Beyond blocks: Accessible and defined memory If those macros are embedded deeper in the allocator code, stack traces will include parts of your allocator internals, which can confuse users.Īnother macro, VALGRIND_RESIZEINPLACE_BLOCK, functions similarly to the realloc system call. This practice allows Memcheck to provide the most accurate warnings when it detects an issue. Use VALGRIND_FREELIKE_BLOCK as early in your program as possible and VALGRIND_MALLOCLIKE_BLOCK as late as possible. VALGRIND_MALLOCLIKE_BLOCK (p, size, 0, 0) VALGRIND_FREELIKE_BLOCK takes two arguments: the starting address of the block and any redzone bytes added as padding. The redzone can be eliminated by designating its size as zero but if your allocator can reserve bytes before and after the block (Memcheck by default uses 16 bytes when overriding malloc), those extra bytes help Valgrind detect reads and writes that overrun or underrun memory. VALGRIND_MALLOCLIKE_BLOCK takes four arguments: the starting address of the block, the size of the block in bytes, the size of the redzone (the padding around the block) in bytes, and whether the block's contents have been initialized (with zero or a known pattern). But when run under Valgrind, they let Memcheck track memory usage from your custom allocator. Like other valgrind.h macros, these do nothing when your program is not running under Valgrind. If you have your own library for managing memory with unique function names, there is a different way to make Valgrind track memory while your own allocator hands out and retrieves blocks: the VALGRIND_MALLOCLIKE_BLOCK and VALGRIND_FREELIKE_BLOCK macros. And now Memcheck can report memory leaks, use-after-free errors, undefined memory use, buffer overruns, and more for all blocks allocated through my_alloc and my_free without you having to add any extra instrumentation to your own allocator. And now when running under Valgrind, all that work is thrown away by simply calling the malloc and free functions you were trying to avoid.īut if you are running under Valgrind, you don't do it for efficiency: You do it to catch memory issues. You put a lot of work into your own allocator, which you believe to be way more efficient for your application than the GNU C library allocator. Now, when running under Valgrind (and only when running under Valgrind), your allocation functions can simply use malloc and free, and Memcheck can track all memory usage as normal. */Ĭompile this code with: $ gcc -I/usr/include/valgrind -O2 -g -c my_alloc.c * Some global structures for the real allocator. If you have defined your own allocation and deallocation functions, you could use the macro as follows: #include The simplest macro is RUNNING_ON_VALGRIND, which is 0 if running natively and 1 when running under Valgrind. These instructions provide near-zero overhead in normal use but when run under Valgrind, they are recognized as "magic sequences" that instruct Valgrind to do something special at that place in the code. That package provides the /usr/include/valgrind/valgrind.h file, which defines some basic macros that annotate your code using instructions that look like they don't do anything. Make sure you have the valgrind-devel package installed to get access to this behavior. If you wrote your own allocator for more specialized use, Valgrind has a way for you to annotate your code so that tools such as Memcheck can keep track of the memory blocks you hand out to the rest of your program. (See the -soname-synonyms option in the Valgrind manual for more on this.) After you give your functions these standard names, Memcheck can provide all normal memory-tracking functions, just as if you were using the system's malloc implementation. If you simply replace the whole GNU C malloc implementation by defining your own functions with the same names (for example, by using tcmalloc or jemalloc), Valgrind will, since version 3.12.0, intercept all your replacement function (like malloc, free, and so on) unless you tell it not to. But Memcheck doesn't know how a program subdivides that memory internally without a little help, and this article will show you how to provide that help through specialized code annotations. But what if you have built your own memory manager? Memcheck keeps track of memory by observing the standard malloc/ free, new/ delete, and mmap/ munmap system calls. Valgrind Memcheck can detect various memory leaks and keep track of whether memory is accessible and defined. Memcheck is the default tool Valgrind uses when you don't ask it for another tool. Valgrind is an instrumentation framework for building dynamic analysis tools that check C and C++ programs for errors.
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