Connect and share knowledge within a single location that is structured and easy to search. Why doesn't the compiler give me an error telling me there isn't enough memory allocated? I read a couple of questions that ask how to check how much memory malloc actually allocates but I didn't find a concrete answer. Shouldn't the free function have to know exactly how much memory is allocated to buffer? The compiler doesn't know. This is the joy and terror of C.
Tools like valgrind detect some of these errors. Other programming languages make it harder to shoot yourself in the foot. Not C. No production malloc implementation should prevent you from trying to write past what you allocated. It is assumed that if you allocate bytes, you will use all or less than what you allocated. Using memory that you didn't explicitly and successfully ask malloc to give you is undefined behavior. Or you could be writing to a black hole.
You never can know, that's why it's undefined behavior. There are malloc implementations that give you built in statistics and debugging , however these need to be used in lieu of the standard malloc facility just like you would if you were using a garbage collected variety.
That argument expects a structure that contains the statistical data. Other tools like electric fence will simply halt your program on the exact instruction that resulted in an overrun or access to invalid blocks. In all honesty or as they say 'at the end of the day' - it's much easier to use a heap profiler such as Valgrind and its associated tools massif in this case which will give you quite a bit of information.
In this particular case, Valgrind would have pointed out the obvious - you wrote past the allocated boundary.
We used a variant of the library that I linked in HelenOS I'm not sure if they're still using it for quite a while, as debugging at the VMM was known to cause insanity. Still, think hard about future ramifications when considering a drop in replacement, when it comes to the malloc facility you almost always want to use what the system ships.
How much malloc internally allocates is implementation-dependent and OS-dependent e. Your writing into the un-allocated bytes may lead to overwriting other variable's values even if your compiler and run-time dont detect the error. The free-function remembers the number of bytes allocated separate from the allocated region, for example in a free-list. C does not block you from using memory you should not.
You can use that memory, but it is bad and result in Undefined Behaviour. You are writing in a place you should not.
This program might appear as running correctly, but might later crash. This is UB. This is what is happening with your strcpy. Here's a complete example, including looping and printing the array, equivalent to the above C code:. We've been using the stack all semester, first in the form of the "call" and "ret" instructions, and later in the form of "push" and "pop" instructions. Internally, the stack is represented via a pointer stored in the register rsp.
The general rules for use of this stack pointer register are:. Exactly like "call" corresponds to pushing the return address and jumping to the called function, and "ret" corresponds to popping the return address and jumping there, you can fake a push thingy by allocating 8 bytes of stack space and then mov QWORD [rsp], thingy , and you can fake a pop thingy by mov thingy , QWORD [rsp] and then release 8 bytes of stack space. More generally, if you need to allocate dynamic storage for use during a single function call, the stack is an easy and efficient place to do it.
The only downside of stack allocation is you MUST release your stack space before you can return from your function. This means long-lived data structures, like a user info class used for the rest of the program, need to be allocated on the heap with malloc. This is despite the most recent allocation being called the "top of the stack", when it's really the smallest address--this kinda makes sense if you think of writing out all the memory values at increasing addresses from top to bottom, but that's not usually how memory is shown!
Regardless, the pointers or other data to make that work have to go somewhere, and inside the just-freed block is a natural choice. In dlmalloc , the smallest allowed allocation is 32 bytes on a bit system. Going back to the malloc 1 question, 8 bytes of overhead are added to our need for a single byte, and the total is smaller than the minimum of 32, so that's our answer: malloc 1 allocates 32 bytes.
Now we can approach the case of allocating zero bytes. It turns out there's a silly debate about the right thing to do, and it hasn't been resolved, so technically allocating zero bytes is implementation-specific behavior. One side thinks that malloc 0 should return a null pointer and be done with it.
It works, if you don't mind a null return value serving double duty. It can either mean "out of memory" or "you didn't request any memory.
0コメント