pmap

What is pmap?

pmap displays the memory map of a specific process. This memory map details how the operating system has allocated virtual memory to that process, including details like:

• Address Ranges: The starting and ending virtual addresses of each memory segment.
• Permissions: The access permissions for each segment (read, write, execute).
• Offset: The offset into the file (if applicable).
• Device: The device from which the segment is mapped (e.g., a file or shared memory).
• Path: The path to the file if the memory is mapped from a file.

Basic Usage

The simplest way to use pmap is to provide the process ID (PID) as an argument:

pmap <PID>

For example, to examine the memory map of the process with PID 1234:

pmap 1234

This will output a table showing the memory segments allocated to process 1234. Each line represents a different segment, and the columns typically include the address range, permissions, offset, device, and path.

Interpreting the Output

The output of pmap can seem dense at first, but understanding the columns is key. Let’s break down a typical line:

address           perms offset  dev   inode       pathname
00400000-00401000 r-xp 00000000 00:00 1234567     /usr/bin/myprogram

• 00400000-00401000: The virtual address range occupied by this segment.
• r-xp: Permissions: r (read), - (no write), x (execute), p (private, meaning this memory is not shared with other processes).
• 00000000: Offset into the file.
• 00:00: Device major and minor numbers.
• 1234567: Inode number.
• /usr/bin/myprogram: Path to the executable file.

Different permissions combinations are possible (e.g., rw-p, rwxp, ---p). shared instead of private indicates shared memory.

Advanced Usage: Targeting Specific Processes

You can identify the PID of a process using other commands like ps:

ps aux | grep myprogram

This will show information about processes containing “myprogram” in their name. Then, copy the PID and use it with pmap:

Handling Multiple Processes

If you need to examine memory usage across multiple processes, you can use ps with pmap for efficient monitoring:

ps aux | while read line; do PID=$(echo $line | awk '{print $2}'); pmap $PID; done

This is an example and might need adjustments based on your system’s ps output format. It’s important to understand the output of your ps command to extract the PID correctly.

Identifying Memory Leaks

By observing changes in the memory map over time, pmap can indirectly help identify potential memory leaks. Repeatedly running pmap on a suspect process can reveal if certain segments are growing excessively, hinting at a memory management problem.

pmap Limitations

Keep in mind that pmap provides a snapshot of the memory map at a specific moment. It doesn’t show dynamic changes in memory allocation. Also, interpretation of the output requires some familiarity with operating system memory management concepts.