This article contains instructions, advice, or how-to content. (March 2015) |
mtrace
is the memory debugger included in the GNU C Library.
Use
editNote that mtrace
tool works only with single threaded applications. One thread could temporarily remove the hook while another thread could malloc memory at the sametime leading to missed allocations in a multithreaded application!
The function mtrace
installs handlers for malloc
, realloc
and free
; the function muntrace
disables these handlers. Their prototypes, defined in the header file mcheck.h
, are
voidmtrace
(void
);
voidmuntrace
(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 called mtrace
, 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
editBad source code
editThe 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.
#include <stdlib.h>
int main(void) {
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 statement “free(a)” */
}
MTrace usage
edit- Set the environment variable
MALLOC_TRACE
to the pathname of the desired output file. Setting environment variables is slightly different in each shell. In Bourne Shell-compatible shells, like Bash, the command is as follows:$ MALLOC_TRACE=/home/YourUserName/path/to/program/MallocTraceOutputFile.txt $ export MALLOC_TRACE
- Include
mcheck.h
in the source code. This is done, for example, by adding the following line to the top of a C or C++ file, as shown below:#include <mcheck.h>
- Call the function
mtrace()
before you start allocating memory. It is usually easiest to callmtrace()
at the very beginning of themain()
function:mtrace();
- To delineate the end of the code that should be traced, call the function
muntrace()
. This is usually done at the end of themain()
function: muntrace();
- Compile and run the program as usual. Note that you need to compile with the
-g
option to get useful output. In GCC on Linux, this can be done using the following commands for a C program:$ gcc yourProgram.c -g $ ./a.out
- 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 calledmtrace
, that converts the computer readable format into human-readable text as shown below. If you do not have access to this console command, there is a Perl script, of the same name, that can be downloaded to accomplish the same task. Themtrace
syntax is as follows:$ mtrace <exec_file_name> <malloc_trace_filename>
- For example:
$ mtrace a.out MallocTraceOutputFile.txt
mtrace
can be used with parallel computing but one process at a time, using a condition on the rank like:if (my_rank==0) mtrace();
MTrace output
editIf 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 Caller 0x08049910 0x4 at /home/sureshsathiah/tips/leak.c:9
Good source code
editThe following is an example of good source code. It releases memory after it is allocated, and it uses mtrace to notify the programmer if there are memory leaks.
#include <stdlib.h>
#include <mcheck.h>
int main(void) {
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 */
}