Debugging Memory on Linux

All programs use memory, even ones that do nothing. Memory misuse results in a good portion of fatal program errors, such as program termination and unexpected behavior.

Memory is a device for handling information. Program memory is usually associated with the amount of physical memory a computer has but can also reside on secondary storage, such as disk drives, when not in use. Memory for users is managed by two devices: the kernel itself and the actual program using calls to memory functions such as malloc().

The operating system kernel manages all the memory requirements for a particular program, or instances of a program (because operating systems can execute several instances of a program simultaneously). When a user executes a program, the kernel allocates an area of memory for the program. This program then manages the area of memory by splitting it into several areas:

Figure 1. Memory Associated with an Instance of a Program

User-allocatable memory is located in the heap in the memory arena. The memory arena is managed by the routines malloc(), realloc(), free() and calloc(). They are part of libc. However, it is possible to substitute these functions with another implementation that may provide better performance for a particular use. See sidebar for a list of alternate memory functions.

Alternate Memory Functions

On Linux systems, programs expand the size of the memory arena in precalculated increments, usually one memory page in size or aligned with a boundary. Once the heap requires more than what is available in the memory arena, the memory routines call the brk() system call that requests additional memory from the kernel. The actual increment size can be set by the sbrk() call.

To view the current stack and memory arena of any process, look at the contents of /proc/<pid>/maps for a particular process, where pid is the process id (see Listing 1).

Listing 1. Output from /proc/<pid>/maps

Each time new memory is allocated with malloc(), a little more memory is obtained than requested. The memory routines use this extra memory for maintenance. To obtain the real amount of memory allocated for user manipulation, use the function call malloc_usable_space(). The real memory chunk is usually eight bytes larger.

The structure of a memory chunk has the size of the chunk prepended and added to the end of the chunk (see Figure 2). The size value also has a bit flag that indicates whether the memory management system maintains the memory chunk immediately before the current one.

Figure 2. The Memory Chunk Structure

The memory routines in GNU libc use bins to store memory chunks of similar size to assist in improving performance and preventing fragmented memory areas, where you have unused memory gaps throughout the memory arena. These memory routines are also threadsafe. Though these routines are quick and stable, there may be areas of possible improvement, such as speed and memory coverage.