Can't delete memory

This is a discussion on Can't delete memory within the C++ Programming forums, part of the General Programming Boards category; Valgrind is giving me something interesting. Process terminating with default action of signal 11 (SIGSEGV) Access not within mapped region ...

  1. #16
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    Valgrind is giving me something interesting.

    Process terminating with default action of signal 11 (SIGSEGV)
    Access not within mapped region at address 0x0

    It's pointing to this though:
    Code:
    max_disk_queue = atoi(argv[1]);
    This is in a completely different place though and happens way before. Also, it doesn't actually segfault at that line.
    Last edited by jcafaro10; 02-04-2009 at 09:11 AM.

  2. #17
    C++まいる!Cをこわせ! Elysia's Avatar
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    Likely, argv[1] is NULL.
    Check argc to see how many arguments are passed.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #18
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    Have you considered using containers and smart pointers instead? For example, if you need a stack of chars, use a std::stack<char> (which is a container adapter that provides the interface of a stack).
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    Quote Originally Posted by jcafaro10 View Post
    How do I determine if the pointer is valid? GDB+KDevelop tells me the memory address of the pointer. When I allocate the stack the pointer gets a memory address and it keeps that memory address so I don't think that's the problem.

    Something that is a little curious is, on my watch list it says stack, and gives me a hex memory address, and under that is *stack and thats got 0x0. I'm not sure what that means though.
    It means that the stack points at memory address <hex number> and that it's content is 0x0.

    It is very hard to visually determine the validity of an address - with a lot of viewing memory addresses, you can get a feel for what is valid and isn't on a particular system - but essentially, on Linux, anything from 128MB to 3GB is theoretically correct - but you don't get a completely RANDOM value in that range when allocating memory.

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  5. #20
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    Quote Originally Posted by Elysia View Post
    Likely, argv[1] is NULL.
    Check argc to see how many arguments are passed.
    My program wouldn't get very far at all if argv[1] is null. max_disk_queue gets a value from the argument and the program continues. GDB says it's getting the value 3 which is good because thats the argument I passed. Right before that valgrind says "invalid read of size 1" and points to that line as well.

  6. #21
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    If I comment out the delete of stack, my program doesn't seg fault but the weird valgrind error is still there

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    Quote Originally Posted by laserlight View Post
    Have you considered using containers and smart pointers instead? For example, if you need a stack of chars, use a std::stack<char> (which is a container adapter that provides the interface of a stack).
    I think I have to do it this way because I'm using ucontext which is a linux thing and requires things to be set up a certain way.

  8. #23
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    Quote Originally Posted by jcafaro10
    I think I have to do it this way because I'm using ucontext which is a linux thing and requires things to be set up a certain way.
    What is the "certain way"? Note that if you need a pointer to the first element of a dynamically allocated array of objects you can use a std::vector<char> and then pass &v[0], where v is the name of the vector.
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  9. #24
    C++まいる!Cをこわせ! Elysia's Avatar
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    Doubtful. A stack is a stack, and a pointer is a pointer, and a smart pointer can handle a raw pointer.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #25
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    Quote Originally Posted by jcafaro10 View Post
    If I comment out the delete of stack, my program doesn't seg fault but the weird valgrind error is still there
    Yes, it won't free the memory either, and since (I'm pretty sure) the problem with the stack delete operation is that the administration block of a previous allocation is being overwritten, it obviously won't fail if you don't call the function that tries to USE that bit of memory.

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  11. #26
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    Hmm well valgrind doesn't seem to be reporting anything that conflicts with the stack allocation. I tagged the lines that valgrind mentioned and the memory addresses are all different than the one for the stack.

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    So I ran the debugger again and the time that I ran it, the stack was allocated to memory address 0xb7c34014, so I tagged 0xb7c34012 and 0xb7c34016 (those are a byte away right? I'm a bit new to this) Neither of the values in those locations were overwritten after the allocation of the stack. The stack is given a new memory address everytime I run the program so I have no idea what memory addresses to check before the stack is allocated.

  13. #28
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    So, you need to check where your stack is actually being overwritten.

    What do you use the stack for?

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  14. #29
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    I'm implementing a thread library so I believe the stack is necessary for the operations that each thread performs.

    According to GDB, the stack isn't being overwritten because none of the values in the memory locations surrounding the memory address, change between the allocation and delete of the stack...or at least thats how I interpret it

  15. #30
    C++まいる!Cをこわせ! Elysia's Avatar
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    There is std::stack and std::vector and std::tr1::smart_ptr. Try those before doing manual allocation.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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