What does it matter what argument I give malloc? [duplicate] - c

This question already has answers here:
I can use more memory than how much I've allocated with malloc(), why?
(17 answers)
Closed 6 years ago.
C noob here. What does it matter what argument I give malloc when I can pass whatever size string to it later?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
char *str;
str = malloc(1*sizeof(char));
strcpy(str, "abcd");
printf(str);
printf("\n");
return 0;
}
This works fine. I would have thought I wouldn't be able to store more than 1 char in str from my understanding of what malloc is supposed to be.

malloc can end up actually allocating more than expected to maintain alignment/simplify the allocator.
What you're doing is undefined behavior, and among other things, "undefined" can mean "works, sometimes". Don't do this though, because the other options are not nearly so good. Some of the time, it will crash. Some of the time, it will appear to work, but it turns out you corrupted the heap, and at some later point, using or freeing some completely different allocation, you'll get "inexplicable" data or heap corruption related errors that aren't tied to the overflow in any obvious way.
It's a terrible idea, never rely on having even one byte more than you requested.

Related

Why is struct in C working even with less memory allocation? [duplicate]

This question already has answers here:
Writing to pointer out of bounds after malloc() not causing error
(7 answers)
Closed 2 years ago.
Malloc function in C allocates the size of memory passed into it's argument in bytes.
Here my struct variable has two integer values.So size of struct should be 8 bytes. I am only allocating 1 bytes and it's still working.
Why?
C (gcc)
#include <stdio.h>
#include <stdlib.h>
struct node{
int a;
int b;
}ab;
int main(){
struct node * nl=(struct node *)malloc(1); //I am allocating memory here
nl->b=89;
nl->a=45;
printf("%d %d %ld",nl->a,nl->b,sizeof(int));
}
Try it online!
When you write past the end of allocated memory, you invoke undefined behavior, which means you can't reliably predict what the program will do. It could crash, it could output strange results, or (as in this case) it can appear to work properly.
How undefined behavior manifests itself can change by making a seemingly unrelated change, such as adding an unused local variable or adding a call to printf for debugging. It could also change by compiling with different optimization settings or with a different compiler.
Just because the program could crash doesn't mean it will.
On most architectures, memory is allocated in 4k "pages" and no smaller. Malloc is placing your single requested byte somewhere in either one of these pages or just beyond the program break.
As long as you don't write beyond the allocated page, the virtual memory system is unaware of your transgression and won't segfault you. Of course, there's no way of knowing if or when an illegal access will happen, and thus in the C standard this behavior is undefined.

Why am I able to copy more bytes than defined in the char array? [duplicate]

This question already has answers here:
Why doesn't my program crash when I write past the end of an array?
(9 answers)
Closed 3 years ago.
I have the following code:
#include <stdio.h>
#include <stdlib.h>
int main()
{
char buffer[2];
strcpy(buffer, "12345678910");
printf("%s\n", buffer);
return 0;
}
Since, I have already defined the char array with size 2, I shouldn't be able to put in more than 2 char plus null terminating character. Yet, it is able to take more than that without any buffer overflows or segmentation faults. Even if I copy the string strcpy(buffer, "123456789101298362936129736129369182");, it works fine. The error is generated when I push strcpy(buffer, "1234567891012983629361297361293691823691823869182632918263918");.
More of a theroetical question than a practical, but I hope it helps the new and the experienced programmers alike since it talks about the fundamentals, and helps improving coding ethics. Thanks in advance.
The simple answer is that C does not protect you from yourself. It's YOUR responsibility to check boundaries. The program will happily read and write wherever you instruct it to. However, the operating system may say something if you do this, which is usually a "segmentation fault". A worse scenario is that it may overwrite other variables.
This is a source of many bugs in C, so be careful. Whenever you're writing outside outside a buffer, you're invoking undefined behavior and these can manifest themselves in various ways, including the program working as it should, overwriting variables and segmentation faults.
I shouldn't be able to put in more than 2 char plus null terminating character
This is a common bug. It's NOT "plus null terminating character". It's INCLUDING null terminating character.

Why does C correctly output an integer when I only allocate 1 byte for it? [duplicate]

This question already has answers here:
How dangerous is it to access an array out of bounds?
(12 answers)
Writing to pointer out of bounds after malloc() not causing error
(7 answers)
Why is it that we can write outside of bounds in C?
(7 answers)
What happens if I try to access memory beyond a malloc()'d region?
(5 answers)
Why does int pointer '++' increment by 4 rather than 1?
(5 answers)
Closed 3 years ago.
I've been digging into memory allocation and pointers in C. I was under the impression that if you do not allocate enough memory for a value and then try to put that value in that memory cell, the program would either crash or behave incorrectly.
But what I get is a seemingly correct output where I'd expect something else.
#include <stdio.h>
#include <stdlib.h>
int main()
{
// Here we intentionally allocate only 1 byte,
// even though an `int` takes up 4 bytes
int * address = malloc(1);
address[0] = 16777215; // this value surely takes more than 3 bytes. It cannot fit 1 byte.
address[1] = 1337; // just for demo, let's put a random other number in the next memory cell.
printf("%i\n", address[0]); // Prints 16777215. How?! Didn't we overwrite a part of the number?
return 0;
}
Why does this work? Does malloc actually allocate more than the number of bytes that we pass to it?
EDIT
Thanks for the comments! But I wish to note that being able to write to unassigned memory is not the part that surprises me and it's not part of the question. I know that writing out of bounds is possible and it is "undefined behavior".
For me, the unexpected part is that the line address[1] = 1337; does not in any way corrupt the int value at address[0].
It seems that the explanations for this diverge, too.
#Mini suggests that the reason for this is that malloc actually allocates more than what's passed, because of cross-platform differences.
#P__J__ in the comments says that address[1] for some reason points to the next sizeof(int) byte, not to the next byte. But I don't think I understand what controls this behavior then, because malloc doesn't seem to know about what types we will put into the allocated blocks.
EDIT 2
So thanks to the comments, I believe I understand the program behavior now.
The answer lies in the pointer arithmetic. The program "knows" that an address pointer is of type int, and therefore adding 1 to it (or accessing via address[1]) gives an address of the block that lies 4 (sizeof(int)) bytes ahead.
And if we really wanted, we could move just one byte and really corrupt the value at address[0] by coercing address to char * as described in this answer
Thanks to all and to #P__J__ and #Blastfurnace in particular!
malloc often allocates more than you actually ask for (all system/environment/OS dependent), which is why it works in you scenario (sometimes). However, this is still undefined behavior it can actually allocate only 1 byte (and you are writing to what may not be allocated heap memory).
C doesn't mandate any kinds of bounds checking on array accesses, and it's possible to overflow storage and write into memory you don't technically own. As long as you don't clobber anything "important", your code will appear to work as intended.
However, the behavior on buffer overruns is undefined, so the results will not generally be predictable or repeatable.

Strings and Dynamic allocation in C [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Undefined, unspecified and implementation-defined behavior
This should seg fault. Why doesn't it.
#include <string.h>
#include <stdio.h>
char str1[] = "Sample string. Sample string. Sample string. Sample string. Sample string. ";
char str2[2];
int main ()
{
strcpy (str2,str1);
printf("%s\n", str2);
return 0;
}
I am using gcc version 4.4.3 with the following command:
gcc -std=c99 testString.c -o test
I also tried setting optimisation to o (-O0).
This should seg fault
There's no reason it "should" segfault. The behaviour of the code is undefined. This does not mean it necessarily has to crash.
A segmentation fault only occurs when you perform an access to memory the operating system knows you're not supposed to.
So, what's likely going on, is that the OS allocates memory in pages (which are typically around 4KiB). str2 is probably on the same page as str1, and you're not running off the end of the page, so the OS doesn't notice.
That's the thing about undefined behavior. Anything can happen. Right now, that program actually "works" on your machine. Tomorrow, str2 may be put at the end of a page, and then segfault. Or possibly, you'll overwrite something else in memory, and have completely unpredictable results.
edit: how to cause a segfault:
Two ways. One is still undefined behavior, the other is not.
int main() {
*((volatile char *)0) = 42; /* undefined behavior, but normally segfaults */
}
Or to do it in a defined way:
#include <signal.h>
int main() {
raise(SIGSEGV); /* segfault using defined behavior */
}
edit: third and fourth way to segfault
Here is a variation of the first method using strcpy:
#include <string.h>
const char src[] = "hello, world";
int main() {
strcpy(0, src); /* undefined */
}
And this variant only crashes for me with -O0:
#include <string.h>
const char src[] = "hello, world";
int main() {
char too_short[1];
strcpy(too_short, src); /* smashes stack; undefined */
}
Your program writes beyond the allocated bounds of the array, this results in Undefined Behavior.
The program is ill-formed and It might crash or may not.An explanation may or may not be possible.
It probably doesn't crash because it overwrites some memory beyond the array bounds which is not being used, bt it will once the rightful owner of that memory tries to access it.
A seg-fault is NOT guaranteed behavior.It is one possible (and sometimes likely) outcome of doing something bad.Another possible outcome is that it works by pure luck.A third possible outcome is nasal demons.
if you really want to find out what this might be corrupting i would suggest you see what follows the over-written memory generate a linker map file that should give you a fair idea but then again this all depends on how things are layed out in memory, even can try running this with gdb to reason why it does or does not segfault, that being said, the granularity for built checks in access violations (HW assisted) cannot be finer than a page unless some software magic is thrown in (even with this page granularity access checking it may happen that the immediately next page does really point to something else for the program which you are executing and that it is a Writable page), someone who knows about valgrind can explain how it is able to detect such access violations (also libefence), most likely (i might be very wrong with this explanation, Correct me if i am wrong!) it uses some form of markers or comparisons for checking if out of bounds access has happened.

the function of malloc(using malloc correctly)

so I'm quite new in this, sorry if it sound like a dumb question
I'm trying to understand malloc, and create a very simple program which will print "ABC" using ASCII code
here is my code (what our professor taught us) so far
char *i;
i = malloc(sizeof(char)*4);
*i = 65;
*(i+1) = 66;
*(i+2) = 67;
*(i+3) = '\0';
what I don't understand is, why do I have to put malloc there?
the professor told us the program won't run without the malloc,
but when I tried and run it without the malloc, the program run just fine.
so what's the function of malloc there?
am I even using it right?
any help and or explanation would be really appreciated
the professor told us the program won't run without the malloc
This is not quite true, the correct wording would be: "The program's behavior is undefined without malloc()".
The reason for this is that
char *i;
just declares a pointer to a char, but there's no initialization -- this pointer points to some indeterminate location. You could be just lucky in that writing values to this "random" location works and won't result in a crash. I'd personally call it unlucky because this hides a bug in your program. undefined behavior just means anything can happen, including a "correct" program execution.
malloc() will dynamically request some usable memory and return a pointer to that memory, so after the malloc(), you know i points to 4 bytes of memory you can use. If malloc() fails for some reason (no more memory available), it returns NULL -- your program should test for it before writing to *i.
All that said, of course the program CAN work without malloc(). You could just write
char i[4];
and i would be a local variable with room for 4 characters.
Final side note: sizeof(char) is defined to be 1, so you can just write i = malloc(4);.
Unfortunately, "runs fine" criterion proves nothing about a C program. Great deal of C programs that run to completion have undefined behavior, which does not happen to manifest itself on your particular platform.
You need special tools to see this error. For example, you can run your code through valgrind, and see it access uninitialized pointer.
As for the malloc, you do not have to use dynamic buffer in your code. It would be perfectly fine to allocate the buffer in automatic memory, like this:
char buf[4], *i = buf;
You have to allocate space for memory. In the example below, I did not allocate for memory for i, which resulted in a segmentation fault (you are trying to access memory that you don't have access to)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
char *i;
strcpy(i, "hello");
printf("%s\n", i);
return (0);
}
Output: Segmentation fault (core dumped)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
char *i;
/*Allocated 6 spots with sizeof char +1 for \0 character*/
i = malloc(sizeof(char) * 6);
strcpy(i, "hello");
printf("%s\n", i);
return (0);
}
Result: hello
Malloc allows you to create space, so you can write to a spot in memory. In the first example, "It won't work without malloc" because i is pointing to a spot in memory that doesn't have space allocated yet.

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