- Mtrace
mtrace
is thememory debugger included in theGNU C Library .Use
The function
mtrace
installs handlers formalloc
,realloc
andfree
; the functionmuntrace
disables these handlers. Their prototypes, defined in the header filemcheck.h
, arevoid
mtrace
(void
);void
muntrace
(void
);The handlers log all memory allocations and frees to a file defined by the
environment variable MALLOC_TRACE (if the variable is unset, describes an invalid filename, or describes a filename the user does not have permissions to, the handlers are not installed).A
perl script calledmtrace
, not to be confused with the function of the same name, is also distributed with the GNU C Library; the script parses through the output file and reports all allocations that were not freed.Usage example
Bad Source Code
The following is an example of bad source code. The problem with the program is that it allocates memory, but doesn’t free the memory before exiting.
#includeint main() {
int * a;
a = malloc(sizeof(int)); /* allocate memory and assign it to the pointer */ return 0; /* we exited the program without freeing memory */
/* we should have released the allocated memory with the command “free(a)” */
}
MTrace Usage
1. The environment variable “MALLOC_TRACE” must be set to the path of the output file. Setting environment variables is slightly different on every shell type. In the BASH shell on Linux, the command is as follows:
2. The required library (“mcheck.h”) must be included in the source code. This is done by adding the following line to the top of the .c or .cpp file, as shown below:MALLOC_TRACE=/home/YourUserName/path/to/program/MallocTraceOutputFile.txtexport MALLOC_TRACE;
3. The function “mtrace()” must be called before you start allocating memory. It is usually easiest to call mtrace() at the very beginning of the main function.
#include
4. The program should be compiled and run. Note that you need to use the -g option to enable profiling. In GCC on Linux, this can be done using the following commands:mtrace();
5. The memory leak information will be reported in the file specified by the MALLOC_TRACE environment variable. The difficulty is, this file will be in a computer-readable format. Most Linux machines come with a console command called “mtrace”, that converts the computer file into human-readable text as shown below. If you do not have access to this console command, there is a perl script that can be downloaded and will accomplish the same task. mtrace follows the following syntax:gcc yourProgram.c -g./a.out
For example:mtracemtrace a.out MallocTraceOutputFile.txtMTrace Output
If the mtrace command reports “No Memory Leaks”, then all memory that was allocated in the last execution of that program was also released, which is the way it should be. If, on the other hand, mtrace gives output such as that below, it means the programmer still has some work to do.
Memory not freed:----------------- Address Size Caller0x08049910 0x4 at /home/sureshsathiah/tips/leak.c:9Good Source Code
The following is an example of great source code. It releases memory after it is allocated, and it uses mtrace to notify the programmer if there are memory leaks.
#include
#includeint main() {
mtrace(); /* Starts the recording of memory allocations and releases */
int* a = NULL;
a = malloc(sizeof(int)); /* allocate memory and assign it to the pointer */ if (a = NULL) { return 1; /* error */ }
free(a); /* we free the memory we allocated so we don't have leaks */ muntrace(); return 0; /* exit */
}
See also
*
malloc External links
* [http://www.gnu.org/software/libc/manual/html_node/Allocation-Debugging.html GNU C Library manual: Allocation debugging]
* [http://www.sourcentral.org/man/ubuntu610/3+mtrace Manual page formtrace
]
* [http://www.sourcentral.org/man/ubuntu610/1+mtrace Manual page for the perl scriptmtrace
]
* [http://www.linuxjournal.com/article/6059 Linux Journal: Memory Leak Detection in Embedded Systems]
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