What is the 'soname' option for building shared libraries for?

I learned the "Program Library HOWTO". It mention that using soname to manage the version like follow.

gcc -shared -fPIC -Wl,-soname,libfoo.so.1  -o libfoo.so.1.0.0 foo.c
ln -s libfoo.so.1.0.0  libfoo.so.1
ln -s libfoo.so.1 libfoo.so

And I get the information that if the soname is not set. it will be equal to libfoo.so.1.0.0 ,see the answer from here.

And I find that it also can work without soname , like following

 gcc -shared -fPIC -o libfoo.so.1.0.0 foo.c
 ln -s libfoo.so.1.0.0  libfoo.so.1
 ln -s libfoo.so.1 libfoo.so

So I think that the only one useful point is that the soname option can tell you the version of the shared library when you use readelf -d libfoo.so command to check it.

What else can it do?

soname is used to indicate what binary api compatibility your library support.

SONAME is used at compilation time by linker to determine from the library file what actual target library version. gcc -lNAME will seek for libNAME.so link or file then capture its SONAME that will certainly be more specific ( ex libnuke.so links to libnuke.so.0.1.4 that contains SONAME libnuke.so.0 ).

At run time it will link with this is then set into ELF dynamic section NEEDED, then a library with this name ( or a link to it ) should exists. At Run time SONAME is disregarded, so only the link or the file existence is enough.

Remark: SONAME is enforced only at link/build time and not at run time.

'SONAME' of library can be seen with 'objdump -p file |grep SONAME'. 'NEEDED' of binaries can be seen with 'objdump -p file |grep NEEDED'.

[EDIT] WARNING Following is a general remark, not the one deployed in linux. See at the end.

Let's assume you have a library with libnuke.so.1.2 name and you develop a new libnuke library :

• if your new library is a fix from previous without api change, you should just keep same soname, increase the version of filename. ie file will be libnuke.so.1.2.1 but soname will still be libnuke.so.1.2.
• if you have a new library that only added new function but didn't break functionality and is still compatible with previous you would like to use same soname than previous plus a new suffix like .1. ie file and soname will be libnuke.so.1.2.1. Any program linked with libnuke.1.2 will still work with that one. New programs linked with libnuke.1.2.1 will only work with that one ( until new subversion come like libnuke.1.2.1.1 ).
• if your new library is not compatible with any libnuke : libnuke.so.2
• if your new library is compatible with bare old version : libnuke.so.1.3 [ ie still compatible with libnuke.so.1 ]

[EDIT] to complete : linux case.

In linux real life SONAME as a specific form : lib[NAME][API-VERSION].so.[major-version] major-version is only one integer value that increase at each major library change. API-VERSION is empty by default

ex libnuke.so.0

Then real filename include minor versions and subversions ex : libnuke.so.0.1.5

I think that not providing a soname is a bad practice since renaming of file will change its behavior.

You created a dynamic library named libx.1.0.0 in naming tradition libname.{a}.{b}.{c}

{a} stand for primary version, should changes when APIs changes(which making things incompatible).
{b} stand for sub version, should changes by adding APIs.
{c} stand for mirror version, should changes by bug fixing or optimizing

Now you are releasing libx.1.2.0, and you need to declare that libx.1.2.0 is compatible with libx.1.0.0 since just adding functions and people's executable would not crashes, just link it like in the old time by:

Setting libx.1.0.0 and libx.1.2.0 to have the same soname, for example libx.1

This's what soname does.

Here is an example supporting Johann Klasek's answer.

In short, SONAME is needed at run time. At compilation time, only a linker name or real name is needed (e.g. g++ main.cpp -L. -ladd or g++ main.cpp -L. -l:libadd.so.1.1). The definitons of linker name and real name follow Program Library HOWTO: 3. Shared Libraries.

Source tree:

├── add.cpp
├── add.h
├── main.cpp
└── Makefile

Makefile:

SOURCE_FILE=add.cpp
# main.cpp includes `add.h`, whose implementation is `add.cpp`
MAIN_FILE=main.cpp
SONAME=libadd.so.1
REAL_NAME=libadd.so.1.1
LINKER_NAME=libadd.so
OUTPUT_FILE=a.out

all:
   g++ -shared -fPIC -Wl,-soname,${SONAME} -o ${REAL_NAME} ${SOURCE_FILE}
   ln -s ${REAL_NAME} ${LINKER_NAME}
   g++ main.cpp -I. -L. -ladd -o ${OUTPUT_FILE}	
   # Same as `ldconfig -n .`, creates a symbolic link
   ln -s ${REAL_NAME} ${SONAME}
   #./a.out: error while loading shared libraries: libadd.so.1: cannot open 
   # shared object file: No such file or directory
   LD_LIBRARY_PATH=. ./${OUTPUT_FILE}
clean:
   rm ${SONAME} ${REAL_NAME} ${LINKER_NAME} ${OUTPUT_FILE}

Let's assume libA.so depends on libB.so, and they all in a directory(of course the directory cannot be found by dynamic linker). If you didn't set soname then dlopen doesn't work:

auto pB = dlopen("./libB.so", RTLD_LAZY | RTLD_GLOBAL);
auto pA = dlopen("./libA.so", RTLD_LAZY | RTLD_GLOBAL);

Because runtime linker cannot find libB.so, so pA is set to NULL.

In this case soname will save you from hell...

Another aspect: At least on Linux the SONAME entry provides a hint for the runtime-linker system on how to create appropriate links in /lib, /lib64 etc. Running the command ldconfig tries to create a symbolic link named with SONAME which is also taken into the run-time linker cache. The newest one of the libraries tagging the same SONAME wins the link-race. If some software relies on the specific SONAME and you want to renew a library you have to provide this SONAME to get ldconfig stick on this new library (if ldconfig is used to rebuild the cache and the links). E.g. libssl.so.6 and libcrypto.so.6 are such cases.