What is the "shared:" column in the output of pmap -d command?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux

Issue

  • What is the shared: column in the output of pmap -d command?
  • shared: column in the output of pmap -d command is lesser than the total shared library size.

Resolution

Shared in the pmap -d output is calculated by checking the kernel flags of the memory mappings of the process. This is the combined size of data pages which are shared between processes and not the size of total shared libraries. For example, following segments would be counted under shared as they does have s flag set.

00007fe9cab19000 12 rw-s- 0000000000000000 000:00009 [ anon ]
00007fe9cab1c000 12 rw-s- 0000000000000000 000:00009 [ anon ]
00007fe9cab1f000 12 rw-s- 0000000000000000 000:00009 [ anon ]
00007fe9cab22000 12 rw-s- 0000000000000000 000:00009 [ anon ]

Diagnostic Steps

    diff = end-start;
    if(flags[3]=='s') total_shared  += diff;
    if(flags[3]=='p'){
      flags[3] = '-';
      if(flags[1]=='w') total_private_writeable += diff;
      else              total_private_readonly  += diff;
    }

[..]
if(d_option){
      const char *cp = mapping_name(p, start, diff, mapbuf, 0, dev_major, dev_minor, inode);
      printf(
        (sizeof(KLONG)==8)
          ? "%016"KLF"x %7lu %s %016Lx %03x:%05x %s\n"
          :      "%08lx %7lu %s %016Lx %03x:%05x %s\n",
        start,
        (unsigned long)(diff>>10),
        flags,
        file_offset,
        dev_major, dev_minor,
        cp
      );
    }

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