The code below is said to give a segmentation violation:
#include <stdio.h>
#include <string.h>
void function(char *str) {
char buffer[16];
strcpy(buffer,str);
}
int main() {
char large_string[256];
int i;
for( i = 0; i < 255; i++)
large_string[i] = 'A';
function(large_string);
return 1;
}
It's compiled and run like this:
gcc -Wall -Wextra hw.cpp && a.exe
But there is nothing output.
NOTE
The above code indeed overwrites the ret address and so on if you really understand what's going underneath.
The ret address will be 0x41414141 to be specific.
Important
This requires profound knowledge of stack
You're just getting lucky. There's no reason that code has to generate a segmentation fault (or any other kind of error). It's still probably a bad idea, though. You can probably get it to fail by increasing the size of large_string.
Probably in your implementation buffer is immediately below large_string on the stack. So when the call to strcpy overflows buffer, it's just writing most of the way into large_string without doing any particular damage. It will write at least 255 bytes, but whether it writes more depends what's above large_string (and the uninitialised value of the last byte of large_string). It seems to have stopped before doing any damage or segfaulting.
By fluke, the return address of the call to function isn't being trashed. Either it's below buffer on the stack or it's in a register, or maybe the function is inlined, I can't remember what no optimisation does. If you can't be bothered to check the disassembly, I can't either ;-). So you're returning and exiting without problems.
Whoever said that code would give a segfault probably isn't reliable. It results in undefined behaviour. On this occasion, the behaviour was to output nothing and exit.
[Edit: I checked on my compiler (GCC on cygwin), and for this code it is using the standard x86 calling convention and entry/exit code. And it does segfault.]
You're compiling a .cpp (c++) program by invoking gcc (instead of g++)... not sure if this is the cause, but on a linux system (it appears your running on windows due to the default .exe output) it throws the following error when trying to compile as you have stated:
/tmp/ccSZCCBR.o:(.eh_frame+0x12): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status
Its UB ( undefined behavior).
Strcpy might have copied more bytes into memory pointed by buffer and it might not cause problem at that moment.
It's undefined behavior, which means that anything can happen. The program can even appear to work correctly.
It seem that you just happen to not overwrite any parts of memory that are still needed by the rest of the (short) program (or are out of the programs address space/write protected/...), so nothing special happens. At least nothing that would lead to any output.
There's a zero byte on the stack somewhere that stops the strcpy() and there's enough room on the stack not to hit protected page. Try printing out strlen(buffer) in that function. In any case the result is undefined behavior.
Get into habit of using strlcpy(3) family of functions.
You can test this in other ways:
#include <stdlib.h>
int main() {
int *a=(int *)malloc(10*sizeof(int));
int i;
for (i=0;i<1000000; i++) a[i] = i;
return 0;
}
In my machine, this causes SIGSEGV only at around i = 37000! (tested by inspecting the core with gdb).
To guard against these problems, test your programs using a malloc debugger... and use lots of mallocs, since there are no memory debugging libraries that I know of that can look into static memory. Example: Electric Fence
gcc -g -Wall docore.c -o c -lefence
And now the SIGSEGV is triggered as soon as i=10, as would be expected.
As everyone says, your program has undefined behaviour. In fact your program has more bugs than you thought it did, but after it's already undefined it doesn't get any further undefined.
Here's my guess about why there was no output. You didn't completely disable optimization. The compiler saw that the code in function() doesn't have any defined effect on the rest of the program. The compiler optimized out the call to function().
Odds are that the long string is, in fact, terminated by the zero byte in i. Assuming that the variables in main are laid out in the order they are declared -- which isn't required by anything in the language spec that I know of but seems likely in practice -- then large_string would be first in memory, followed by i. The loop sets i to 0 and counts up to 255. Whether i is stored big-endian or little-endian, either way it has a zero byte in it. So in traversing large_string, at either byte 256 or 257 you'll hit a null byte.
Beyond that, I'd have to study the generated code to figure out why this didn't blow. As you seem to indicate, I'd expect that the copy to buffer would overwrite the return address from the strcpy, so when it tried to return you'd be going into deep space some where and would quickly blow up on something.
But as others say, "undefined" means "unpredictable".
There may be anything in your 'char buffer[16]', including \0. strcpy copies till it finds first \0 - thus not going above your boundary of 16 characters.
Related
I have this code in C which takes in bunch of chars
#include<stdio.h>
# define NEWLINE '\n'
int main()
{
char c;
char str[6];
int i = 0;
while( ((c = getchar()) != NEWLINE))
{
str[i] = c;
++i;
printf("%d\n", i);
}
return 0;
}
Input is: testtesttest
Output:
1
2
3
4
5
6
7
8
117
118
119
120
My questions are:
Why don't I get an out of bounds (segmentation fault) exception although I clearly exceed the capacity of the array?
Why do the numbers in the output suddenly jump to very big numbers?
I tried this in C++ and got the same behavior. Could anyone please explain what is the reason for this?
C doesn't check array boundaries. A segmentation fault will only occur if you try to dereference a pointer to memory that your program doesn't have permission to access. Simply going past the end of an array is unlikely to cause that behaviour. Undefined behaviour is just that - undefined. It may appear to work just fine, but you shouldn't be relying on its safety.
Your program causes undefined behaviour by accessing memory past the end of the array. In this case, it looks like one of your str[i] = c writes overwrites the value in i.
C++ has the same rules as C does in this case.
When you access an array index, C and C++ don't do bound checking. Segmentation faults only happen when you try to read or write to a page that was not allocated (or try to do something on a page which isn't permitted, e.g. trying to write to a read-only page), but since pages are usually pretty big (multiples of a few kilobytes; on Mac OS, multiples of 4 KB), it often leaves you with lots of room to overflow.
If your array is on the stack (like yours), it can be even worse as the stack is usually pretty large (up to several megabytes). This is also the cause of security concerns: writing past the bounds of an array on the stack may overwrite the return address of the function and lead to arbitrary code execution (the famous "buffer overflow" security breaches).
The values you get when you read are just what happens to exist at this particular place. They are completely undefined.
If you use C++ (and are lucky enough to work with C++11), the standard defines the std::array<T, N> type, which is an array that knows its bounds. The at method will throw if you try to read past the end of it.
C does not check array bounds.
In fact, a segmentation fault isn't specifically a runtime error generated by exceeding the array bounds. Rather, it is a result of memory protection that is provided by the operating system. It occurs when your process tries to access memory that does not belong to it, or if it tries to access a memory address that doesn't exist.
Writing outside array bounds (actually even just performing the pointer arithmetic/array subscripting, even if you don't use the result to read or write anything) results in undefined behavior. Undefined behavior is not a reported or reportable error; it measn your program could do anything at all. It's very dangerous and you are fully responsible for avoiding it. C is not Java/Python/etc.
Memory allocation is more complicated than it seems. The variable "str," in this case, is on the stack, next to other variables, so it's not followed by unallocated memory. Memory is also usually word-aligned (one "word" is four to eight bytes.) You were possibly messing with the value for another variable, or with some "padding" (empty space added to maintain word alignment,) or something else entirely.
Like R.. said, it's undefined behavior. Out-of-bounds conditions could cause a segfault... or they could cause silent memory corruption. If you're modifying memory which has already been allocated, this will not be caught by the operating system. That's why out-of-bounds errors are so insidious in C.
Because C/C++ doesn't check bounds.
Arrays are internally pointers to a location in memory. When you call arr[index] what it does is:
type value = *(arr + index);
The results are big numbers (not necessarily) because they're garbage values. Just like an uninitialized variable.
You have to compile like this:
gcc -fsanitize=address -ggdb -o test test.c
There is more information here.
I have this code in C which takes in bunch of chars
#include<stdio.h>
# define NEWLINE '\n'
int main()
{
char c;
char str[6];
int i = 0;
while( ((c = getchar()) != NEWLINE))
{
str[i] = c;
++i;
printf("%d\n", i);
}
return 0;
}
Input is: testtesttest
Output:
1
2
3
4
5
6
7
8
117
118
119
120
My questions are:
Why don't I get an out of bounds (segmentation fault) exception although I clearly exceed the capacity of the array?
Why do the numbers in the output suddenly jump to very big numbers?
I tried this in C++ and got the same behavior. Could anyone please explain what is the reason for this?
C doesn't check array boundaries. A segmentation fault will only occur if you try to dereference a pointer to memory that your program doesn't have permission to access. Simply going past the end of an array is unlikely to cause that behaviour. Undefined behaviour is just that - undefined. It may appear to work just fine, but you shouldn't be relying on its safety.
Your program causes undefined behaviour by accessing memory past the end of the array. In this case, it looks like one of your str[i] = c writes overwrites the value in i.
C++ has the same rules as C does in this case.
When you access an array index, C and C++ don't do bound checking. Segmentation faults only happen when you try to read or write to a page that was not allocated (or try to do something on a page which isn't permitted, e.g. trying to write to a read-only page), but since pages are usually pretty big (multiples of a few kilobytes; on Mac OS, multiples of 4 KB), it often leaves you with lots of room to overflow.
If your array is on the stack (like yours), it can be even worse as the stack is usually pretty large (up to several megabytes). This is also the cause of security concerns: writing past the bounds of an array on the stack may overwrite the return address of the function and lead to arbitrary code execution (the famous "buffer overflow" security breaches).
The values you get when you read are just what happens to exist at this particular place. They are completely undefined.
If you use C++ (and are lucky enough to work with C++11), the standard defines the std::array<T, N> type, which is an array that knows its bounds. The at method will throw if you try to read past the end of it.
C does not check array bounds.
In fact, a segmentation fault isn't specifically a runtime error generated by exceeding the array bounds. Rather, it is a result of memory protection that is provided by the operating system. It occurs when your process tries to access memory that does not belong to it, or if it tries to access a memory address that doesn't exist.
Writing outside array bounds (actually even just performing the pointer arithmetic/array subscripting, even if you don't use the result to read or write anything) results in undefined behavior. Undefined behavior is not a reported or reportable error; it measn your program could do anything at all. It's very dangerous and you are fully responsible for avoiding it. C is not Java/Python/etc.
Memory allocation is more complicated than it seems. The variable "str," in this case, is on the stack, next to other variables, so it's not followed by unallocated memory. Memory is also usually word-aligned (one "word" is four to eight bytes.) You were possibly messing with the value for another variable, or with some "padding" (empty space added to maintain word alignment,) or something else entirely.
Like R.. said, it's undefined behavior. Out-of-bounds conditions could cause a segfault... or they could cause silent memory corruption. If you're modifying memory which has already been allocated, this will not be caught by the operating system. That's why out-of-bounds errors are so insidious in C.
Because C/C++ doesn't check bounds.
Arrays are internally pointers to a location in memory. When you call arr[index] what it does is:
type value = *(arr + index);
The results are big numbers (not necessarily) because they're garbage values. Just like an uninitialized variable.
You have to compile like this:
gcc -fsanitize=address -ggdb -o test test.c
There is more information here.
I have this code in C which takes in bunch of chars
#include<stdio.h>
# define NEWLINE '\n'
int main()
{
char c;
char str[6];
int i = 0;
while( ((c = getchar()) != NEWLINE))
{
str[i] = c;
++i;
printf("%d\n", i);
}
return 0;
}
Input is: testtesttest
Output:
1
2
3
4
5
6
7
8
117
118
119
120
My questions are:
Why don't I get an out of bounds (segmentation fault) exception although I clearly exceed the capacity of the array?
Why do the numbers in the output suddenly jump to very big numbers?
I tried this in C++ and got the same behavior. Could anyone please explain what is the reason for this?
C doesn't check array boundaries. A segmentation fault will only occur if you try to dereference a pointer to memory that your program doesn't have permission to access. Simply going past the end of an array is unlikely to cause that behaviour. Undefined behaviour is just that - undefined. It may appear to work just fine, but you shouldn't be relying on its safety.
Your program causes undefined behaviour by accessing memory past the end of the array. In this case, it looks like one of your str[i] = c writes overwrites the value in i.
C++ has the same rules as C does in this case.
When you access an array index, C and C++ don't do bound checking. Segmentation faults only happen when you try to read or write to a page that was not allocated (or try to do something on a page which isn't permitted, e.g. trying to write to a read-only page), but since pages are usually pretty big (multiples of a few kilobytes; on Mac OS, multiples of 4 KB), it often leaves you with lots of room to overflow.
If your array is on the stack (like yours), it can be even worse as the stack is usually pretty large (up to several megabytes). This is also the cause of security concerns: writing past the bounds of an array on the stack may overwrite the return address of the function and lead to arbitrary code execution (the famous "buffer overflow" security breaches).
The values you get when you read are just what happens to exist at this particular place. They are completely undefined.
If you use C++ (and are lucky enough to work with C++11), the standard defines the std::array<T, N> type, which is an array that knows its bounds. The at method will throw if you try to read past the end of it.
C does not check array bounds.
In fact, a segmentation fault isn't specifically a runtime error generated by exceeding the array bounds. Rather, it is a result of memory protection that is provided by the operating system. It occurs when your process tries to access memory that does not belong to it, or if it tries to access a memory address that doesn't exist.
Writing outside array bounds (actually even just performing the pointer arithmetic/array subscripting, even if you don't use the result to read or write anything) results in undefined behavior. Undefined behavior is not a reported or reportable error; it measn your program could do anything at all. It's very dangerous and you are fully responsible for avoiding it. C is not Java/Python/etc.
Memory allocation is more complicated than it seems. The variable "str," in this case, is on the stack, next to other variables, so it's not followed by unallocated memory. Memory is also usually word-aligned (one "word" is four to eight bytes.) You were possibly messing with the value for another variable, or with some "padding" (empty space added to maintain word alignment,) or something else entirely.
Like R.. said, it's undefined behavior. Out-of-bounds conditions could cause a segfault... or they could cause silent memory corruption. If you're modifying memory which has already been allocated, this will not be caught by the operating system. That's why out-of-bounds errors are so insidious in C.
Because C/C++ doesn't check bounds.
Arrays are internally pointers to a location in memory. When you call arr[index] what it does is:
type value = *(arr + index);
The results are big numbers (not necessarily) because they're garbage values. Just like an uninitialized variable.
You have to compile like this:
gcc -fsanitize=address -ggdb -o test test.c
There is more information here.
I have this code in C which takes in bunch of chars
#include<stdio.h>
# define NEWLINE '\n'
int main()
{
char c;
char str[6];
int i = 0;
while( ((c = getchar()) != NEWLINE))
{
str[i] = c;
++i;
printf("%d\n", i);
}
return 0;
}
Input is: testtesttest
Output:
1
2
3
4
5
6
7
8
117
118
119
120
My questions are:
Why don't I get an out of bounds (segmentation fault) exception although I clearly exceed the capacity of the array?
Why do the numbers in the output suddenly jump to very big numbers?
I tried this in C++ and got the same behavior. Could anyone please explain what is the reason for this?
C doesn't check array boundaries. A segmentation fault will only occur if you try to dereference a pointer to memory that your program doesn't have permission to access. Simply going past the end of an array is unlikely to cause that behaviour. Undefined behaviour is just that - undefined. It may appear to work just fine, but you shouldn't be relying on its safety.
Your program causes undefined behaviour by accessing memory past the end of the array. In this case, it looks like one of your str[i] = c writes overwrites the value in i.
C++ has the same rules as C does in this case.
When you access an array index, C and C++ don't do bound checking. Segmentation faults only happen when you try to read or write to a page that was not allocated (or try to do something on a page which isn't permitted, e.g. trying to write to a read-only page), but since pages are usually pretty big (multiples of a few kilobytes; on Mac OS, multiples of 4 KB), it often leaves you with lots of room to overflow.
If your array is on the stack (like yours), it can be even worse as the stack is usually pretty large (up to several megabytes). This is also the cause of security concerns: writing past the bounds of an array on the stack may overwrite the return address of the function and lead to arbitrary code execution (the famous "buffer overflow" security breaches).
The values you get when you read are just what happens to exist at this particular place. They are completely undefined.
If you use C++ (and are lucky enough to work with C++11), the standard defines the std::array<T, N> type, which is an array that knows its bounds. The at method will throw if you try to read past the end of it.
C does not check array bounds.
In fact, a segmentation fault isn't specifically a runtime error generated by exceeding the array bounds. Rather, it is a result of memory protection that is provided by the operating system. It occurs when your process tries to access memory that does not belong to it, or if it tries to access a memory address that doesn't exist.
Writing outside array bounds (actually even just performing the pointer arithmetic/array subscripting, even if you don't use the result to read or write anything) results in undefined behavior. Undefined behavior is not a reported or reportable error; it measn your program could do anything at all. It's very dangerous and you are fully responsible for avoiding it. C is not Java/Python/etc.
Memory allocation is more complicated than it seems. The variable "str," in this case, is on the stack, next to other variables, so it's not followed by unallocated memory. Memory is also usually word-aligned (one "word" is four to eight bytes.) You were possibly messing with the value for another variable, or with some "padding" (empty space added to maintain word alignment,) or something else entirely.
Like R.. said, it's undefined behavior. Out-of-bounds conditions could cause a segfault... or they could cause silent memory corruption. If you're modifying memory which has already been allocated, this will not be caught by the operating system. That's why out-of-bounds errors are so insidious in C.
Because C/C++ doesn't check bounds.
Arrays are internally pointers to a location in memory. When you call arr[index] what it does is:
type value = *(arr + index);
The results are big numbers (not necessarily) because they're garbage values. Just like an uninitialized variable.
You have to compile like this:
gcc -fsanitize=address -ggdb -o test test.c
There is more information here.
I am trying to get my feet into C, and wrote this program that displays a kb of my RAM in a random location. Here is the code, and it works fine:
#include <stdio.h>
int main(){
char *mem;
for(int i =0; i < 1024; i++){
mem++;
printf("%c", *mem);
}
return 0;
}
After that, I did the following change in my code, and I get segfaults every time I run my program:
#include <stdio.h>
// Just added this signature
int main(int argc, char *argv[]){
char *mem;
for(int i =0; i < 1024; i++){
mem++;
printf("%c", *mem);
}
return 0;
}
My spider senses tell me that the segfaults I get are random, and should also be caused in the first example, but running the different programs again and again makes it look like predictable behaviour.
$ gcc -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
Both your snippets invoke undefined behavior as you try to
Go out of bound (mem++;, with no allocation)
use uninitialized values (accessing *mem )
with the current version.
Remember, pointers do not magically inherit (or acquire) memory, you need to make a pointer point to something valid, in general.
The value of mem is undefined (not initialized), but not random. If before main is called, other C runtime functions, are called, then the slot of stack used by mem may have a valid pointer within it. Adding parameters to main changes which slot is used and changes behaviour. This can mean the code doesn't crash, although it is not correct.
You need to initialize mem. I guess you're trying to just read random memory, but that isn't allowed. For example, you may be trying to read memory that's used by a different process, or you may be trying to read some address that doesn't even exist in your computer.
By changing the signature for main, you've changed what random junk value is in mem to start with. The way it probably works is that mem is taking a random value from some register. When you modified the function signature, argc and argv are using those registers instead. Therefor mem is getting a different junk register value of a junk stack value. In any case, you shouldn't try to follow a junk pointer.
Just because it works in one example, only means you got lucky. You still should not do it. It's very likely it wouldn't work if any little thing was changed.
You never initialize mem, so its contents are undefined. When you attempt to either increment it with ++ or dereference the pointer, you get undefined behavior.
One of the things that can happen with undefined behavior is that a program may appear to work normally, and making a seemingly unrelated change will cause a crash.