---
name: "mtrace"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/mtrace/3"
---

# mtrace(3)

malloc tracing

## NAME

mtrace, muntrace - malloc tracing

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <mcheck.h>
```

**void mtrace(void);** **void muntrace(void);**

## DESCRIPTION

The **mtrace**() function installs hook functions for the memory-allocation functions ([**malloc**(3)](/malloc/3.md), [**realloc**(3)](/realloc/3.md) [**memalign**(3)](/memalign/3.md), [**free**(3)](/free/3.md)). These hook functions record tracing information about memory allocation and deallocation. The tracing information can be used to discover memory leaks and attempts to free nonallocated memory in a program.

The **muntrace**() function disables the hook functions installed by **mtrace**(), so that tracing information is no longer recorded for the memory-allocation functions. If no hook functions were successfully installed by **mtrace**(), **muntrace**() does nothing.

When **mtrace**() is called, it checks the value of the environment variable **MALLOC\_TRACE**, which should contain the pathname of a file in which the tracing information is to be recorded. If the pathname is successfully opened, it is truncated to zero length.

If **MALLOC\_TRACE** is not set, or the pathname it specifies is invalid or not writable, then no hook functions are installed, and **mtrace**() has no effect. In set-user-ID and set-group-ID programs, **MALLOC\_TRACE** is ignored, and **mtrace**() has no effect.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **mtrace**(), **muntrace**() | Thread safety | MT-Unsafe |

## STANDARDS

GNU.

## NOTES

In normal usage, **mtrace**() is called once at the start of execution of a program, and **muntrace**() is never called.

The tracing output produced after a call to **mtrace**() is textual, but not designed to be human readable. The GNU C library provides a Perl script, [**mtrace**(1)](/mtrace/1.md), that interprets the trace log and produces human-readable output. For best results, the traced program should be compiled with debugging enabled, so that line-number information is recorded in the executable.

The tracing performed by **mtrace**() incurs a performance penalty (if **MALLOC\_TRACE** points to a valid, writable pathname).

## BUGS

The line-number information produced by [**mtrace**(1)](/mtrace/1.md) is not always precise: the line number references may refer to the previous or following (nonblank) line of the source code.

## EXAMPLES

The shell session below demonstrates the use of the **mtrace**() function and the [**mtrace**(1)](/mtrace/1.md) command in a program that has memory leaks at two different locations. The demonstration uses the following program:

$ **cat t\_mtrace.c**

```
#include <mcheck.h>
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
    mtrace();
    for (unsigned int j = 0; j < 2; j++)
        malloc(100);            /* Never freed--a memory leak */
    calloc(16, 16);             /* Never freed--a memory leak */
    exit(EXIT_SUCCESS);
}
```

When we run the program as follows, we see that **mtrace**() diagnosed memory leaks at two different locations in the program:

```
$ cc -g t_mtrace.c -o t_mtrace
$ export MALLOC_TRACE=/tmp/t
$ ./t_mtrace
$ mtrace ./t_mtrace $MALLOC_TRACE
Memory not freed:
-----------------
   Address     Size     Caller
0x084c9378     0x64  at /home/cecilia/t_mtrace.c:12
0x084c93e0     0x64  at /home/cecilia/t_mtrace.c:12
0x084c9448    0x100  at /home/cecilia/t_mtrace.c:16
```

The first two messages about unfreed memory correspond to the two [**malloc**(3)](/malloc/3.md) calls inside the *for* loop. The final message corresponds to the call to [**calloc**(3)](/calloc/3.md) (which in turn calls [**malloc**(3)](/malloc/3.md)).

## SEE ALSO

[**mtrace**(1)](/mtrace/1.md), [**malloc**(3)](/malloc/3.md), [**malloc\_hook**(3)](/malloc_hook/3.md), [**mcheck**(3)](/mcheck/3.md)