2.7. Commands when manipulating objects and libraries

This section explains how to use the following commands, which are necessary to manipulate objects and libraries on AIX by providing the following subsections:

• Section 2.7.1, “dump” on page 86

• Section 2.7.2, “genkld” on page 88

• Section 2.7.3, “ldd” on page 90

• Section 2.7.4, “nm” on page 90

• Section 2.7.5, “rtl_enable” on page 91

• Section 2.7.6, “slibclean” on page 91

2.7.1. dump

The dump command is used to examine the header information of executable files and shared objects. The main options that are useful when working with shared libraries are the -H option and the -Tv options.

Displaying header information with dump -H

Use the dump -H command to determine which shared objects an executable or shared object depends on for symbol resolution at run time. The interesting information is in the last section of output, as shown under the ***Import File Strings*** header in Example 2-18.

Example 2-18. Sample dump -H output

$ dump -H func1.so

func1.so:


                        ***Loader Section***
                      Loader Header Information
VERSION#         #SYMtableENT     #RELOCent        LENidSTR
0x00000001       0x00000003       0x00000006       0x00000023

#IMPfilID        OFFidSTR         LENstrTBL        OFFstrTBL
0x00000003       0x000000b0       0x00000000       0x00000000


                        ***Import File Strings***
INDEX  PATH                          BASE                MEMBER
0      /usr/lib:/lib
1                                    libc.a              shr.o
2                                    ..

The number of INDEX entries will depend on how many shared objects the target depends on for symbol resolution.

The INDEX 0 entry is a colon separated list of directories shown as /usr/lib:/lib in Example 2-18. If the LIBPATH environment variable is not set when the executable is started, the directories listed in the first (index 0) entry will be used in order to look for the shared objects listed in subsequent entries. If LIBPATH is defined, the directories listed in it will be used before the directories in the first (index 0) entry.

The format of the interesting columns is as follows:

Displaying symbol definition with dump -Tv

Use the dump -Tv command to examine the symbol information of a shared object or executable. It displays information on the symbols the object is exporting. It also lists the symbols the object or executable will try and import at load time and, if known, the name of the shared object that contains those symbols. The interesting columns are IMEX, IMPid, and Name, as highlighted in Example 2-19.

Example 2-19. Sample dump -Tv output

$ dump -Tv func1.so

func1.so:


                       ***Loader Section***

                       ***Loader Symbol Table Information***
[Index]      Value     Scn     IMEX Sclass   Type           IMPid Name

[0]     0x00000000   undef      IMP     DS EXTref   libc.a(shr.o) printf
[1]     0x00000050   .data      EXP     DS SECdef        [noIMid] func1
[2]     0x00000000   undef      IMP     DS EXTref              .. func3

The values in these columns have the following meaning:

2.7.2. genkld

The genkld command is used to list the shared objects that are loaded in the system shared library segment. The output is quite lengthy and has three columns, the virtual address of the object within the system segment, the size of the object, and the name of the file that was loaded, as shown in Example 2-20.

Example 2-20. genkld output example (1)

$ genkld | pg
Virtual Address              Size File

        d1febce0            19907 /usr/lib/libcurses.a/shr.o
... rest of the output is omitted on purpose ...

If the file name does not have a slash character at its end, then it is a shared archive member of a library archive. In Example 2-20, shr.o is a shared archive member of /usr/lib/libcurses.a.

If the file name has a slash character at its end, it is a shared object file, as shown in Example 2-21.

Example 2-21. genkld output example (2)

$ genkld | head -1; genkld | grep NIS
Virtual Address              Size File
        d00e7000             2237 /usr/lib/security/NIS/
$ ls -l /usr/lib/security/NIS
-r-xr-xr-x   1 root     system         8760 Sep 15 2002  /usr/lib/security/NIS
$ file /usr/lib/security/NIS
/usr/lib/security/NIS:  executable (RISC System/6000) or object module

The virtual address of a 32-bit shared object starts from 0xD, whereas the address of a 64-bit shared object typically starts from 0x90000000, as shown in Example 2-22.

Example 2-22. genkld output example (3)

$ genkld | head -1; genkld | grep 'libc.a'
Virtual Address              Size File
        d01bb3b8              1df /usr/lib/libc.a/dl.o
        d014d0c2              1da /usr/lib/libc.a/pse.o
        d01d0be0           1e5907 /usr/lib/libc.a/shr.o
 90000000022aca0           20d75e /usr/lib/libc.a/shr_64.o

The 0xD segment is the system-wide 32-bit shared text segment, which is shared by all the 32-bit processes on the system. As for 64-bit processes, shared objects can be loaded into the system-wide 64-bit shared text segments, which can be located from 0x9000_0000_0000_0000 to 0x9FFF_FFFF_FFFF_FFFF (576 - 640 PB); however, they are most likely loaded into the first segment in this address range.

For further information about the system-wide shared text segments, see 3.2, “The 32-bit user process model” on page 109 and 3.3, “The 64-bit user process model” on page 130.

2.7.3. ldd

The ldd^[17] command lists the shared objects and archive members that will be loaded to start the executable. The following example shows what shared object modules and shared libraries (dependencies) are required to run the executable file helloworld:

^[17] The ldd command is supported on AIX 5L Version 5.2 and later.

# ldd helloworld

helloworld needs:
         ./libone.so
         ./libtwo.so
         /usr/lib/libc.a(shr.o)
         /usr/lib/librtl.a(shr.o)

The command reports dependencies by traversing valid XCOFF header information of the specified executable file.

2.7.4. nm

The nm command displays information about symbols in the specified file, which can be an object file, an executable file, or shared object. For example:

$ nm libone.so

.myfunc1          T        144
.myfunc1          t        144     40
.myfunc2          T          0
.myfunc2          T        184
.myfunc2          t        184     40
.printf           T        104
.printf           t        104     40
TOC               d         52
_$STATIC          d          0     40
_$STATIC          d         52      4
glink.s           f          -
glink.s           f          -
glink.s           f          -
myfunc1           U          -
myfunc1           d         60      4
myfunc2           D         40     12
myfunc2           d         64      4
printf            U          -
printf            d         56      4
source1.c         f          -

If the file contains no symbol information, the nm command reports the fact, but does not interpret it as an error condition. The nm command reports numerical values in decimal notation by default. Unlike the dump command, nm does not display the shared object or archive member name that is expected to supply the symbol.

2.7.5. rtl_enable

The rtl_enable command is used to convert a shared library that is not enabled for run-time linking into a run-time linking enabled one.

2.7.6. slibclean

The slibclean command can be used by the root user to unload all shared objects with a use count value of zero from the system shared library segment. This command is useful in an environment when shared libraries are under development. You can run the slibclean command followed by the genkld command to ensure that the shared objects under development are not loaded in the system shared library segment. This means that any application started after this will automatically use the latest version of the shared objects since the system loader will search for and load them. It also prevents multiple versions of the same objects existing in the system segment.

During the development of shared objects, you may sometimes see an error message similar to the following when creating a new version of an existing shared object:

# make libone.so
        cc -O -c source1.c
        cc -berok -G -o libone.so source1.o
ld: 0711-851 SEVERE ERROR: Output file: libone.so
        The file is in use and cannot be overwritten.
make: 1254-004 The error code from the last command is 12.

The error message means that the target shared object file is in use and it has been loaded into the system shared library segment. Once loaded, the file is marked as in use, even if the use count is already zero. Running the slibclean command will unload all of the unused shared objects from the system. An alternative (and simpler) method of avoiding this problem is to use the rm -f command to remove the shared object before creating it.

However, frequent slibclean invocation on production systems should be avoided, because it may affect the system performance by unloading frequently used, but unused when the command is issued, shared objects and libraries on the system. It is recommended to issue slibclean on production systems that are in a software maintenance phase, especially before the deinstallation of no longer required applications or the updating of installed applications.