Using Valgrind to debug memory leaks. Valgrind is a wonderful tool useful mainly to debug memory related problems in CC programs. I dont know a better tool to find memory leaks. Although output of this program is often clear and intuitive its worth to spend some time to get deeper knowledge of how Valgrind works, what exactly its messages mean and what are the problematic cases when tracing a memory leak is harder even with Valgrind. Valgrind is a tool suite that automatically detect many memory and thread related problems with an application. VjcMK.jpg' alt='Valgrind For Windows' title='Valgrind For Windows' />Its composed of few tools, each designed to track different kind of problems. Valgrind can detect bad memory usage reading uninitialized memory, writing past the buffer etc. Ill cover here. CPU cache usage profile program like gprof. All those features can be used by running one command and your programs executable file as an argument. In this article I will cover usage of the memcheck, so in most cases we will run valgrind this way. Its often useful to redirect valgrinds output to a file instead of stderr, you can use log filevalgrind. What is a memory leak Basically its a case when a program no longer uses some chunk of dynamically allocated memory will not need it in its life time but the memory was not deallocated. In practice the real problem is when the program grows, i. Pages/valgrind/images/LaunchConfig.png' alt='Valgrind For Windows' title='Valgrind For Windows' />
Valgrind can automatically detect most of cases where memory leaks and point them in the code. Lets consider an example program that copies its standard input to the standard output include lt stdio. Have less data than neededn return left len ifwrite1, buf, reslt 0perror writeexit 1 left res free buf return len int main int argc,charargvcopydata 1. To run it under valgrind use the following command. Valgrind will show someting like that omitting the introduction text. ERROR SUMMARY 0 errors from 0 contexts suppressed 1. For counts of detected errors, rerun with v. All heap blocks were freed no leaks are possible. The first line tells us that there were no errors, it means that Valgrind didnt detected any bad memory access etc. More interesting is the summary of memory allocationdeallocation that follows the first line. They are. 1. 58. At the process exit time there was no dynamically allocated memory that was not freed. The program allocated one chunk of memory and also freed one chunk of memory. Valgrind-markers.png' alt='Valgrind For Windows' title='Valgrind For Windows' />Is zlib Y2Kcompliant Yes. Where can I get a Windows DLL version The zlib sources can be compiled without change to produce a DLL. Valgrind is a wonderful tool useful mainly to debug memory related problems in CC programs. I dont know a better tool to find memory leaks. All heap blocks were freed no leaks are possible. Offline Activation Keygen Hardware Identification. This program in the sample run is 1. In practice you will probably not see this message very often. Ill show you later why. Linux_Tools_Project/Valgrind/User_Guide/images/HelgrindOutput2.png' alt='Valgrind For Windows' title='Valgrind For Windows' />Stack Overflow The Worlds Largest Online Community for Developers. Accessoriesmanifest apicouncilfilter Parent for API additions that requires Android API Council approval. BUG b32916152 assetsandroidstudiouxassets Bug. The example program is not perfect, lets see what happens when we try to copy content of devnull to stdout. Have less data than neededERROR SUMMARY 0 errors from 0 contexts suppressed 1. For counts of detected errors, rerun with v. D2. E malloc vgreplacemalloc. LEAK SUMMARY. 1. Wow in case of an error something wrong happened. Valgrind detected that 1. We can see that there was 1 bock, 1. B large that was not deallocated. Valgrind even shows the backtrace at the place when it was allocated and says its confident its a memory leak 6 bytes in 1 blocks are definitely lost. This is a case when Valgrind is 1. Such memory chunks are described as definitely lost. Why its so sure How does it workValgrind tracks each memory allocation when using standard facilities malloc, new operator etc. When it sees that no pointer exists in the program to the allocated memory chunk or its content the program has no way to deallocate it, so there is definitely a memory leak. In our example in case of an error in copydata the buffer pointed to by the local buf variable is not freed. When we return from the function the pointer to the buffer is lost, so Valgrind says there is a memory leak. This is the simplest, most obvious situation, lets look at other examples. The situation above is clear for Valgrind but there are difficult cases. Lets see another example. The code is silly but in practice difficult cases appear in complex programs and its hard to create a short example that makes sense and shows to problem include lt stdio. The program prints its first argument in lower case. Valgrind shows. 2. D2. E malloc vgreplacemalloc. D8. 05. F strdup in libtlsi. F main test. 2. This time memory is possibly lost. Why its not sure Because at the program exit time we didnt completely lost the pointer to the allocated memory, weve only advanced it to print the lower string in a funny way. Its theoretically possible that we have a variable, a counter that tells us how much weve advanced, so we could compute the pointer to the memory to free it. A different case is when we modify the main function this way int main int argc,charrgvlower tolower argv1puts lower return. This way Valgrind doesnt even tell us that there is a memory leak Its worth to point again that the definition of a memory leak used by Valgrind is the case when the program loses the pointer to a dynamically allocated memory. In the above example the pointer was not lost. Despite this fact Valgrind isnt completely silent in this case. It tells us. 2. LEAK SUMMARY. Reachable blocks those to which a pointer was found are not shown. To see them, rerun with leak checkfull show reachableyes. The problematic memory chunk is called reachable block. Its memory that was not freed, but a pointer to it still exists at the programs exit time. If we add the suggested options to the Valgrinds invocation we will see. D2. E malloc vgreplacemalloc. D8. 05. F strdup in libtlsi. C4 tolower test. F main test. Just like when the memory was definitely lost we see the palce in the program where the problematic block was allocated. Judging if its really a memory leak is left to the programmer. There are situations when debugging memory leaks is harder that usual. First of all Valgrind understands only standard allocationdeallocation routines like mallocnew and if you or a library you are using use some custom memory allocators Valgrind cant track memory usage. One example is glib library. If something allocates memory using its routines Valgrind doesnt work properly by default. In this case authors of the library created an easy mechanism to overcome the problem you can set some environment variables GDEBUG set to include gc friendly, GSLICEalways malloc to switch glib custom memory allocation to use systems functions directly. Another solution is to teach Valgrind how your allocators works using its mechanism to describe custom memory allocators. This is useful if you are writing your own memory allocator. Sometimes you see the program grows because standard system monitoring utilities show that the process allocates more memory than you expect but Valgrind shows nothing. The reason could be that memory is deallocated but not as soon as possible. Imagine that you are creating new objects in some other master object, all pointers are stored in a vector so they can be freed in the destructor. Unfortunately your object that does such things has a long life time probably is destroyed at the programs exit time and allocated objects are not freed as soon as they are no longer needed but when the program exits. Its not a memory leak for Valgrind, but its definitely a bug. I remember I was using the std vector container in improper way.