will always be I/O bound. Despite its apparent complexity, scanf (and company) are usually code that's been used and carefully optimized over years of use. In this case, the hand-rolled loop may be quite a bit slower (e.g., if the code for getchar doesn't get expanded inline) or it may be about the same speed. The only way it stands any chance of being significantly faster is if the person who wrote your standard library was incompetent.
As far maintainability: IMO, anybody who claims to know C should know the scan set conversion for scanf. This is neither new nor rocket science. Anybody who doesn't know it really isn't a competent C programmer.
The first 2 examples use a feature of scanf that I didn't even know existed, and I'm sure a lot of other people didn't know. Being able to support a feature in the future is important. Even if it was a well known feature, it will be less efficient and harder to read the format string than your 3rd example.
The third example looks fine.
(edit history: I made a mistake saying that ANSI-C did not guarantee left-to-right evaluation of && and proposed a change. However, ANSI-C does guarantee left-to-right evaluation of &&. I'm not sure about K&R C, but I can't find any reference to it and no one uses it anyways...)
Many other solutions have the problem that they cause the program to hang and wait for input when there is nothing left to flush. Waiting for EOF is wrong because you don't get that until the user closes the input completely!
On Linux, the following will do a non-blocking flush:
// flush any data from the internal buffers
fflush (stdin);
// read any data from the kernel buffers
char buffer[100];
while (-1 != recv (0, buffer, 100, MSG_DONTWAIT))
{
}
The Linux man page says that fflush on stdin is non-standard, but "Most other implementations behave the same as Linux."
The MSG_DONTWAIT flag is also non-standard (it causes recv to return immediately if there is no data to be delivered).
You should use getline/getchar:
#include <stdio.h>
int main()
{
int bytes_read;
int nbytes = 100;
char *my_string;
puts ("Please enter a line of text.");
/* These 2 lines are the heart of the program. */
my_string = (char *) malloc (nbytes + 1);
bytes_read = getline (&my_string, &nbytes, stdin);
if (bytes_read == -1)
{
puts ("ERROR!");
}
else
{
puts ("You typed:");
puts (my_string);
}
return 0;
I think if you see carefully at right hand side of this page you will see many questions similar to yours. You can use fflush() on windows.

Very simple C question using getchar() and putchar()

Hello I am teaching myself C and going through the K & R book and I am having some trouble (I am running OS X). This is from section 1.5.1 "File Copying" which is supposed to take a character as input, then output the character. Here is the code:
#include <stdio.h>
/* -- Copy input to output -- */
int main(void)
{
int c;
c = getchar();
while ( c != EOF ) {
putchar(c);
c = getchar;
}
}
So, I think my problem is not with the code itself but with compling and running. First of all, when compiling I get the following errors
/Volumes/Goliath/Dropbox/C programs/prog1_5_1.c: In function ‘main’:
/Volumes/Goliath/Dropbox/C programs/prog1_5_1.c:12: warning: assignment makes integer from pointer without a cast
/Volumes/Goliath/Dropbox/C programs/prog1_5_1.c:16: warning: control reaches end of non-void function
Then when I run the output file (in terminal) it has a small space, then when I input a letter, say I type
a
then I hit Return
and I get a new line. If I then hit a new key, the screen starts going crazy with question marks all over the place.
I am not sure if I am making much sense but I am finding this an odd problem to have. Thank you very much in advance
The second assignment should be c = getchar();. By leaving out the parentheses, you're assigning the address of the getchar function to c, which is very much not what you want.
Also, at the end of main you need the line return 0; or similar in order to get rid of the "control reaches end of non-void function" warning.
you missed the () on getchar on line 12. without parenthesis, "getchar" evaluates to the address of the function, which is why you get the pointer-cast-to-int warning
You're missing parenthesis after the 2nd getchar.
This means you're assigning the location in memory of the method to the variable c, which causes an infinite loop as it's never equal to EOF.

Learning C (via K&R) using xcode

I'm learning C with The C Programming Language (K&R).
Since I don't particularly want to bob back and forth between a text editor and running gcc, I've decided to use xcode as an IDE. So far, I've been able to follow the book's examples without a problem up until section 1.5.2.
When given the valid (?) program...
#include <stdio.h>
void main()
{
long nc;
nc = 0;
while (getchar() != EOF)
++nc;
printf("%ld\n", nc);
}
...I receive no final output telling me how many characters were in my input. I am entering my input via the xcode console window.
Upon some debugging, it looks like my program gets stuck in the while loop, and never encounters the EOF token. To accommodate for this, I've instead substituted a newline as the new condition, by replacing EOF with "\n", which also does nothing and gives me a int to pointer comparison warning.
What am I doing wrong here?
Will I be able to follow K&R using xcode?
Type ^D (control-d) to send an EOF.
If you want to break on a newline, you need to compare the return value of getchar to '\n', not "\n". The former is the actual char value of a newline; the latter is a pointer to a char with that value. If that doesn't make sense to you yet, don't worry, it will once you've read more.

why does this happen (see image)?

Why does the following have the effect it does - it prints a terminal full of random characters and then exits leaving a command prompt that produces garbage when you type in it. (I tried it because I thought it would produce a seg fault).
#include <stdio.h>
int main(){
char* s = "lololololololol";
while(1){
printf("%c", *s);
s++;
}
}
it was compiled with:
gcc -std=c99 hello.c
It will eventually seg fault, but before that it'll print out whatever bytes are in the same page. That's why you see random chars on the screen.
Those may well include escape sequences to change (say) the character encoding of the console. That's why you end up with gibberish when you type on the console after it's exited, too.
Because you have an infinite loop (while(1)), and you keep getting the current value of pointer (*s), and then moving the pointer one char forward (s++). This has the effect of marching well past the end of the string into "garbage" (uninitialized memory), which gets printed to the console as a result.
In addition to what everyone else said in regards to you ignoring the string terminal character and just printing willy-nilly what's in memory past the string, the reason why your command prompt is also "garbage" is that by printing a particular "unprintable" character, your terminal session was left in a strange character mode. (I don't know which character it is or what mode change it does, but maybe someone else can pipe in about it that knows better than I.)
You are just printing out what is in memory because your loop doesn't stop at the end of the string. Each random byte is interpreted as a character. It will seg fault when you reach the end of the memory page (and get into unreadable territory).
Expanding ever so slightly on the answers given here (which are all excellent) ... I ran into this more than once myself when I was just beginning with C, and it's an easy mistake to make.
A quick tweak to your while loop will fix it. Everyone else has given you the why, I'll hook you up with the how:
#include <stdio.h>
int main() {
char *s = "lolololololololol";
while (*s != '\0') {
printf("%c", *s);
s++;
}
}
Note that instead of an infinite loop (while(1)), we're doing a loop check to ensure that the pointer we're pulling isn't the null-terminator for the string, thus avoiding the overrun you're encountering.
If you're stuck absolutely needing while(1) (for example, if this is homework and the instructor wants you to use it), use the break keyword to exit the loop. The following code smells, at least to me, but it works:
#include <stdio.h>
int main() {
char *s = "lolololololololol";
while (1) {
if (*s == '\0')
break;
printf("%c", *s);
s++;
}
}
Both produce the same console output, with no line break at the end:
lolololololololol
Your loop doesn't terminate, so println prints whatever is in the memory after the text you write; eventually it will access memory it is not allowed to read, causing it to segfault.
You can change the loop as the others suggested, or you can take advantage of fact that in c, zero is false and null (which terminates all strings) is also zero, so you can construct the loop as:
while (*s) {
Rather than:
while (*s != '\0')
The first one may be more difficult to understand, but it does have the advantage of brevity so it is often used to save a bit of typing.
Also, you can usually get back to your command prompt by using the 'reset' command, typing blindly of course. (type Enter, reset, Enter)

Resources