int main()
{
char ch = 0;
int c = 0;
scanf("%d", &ch);
printf("%c", ch);
}
I can input the ASCII number and output the correct character. But the program crashes at the end of the code, the } sign.
Using Visual Studio 2019
Run-Time Check Failure #2 - Stack around the variable 'ch' was corrupted.
Since scanf has non-existent type safety, it can't know the type of parameters passed (one reason why the format string is there in the first place).
The stack corruption is caused by you lying to the compiler here: scanf("%d", &ch);. You tell it "trust me, ch is an int". But it isn't. So scanf tries to store 4 bytes in an area where only 1 byte is allocated and you get a crash.
Decent compilers can warn for incorrect format specifiers.
There are multiple problems in your code:
missing #include <stdio.h> ;
unused variable c ;
type mismatch passing a char variable address when scanf() expects an int * for the %d conversion specifier ;
missing test on scanf() return value ;
missing newline at the end of the output ;
missing return 0; (implied as of c99, but sloppy).
Here is a modified version:
#include <stdio.h>
int main() {
int ch;
if (scanf("%d", &ch) == 1) {
printf("%c\n", ch);
}
return 0;
}
I guess your question was not, "What did I do wrong?", or "Why didn't it work?".
I guess your question is, "Why did it work?", or, "Why did it print an error message after seeming to work correctly?"
There are lots of mistakes you can make which will corrupt the stack. This is one of them. Much of the time, these errors are rather inscrutable: If the memory corruption is severe enough that the memory management hardware can detect it, you may get a generic message like "Segmentation violation" at the instant the bad access occurs, but if not, if the damage isn't bad enough to cause any overt problems, your program may seem to work as you expected, despite the error.
It would be prohibitively expensive to perform explicit tests (in software) to check the stack for damage during every operation. Therefore, no attempt is made to do so, and the primary responsibility is placed on you, the programmer, not to do obviously wrong things like telling scanf to store an int in a char-sized box.
In this case, your system did make a check for stack damage, but as some kind of a one-time operation, only after the main function had returned. That's why the scanf and the printf seemed to work correctly, and the error message seemed to coincide with the closing } at the end of main.
Echoing an analogy I made in a comment, this is sort of like having a policeman write you a ticket for having done something dangerous that did not cause an accident. Why not write the ticket as you are doing the dangerous thing? Why write the ticket after the fact at all, since your dangerous behavior didn't cause an accident? Well, because sometimes that's just the way the world works.
I think you are trying to input an integer and print its char value so in my opinion this is what you are trying to do:
#include <iostream>
using namespace std;
int main()
{
int c = 0;
scanf("%d", &c);
printf("%c", c);
return 0;
}
Note: This is a C++ code but the syntax inside main function will work for C language as well
Related
For some homework I have to write a calculator in C. I wanted to input some string with scanf and then access it. But when I access the first element I get a segmentation error.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(){
char input1[30];
scanf("%s",input1);
printf("%s",input1);
char current = input1[0];
int counter = 0;
while(current != '\0'){
if(isdigit(current) || current == '+' || current == '-' || current == '*' || current == '/'){
counter++;
current = input1[counter];
}else{
printf("invalid input\n");
exit(1);
}
}
return 0;
}
The printf in line 3 returns the string, but accessing it in line 4 returns a segmentation error (tested in gdb). Why?
There are a few potential causes, some of which have been mentioned in the comments (I won't cover those). It's hard to say which one (or more) is the cause of your problem, so I guess it makes sense to iterate them. However, you may notice that I cite some resources in the process... The information is out there, yet you're not stumbling across it until it's too late. Something needs to change with how you research, because this is slowing your progress down.
On input/output dynamics, just a quick note
printf("%s",input1);
Unless we include a trailing newline, this output may be delayed (or "buffered"), which may have the effect of confusing you about the root of your issues. As an alternative to using a trailing newline (which I'd prefer, personally) you could explicitly force partial lines to be written by invoking fflush(stdout) immediately after each of the relevant output operations, or use setbuf to disable buffering entirely. I think this is unlikely to be your problem, but it may mask your problem, so it's important to realise, when using printf to debug, it might be best to include a trailing newline...
On main entry points
The first potential culprit I see is here:
int main()
I don't know why our education system is still pushing these broken lessons. My only guess is the professors learnt many years back using the nowadays irrelevant Turbo C and don't want to stay up-to-date with tech. We can further reduce this to a simple testcase to work out if this is your segfault, but like I said, it's hard to say whether this is actually your problem...
int main() {
char input1[30];
memset(input1, '\x90', sizeof input1);
return 0; // this is redundant for `main` nowadays, btw
}
To explain what's going on here, I'll cite this page, which you probably ought to go and read (in its entirety) once you're done here:
A common misconception for C programmers, is to assume that a function prototyped as follows takes no arguments:
int foo();
In fact, this function is deemed to take an unknown number of arguments. Using the keyword void within the brackets is the correct way to tell the compiler that the function takes NO arguments.
Simply put, if the linker doesn't know/can't work out how many arguments are required for the entry point, there's probably gonna be some oddness to your callstack, and that's gonna occur at the beginning or end of your program.
On input errors, return values and uninitialised access
#include <assert.h>
#include <stdio.h>
#include <string.h>
int main(void) {
char input1[30];
memset(input1, '\x90', sizeof input1);
scanf("%s",input1); // this is sus a.f.
assert(memchr(input1, '\0', sizeof input1));
}
In my testcase, I actually wrote '\x90' to each byte in the array, to show that if the scanf call fails you may end up with an array that has no null terminator. If this is your problem, this assertion is likely to throw (as you can see from the ideone demo) when you run it, which indicates that your loop is likely accessing garbage beyond the bounds of input1. On this note I intended to demonstrate that we (mostly) cannot rely upon scanf and friends unless we also check their return values! There's a good chance your compiler is warning you about this one, so another lesson is uto pay close attention to warning messages, and strive to have none.
On argument expectations for standard library functions
For many standard library functions it may be possible to give input that is outside of the acceptable domain, and so causes instability. The most common form, which I also see in your program, exists in the form of possibly passing invalid values to <ctype.h> functions. In your case, you could change the declaration of current to be an unsigned char instead, but the usual idiom is to put the cast explicitly in the call (like isdigit((unsigned char) current)) so the rest of us can see you're not stuck in this common error, at least while you're learning C.
Please note at this point I'm thinking whichever resources you're using to learn aren't working, because you're stumbling into common traps... please try to find more reputable resources to learn from so you don't fall into more common traps and waste more time later on. If you're struggling, check out the C tag wiki...
It seems like there is problem in scanf_s
Here is my code.
#include <stdio.h>
#include "stack.h"
int main(){
int disk;
int hanoi[3][9];
char input[3] = { 0,0,0 };
int moveDisk;
for (int i = 0; i < 9; i++) {
hanoi[0][i] = i + 1;
hanoi[1][i] = 0;
hanoi[2][i] = 0;
}
printf("Insert the number of disks(1~9): ");
scanf_s("%d", &disk);
while (input[0] != 'q') {
printf("%3c %3c %3c\n", 'A', 'B', 'C');
for (int i = 0; i < disk; i++) {
printf("%3d %3d %3d\n", hanoi[0][i], hanoi[1][i], hanoi[2][i]);
}
scanf_s("%s", &input); //getting moving disk -- LOCATION OF ERROR
}
}
I have no idea how to solve this
No doubt you tried to use scanf() in the normal way and Visual Studio reported an error instructing you to use scanf_s()? It is not a direct replacement. For all %c, %s and %[ format specifiers you must provide two arguments - the target receiving the input, and the size of target (or strictly the number of elements).
In VS2019 even at /W1 warning level, it issues a clear explanation of the problem in this case:
warning C4473: 'scanf_s' : not enough arguments passed for format string
message : placeholders and their parameters expect 2 variadic arguments, but 1 were provided
message : the missing variadic argument 2 is required by format string '%s'
message : this argument is used as a buffer size
Don't ignore the warnings, and certainly don't disable them globally (/W0).
So in this case:
scanf_s("%s", input, sizeof(input) ) ;
again more strictly:
scanf_s("%s", input, sizeof(input)/sizeof(*input) ) ;
but the latter is really only necessary for wscanf_s (wide characters). In both cases you could use the _countof() macro, but it is Microsoft specific.
scanf_s("%s", input, _countof(input) ) ;
Note also the lack of an & before input. You don't need it for an argument that is already array or pointer. That is true of scanf() too.
Whilst there are arguments for using scanf_s() over scanf() (which is intrinsically more dangerous), it can just make life difficult if you are learning from standard examples or using a different toolchain. The simpler solution is just to disable the warning, and understand that it is unsafe:
You cited the line
scanf_s("%s", &input);
There are several things wrong with this line:
You are reading a string into a character array. This is an exception to the normal pattern for scanf, in that you do not need the &.
You are using the semistandard scanf_s, instead of the normal scanf. scanf_s is supposed to be "safer", but in order for it to provide its safetiness guarantees you have to call it differently than normal scanf, too. You have to tell it the size of the array you're reading the string into. Combined with #1 above, I believe a more correct call would be scanf_s("%s", input, 3);.
For most purposes, a string of size 3 would be far too small for reading a line of input from the user. Since in this case I guess you're only reading a "line" to give yourself an opportunity to hit RETURN before the program makes another trip through its loop, I guess it's okay.
As I mentioned, scanf_s is not quite Standard, so using it is a mixed bag. Pros: 1. It's allegedly safer. 2. Some people (including perhaps your instructor) will recommend always using it for that reason. Cons: 3. It's nut fully standard (it's an optional part of the standard) meaning that not all C compilers and libraries will support it. 4. Its calling patterns are necessarily quite different than normal scanf; it is not a drop-in replacement, so confusion is likely. (I'm not saying "don't use scanf_s", but you should be aware of its somewhat dubious status.)
If you want to read a line of input from the user before continuing, and if the line might be a "q" or something else, scanf (of any variety) might not be the best choice. In particular, %s wants to read a non-whitespace string, so if you just hit the Return key, it's going to keep waiting. This might or might not be a problem for you. (Or it might not be something you need to worry about right now; you may have bigger fish to fry.)
How can I solve this problem during debugging?
Run your program step by step using the debugger. Then when you get the exception, you've found the line causing it.
Restart your program and go up to the line where the exception will occur. That is stop on that line without execution it.
Then with the debugger, you can look at all variables and try to understand if their value is what you expect.
Does this answered your question?
BTW: The compiler should at least emitted some warnings. You really should first fix those warnings. If you have no warning, make sure you have turned on all warnings in the compiler options.
After compiling my program of Dice Roll, I got this error. What is wrong with the code?
Also before I was using gets() instead of scanf() command, but because of that I got this error - passing argument 1 of 'gets' makes pointer from integer without a cast
So I removed the gets() command and used scanf and then there was no error regarding scanf().
What is the reason for getting these two errors?
Ok, so as per the answer I got to know how I should have used the gets() command and why I shouldn't use it instead should use scanf(). So, I made the changes.
Though I have encountered two new errors, this time it's related to the delay() command that I used.
Errors: undefined reference to delay
|error: ld returned 1 exit status|
OK so I solved my last errors by using Sleep() command from windows.h library instead of Delay() command. The programs was compiled.
But still there is a runtime error in the program, it works well till getting the roll1 but then it just print the next two statement and terminated the programs without taking a input for the guess.
It skips all the code after printf("Will it be Higher/Lower or the same? (press H/L/S)\n"); and directly terminates the program.
Ok So I solved above problem adding a whitespace before the "%c" in scanf(" %c", &nextGuess); statement. (Little things xD)
Now only problem is that my toupper() command is not working.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <windows.h>
int main()
{
int i, roll1=0, roll2=0, NumberOfRolls, RandomNUM1[50], RandomNUM2[50];
char nextGuess;
puts("Welcome to the Dice Roll Game");
puts("How many times do you want to roll a dice?");
scanf("%d", &NumberOfRolls);
for( i=1; i<=NumberOfRolls; i++ ) {
RandomNUM1[i] = ( rand()%6 ) + 1;
roll1 += RandomNUM1[i];
}
printf("\nYou Got %d in your first roll!\n", roll1);
Sleep(3000);
printf("\nLet's see if you can guess the value of next roll.\n");
printf("Will it be Higher/Lower or the same? (press H/L/S)\n");
scanf(" %c", &nextGuess);
toupper(nextGuess);
for( i=1; i<=NumberOfRolls; i++ ) {
RandomNUM2[i] = ( rand()%6 ) + 1;
roll2 += RandomNUM2[i];
}
if(nextGuess=='H'){
if(roll1<roll2){
printf("You are such a player, you guessed it right! It's %d", roll2);
}
else if(roll1>roll2){
printf("Uh-Oh! Bad Luck! First roll was higher, It's %d", roll2);
}
else if(roll1==roll2){
printf("Uh-Oh! Bad Luck! Both the rolls are same, It's %d", roll2);
}
}
if(nextGuess=='L'){
if(roll1>roll2){
printf("You are such a player, you guessed it right! It's %d", roll2);
}
else if(roll1<roll2){
printf("Uh-Oh! Bad Luck! First roll was lower, It's %d", roll2);
}
else if(roll1==roll2){
printf("Uh-Oh! Bad Luck! Both the rolls are same, It's %d", roll2);
}
}
if(nextGuess=='S'){
if(roll1==roll2){
printf("You are such a player, you guessed it right! It's %d", roll2);
}
else if(roll1>roll2){
printf("Uh-Oh! Bad Luck! First roll was higher, It's %d", roll2);
}
else if(roll1<roll2){
printf("Uh-Oh! Bad Luck! Second roll is higher, It's %d", roll2);
}
}
return 0;
}
You have a stray ,
At the 2nd line of your main, you declare char nextGuess, instead of char nextGuess;
The compiler tells you it expects specifiers or ... after , so either you add these, or you end the line properly with;.
And for the other problem you mention:
passing argument 1 of 'gets' makes pointer from integer without a cast
Since gets argument should be char *str and you didn't provide it.
You can fix that by, for example:
char tmp_NumberOfRolls[10];
gets(tmp_NumberOfRolls);
NumberOfRolls = atoi(tmp_NumberOfRolls);
but I prefer the scanf solution
PS: (in a now edited version of the code) ***//Error Line*** is not a comment (at least, not all of it) since the *** is still counted as part of the code and will cause an error. Either move the // a bit to the left or enclose that whole part with /* ... */
The gets function reads a string (till the next newline character) from stdin, therefore it asks for a char pointer (char*) to an area of memory where it can put all the characters read including string terminator. The mistake you made is to pass to that function a pointer to int, therefore not only you get a compiler error because you tried to pass an int pointer to gets (for which there are none implicit conversions), but even if it compiled, it would not have worked as you expected because it would put all the characters (which have a size of one byte - most of the times) in that area of memory encoded as chars. That means that when you try to dereference one using a pointer to int, the characters are "read like they were an int"!
There is an example (supposing 1-byte chars, 4-bytes int and a little endian machine and that there is an implicit cast from int* to char* which does not exist and hopefully will never exist):
int num
gets(&num);
if I input 123 in stdin, the memory area pointer by num is large enough to contain the string, but it would contain (exadecimal representation):
0x31|0x32|0x33|0x00
because the string is 3 characters long, ASCII code for '1' is 0x31, for '2' is 0x32, and for '3' is 0x33 and terminates with '\0'!!
Then, when you try to dereference it you get this int (binary representation - supposing a little endian machine):
00000000|00110001|00110010|00110011
which is not the int value 123 but instead 3224115. Which is not what you wanted to get.
When you use the scanf function, you pass to that function a format string which tells it how to interpret the next arguments passed to it and performs the appropriate conversions between the string read to the right type you specified. That means that you should still pay attention to what you tell to the scanf function while you write the format string (if you tell it to read a string and pass to it a pointer to int, the program will probably crash), but it performs the appropriate conversions for you, if you write the format string correctly.
That's why with scanf everything works perfectly (you're reading an int and you specified "%d" as format string), whereas with gets it does not compile to avoid serious mistakes.
I also would like to remark some points of this answer:
The example is just for didactic purposes and the code provided does not work indeeed. The fact that I supposed that it compiles is for didactic purposes; obviously, the code provided in that example does not compile
If, in that example, we input a string larger than 3 characters (which are four if we include the null terminator) or the int (char) type contains less (more) than 4 (1) byte, the program would have crashed because we corrupted other areas of memory
The reference is more expert than me in techincal stuff, so here are the links to the gets function and the scanf function: gets, scanf (and the ASCII Table is useful too)
You could use the gets function together with the atoi function to parse the string read from gets (using a char pointer to a free area of memory large enough to contain the string, which is pretty hard to allocate (*)) to an int, but scanf is the best approach.
(*) Remember: if you allocate an area of memory that contains 20 chars, the user will input 21 chars. The atoi function fails, but the worst thing is that you have a buffer overflow (and can be a high security issue if your program runs under root permissions).
I came across the following code :
int i;
for(; scanf("%s", &i);)
printf("hello");
As per my understanding, if we provide integer input scanf would be unsuccessful in reading and therefore return 0, thus the loop should not run even once. However, it runs infinitely by accepting all types of inputs as successful reads.
Would someone kindly explain this behaviour?
That is the incorrect format specifier for an int: should be "%d".
It is attempting to read a string into an int variable, probably overwriting memory. As "%s" is specified, all inputs will be read thus scanf() returns a value greater than zero.
(Edit: I don't think this answer should have been accepted. Upvoted maybe, but not accepted. It doesn't explain the infinite loop at all, #hmjd does that.)
(This doesn't actually answer the question, the other answers do that, but it's interesting and good to know.)
As hmjd says, using scanf like this will overwrite memory ("smash the stack"), as it starts writing to i in memory, and then keeps going, even outside the 4 bytes of memory that i takes up (or 8 bytes, on a 64-bit platform).
To illustrate, consider the following bit of code:
#include<stdio.h>
int main() {
char str_above[8] = "ABCDEFG";
int i;
char str_below[8] = "ABCDEFG";
scanf("%s", &i);
printf("i = %d\n", i);
printf("str_above = %s\nstr_below = %s\n", str_above, str_below);
return 0;
}
Compiling and running it, and entering 1234567890 produces the following output:
i = 875770417
str_above = 567890
str_below = ABCDEFG
Some points:
i has little correspondence to the integer 1234567890 (it is related to the values of the characters '1',...,'4' and the endianness of the system).
str_above has been modified by scanf: the characters '5',...,'0','\0' have overrun the end of the block of memory reserved for i and have been written to the memory reserved for str_above.
The stack has been smashed "upwards", i.e. str_above is stored later in memory than i and str_below is stored earlier in memory. (To put it another way &str_above > &i and &str_below < &i.)
This is the basis for "buffer overrun attacks", where values on the stack are modified by writing too much data to an array. And it is why gets is dangerous (and should never be used) and using scanf with a generic %s format specifier should also never be done.
Thank you everybody so far for your input and advice!
Additionally:
After testing and toying further, it seems individual calls to FileReader succeed. But calling FileReader multiple times (these might be separate versions of FileReader) causes the issue to occur.
End Add
Hello,
I have a very unusual problem [please read this fully: it's important] (Code::Blocks compiler, Windows Vista Home) [no replicable code] with the C File Reading functions (fread, fgetc). Now, normally, the File Reading functions load up the data correctly to a self-allocating and self-deallocating string (and it's not the string's issue), but this is where it gets bizarre (and where Quantum Physics fits in):
An error catching statement reports that EOF occurred too early (IE inside the comments section at the start of the text file it's loading). Printing out the string [after it's loaded] reports that indeed, it's too short (24 chars) (but it has enough space to fit it [~400] and no allocation issues). The fgetc loop iterator reports it's terminating at just 24 (the file is roughly 300 chars long) with an EOF: This is where it goes whacky:
Temporarily checking Read->_base reports the entire (~300) chars are loaded - no EOF at 24. Perplexed, [given it's an fgetc loop] I added a printf to display each char [as a %d so I could spot the -1 EOF] at every step so I could see what it was doing, and modified it so it was a single char. It loops fine, reaching the ~300 mark instead of 24 - but freezes up randomly moments later. BUT, when I removed printf, it terminated at 24 again and got caught by the error-catching statement.
Summary:
So, basically: I have a bug that is affected by the 'Observer Effect' out of quantum physics: When I try to observe the chars I get from fgetc via printf, the problem (early EOF termination at 24) disappears, but when I stop viewing it, the error-catch statement reports early termination.
The more bizarre thing is, this isn't the first time it's occurred. Fread had a similar problem, and I was unable to figure out why, and replaced it with the fgetc loop.
[Code can't really be supplied as the code base is 5 headers in size].
Snippet:
int X = 0;
int C = 0;
int I = 0;
while(Copy.Array[X] != EOF)
{
//Copy.Array[X] = fgetc(Read);
C = fgetc(Read);
Copy.Array[X] = C;
printf("%d %c\n",C,C); //Remove/add this as necessary
if(C == EOF){break;}
X++;
}
Side-Note: Breaking it down into the simplest format does not reproduce the error.
This is the oldest error in the book, kind of.
You can't use a variable of type char to read characters (!), since the EOF constant doesn't fit.
You need:
int C;
Also, the while condition looks scary, you are incrementing X in the loop, then checking the (new) position, is that properly initialized? You don't show how Copy.Array is set up before starting the loop.
I would suggest removing that altogether, it's very strange code.
In fact, I don't understand why you loop reading single characters at all, why not just use fread() to read as much as you need?
Firstly, unwind's answer is a valid point although I'm not sure whether it explains the issues you are seeing.
Secondly,
printf("%d %c\n",C,C); //Remove/add this as necessary
might be a problem. The %d and %c format specifiers expect an int to be the parameter, you are only passing a char. Depending on your compiler, this might mean that they are too small.
This is what I think the problem is:
How are you allocating Copy.Array? Are you making sure all its elements are zeroed before you start? If you malloc it (malloc just leaves whatever garbage was in the memory it returns) and an element just happens to contain 0xFF, your loop will exit prematurely because your while condition tests Copy.Array[X] before you have placed a character in that location.
This is one of the few cases where I allow myself to put an assignment in a condition because the pattern
int c;
while ((c = fgetc(fileStream)) != EOF)
{
doSomethingWithC(c);
}
is really common
Edit
Just read your "Additionally" comment. I think it is highly likely you are overrunning your output buffer. I think you should change your code to something like:
int X = 0; int C = 0; int I = 0;
while(X < arraySize && (C = fgetc(Read)) != EOF)
{
Copy.Array[X] = C;
printf("%d %c\n", (int)C, (int)C);
X++;
}
printf("\n");
Note that I am assuming that you have a variable called arraySize that is set to the number of characters you can write to the array without overrunning it. Note also, I am not writing the EOF to your array.
You probably have some heap corruption going on. Without seeing code it's impossible to say.
Not sure if this is your error but this code:
C = fgetc(Read);
Copy.Array[X] = C;
if(C == EOF){break;}
Means you are adding the EOF value into your array - I'm pretty sure you don't want to do that, especially as your array is presumably char and EOF is int, so you'll actually end up with some other value in there (which could mess up later loops etc).
Instead I suggest you change the order so C is only put in the array once you know it is not EOF:
C = fgetc(Read);
if(C == EOF){break;}
Copy.Array[X] = C;
Whilst this isn't what I'd call a 'complete' answer (as the bug remains), this does solve the 'observer effect' element: I found, for some reason, printf was somehow 'fixing' the code, and using std::cout seemed to (well, I can't say 'fix' the problem) prevent the observer effect happening. That is to say, use std::cout instead of printf (as printf is the origin of the observer effect).
It seems to me that printf does something in memory on a lower level that seems to partially correct what does indeed seem to be a memory allocation error.