Understanding Linux /proc/id/maps


Q:I am trying to understand my embedded linux application's memory use. The /proc/pid/maps utility/file seems to be a good resource for seeing the details. Unfortunately I don't understand all the columns and entries.

Is there good resource/documentation for the proc//maps utility/file?

What does the anonymous inode 0 entries mean? These seem to be some of the larger memory segments.::


Each row in /proc/$PID/maps describes a region of contiguous virtual memory in a process or thread. Each row has the following fields:

  • address - This is the starting and ending address of the region in the process's address space
  • permissions - This describes how pages in the region can be accessed. There are four different permissions: read, write, execute, and shared. If read/write/execute are disabled, a '-' will appear instead of the 'r'/'w'/'x'. If a region is not shared, it is private, so a 'p' will appear instead of an 's'. If the process attempts to access memory in a way that is not permitted, a segmentation fault is generated. Permissions can be changed using the mprotect system call.
  • offset - If the region was mapped from a file (using mmap), this is the offset in the file where the mapping begins. If the memory was not mapped from a file, it's just 0.
  • device - If the region was mapped from a file, this is the major and minor device number (in hex) where the file lives.
  • inode - If the region was mapped from a file, this is the file number.
  • pathname - If the region was mapped from a file, this is the name of the file. This field is blank for anonymous mapped regions. There are also special regions with names like [heap][stack], or[vdso][vdso] stands for virtual dynamic shared object. It's used by system calls to switch to kernel mode. Here's a good article about it.

You might notice a lot of anonymous regions. These are usually created by mmap but are not attached to any file. They are used for a lot of miscellaneous things like shared memory or buffers not allocated on the heap. For instance, I think the pthread library uses anonymous mapped regions as stacks for new threads.

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Yes the pthreads are allocating 8Mb blocks for a stack with a smaller section for stack overflow detection(i think). So each pthread created, by default, allocates an inode 0 memory region of 8Mb and a inode 0 region of 4Kb. –  simon  Sep 10 '09 at 17:13