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How does shared library work in NEEDED section#28952

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JinhangZhang asked this question inQ&A
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Hi,

I have two questions regarding to the shared library stated in the NEEDED section for a .so file.
For example

# readelf --dynamic /usr/lib64/libssl.soDynamic section at offset 0xdfbb0 contains 31 entries:  Tag        Type                         Name/Value 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.3] 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6] 0x000000000000000e (SONAME)             Library soname: [libssl.so.3]...

libssl.so needed a shared library named libcrypto.so.3, my first question is: Im not sure is that libcrypto.so.3 a SONAME or a filename? If it is a SONAME how does the libssl.so know where to find it?

Another question

# readelf --dynamic /usr/lib64/libcrypto.soDynamic section at offset 0x5117a0 contains 31 entries:  Tag        Type                         Name/Value 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1] 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6] 0x000000000000000e (SONAME)             Library soname: [libcrypto.so.3]... # readelf --dynamic /usr/lib64/libcrypto.so.3Dynamic section at offset 0x5117a0 contains 31 entries:  Tag        Type                         Name/Value 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1] 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6] 0x000000000000000e (SONAME)             Library soname: [libcrypto.so.3]...

I can see both libcrypto.so and libcrypto.so.3 have the same SONAME, how does other file knows which one should be called?

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Replies: 1 comment 2 replies

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If you want to read all the gory details, theman-page for ld.so is the place to look.

In short, theNEEDED tag gives an SONAME, which is also taken as a file name, so whatever the value is, is whatld.so will search for.

File names likelibssl.so are mainly there forbuilding, and is usually a symbolic link to the appropriate SONAME named file. For example, on my laptop:

ls -l /usr/lib/x86_64-linux-gnu/lib{crypto,ssl}.*-rw-r--r-- 1 root root 11945002 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libcrypto.alrwxrwxrwx 1 root root       14 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libcrypto.so -> libcrypto.so.3-rw-r--r-- 1 root root  6373952 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libcrypto.so.3-rw-r--r-- 1 root root  2095014 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libssl.alrwxrwxrwx 1 root root       11 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libssl.so -> libssl.so.3-rw-r--r-- 1 root root  1101760 sep 30 21:54 /usr/lib/x86_64-linux-gnu/libssl.so.3
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2 replies
@JinhangZhang
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Hi@levitte , thanks for your reply.

In short, the NEEDED tag gives an SONAME, which is also taken as a file name, so whatever the value is, is what ld.so will search for.

Im wondering a special case that:

  1. there is a shared library file named libcrypto.so.3, whose SONAME is libcrypto.so
  2. there is another shared library file named libcrypto.so whose SONAME is libcrypto.so.3

which one is NEEDED for libssl.so?

@levitte
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  1. there is a shared library file named libcrypto.so.3, whose SONAME is libcrypto.so

That sounds wrong. Its SONAME should be libcrypto.so.3

Have you checked whether any of those files are symbolic links?

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