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Document library types in the intro(3) manual page
Add a paragraph about library types to the intro(3)
manual page. Document library types, locations
and versioning.
Reviewed by: emaste, jilles, mhorne, pauamma_gundo.com
Obtained from: OpenBSD (partial)
Differential Revision: https://reviews.freebsd.org/D36594
(cherry picked from commit 54611b7cc6
)
This commit is contained in:
parent
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@ -26,8 +26,7 @@
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)intro.3 8.1 (Berkeley) 6/5/93
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.\"
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.Dd November 7, 2022
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.Dd November 16, 2023
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.Dt INTRO 3
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.Os
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.Sh NAME
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@ -278,13 +277,105 @@ the math library
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.It Pa /usr/lib/libm_p.a
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the math library compiled for profiling
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.El
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.Sh LIBRARY TYPES
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The system libraries are located in
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.Pa /lib
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and
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.Pa /usr/lib .
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A library has the following naming convention:
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.Bd -unfilled -offset indent
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libc.so.7
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.Ed
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.Pp
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Libraries with an
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.Sq .a
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suffix are static.
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When a program is linked against a static library, all necessary library code
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will be included in the binary.
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This means the binary can be run even when the libraries are unavailable.
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However, it can be inefficient with both disk space and memory usage
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during execution.
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The C compiler,
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.Xr cc 1 ,
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can be instructed to link statically by specifying the
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.Fl static
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flag.
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.Pp
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Libraries with a
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.Sq .so.X
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suffix are dynamic libraries.
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When code is linked dynamically, the library code that the application needs
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is not included in the binary.
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Instead, data structures are added containing information about which dynamic
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libraries to link with.
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When the binary is executed, the run-time linker
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.Xr ld.so 1
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reads these data structures and loads them into the
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process virtual address space.
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.Xr rtld 1
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loads the shared libraries when the program is executed.
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.Pp
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.Sq X
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represents the library version number of the library.
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In the example above, a binary linked with
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.Pa libc.so.8
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would not be usable on a system where only
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.Pa libc.so.7
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is available.
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.Pp
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The advantages of dynamic libraries are that multiple instances of the same
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library can share address space, and the physical size of the binary is
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smaller.
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A namespace per shared library is available via hidden visibility,
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allowing multiple compilation units in a library to share things without
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making them available to other libraries.
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It is possible to load libraries dynamically via
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.Xr dlopen 3 .
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The disadvantage is the added complexity that comes with loading the
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libraries dynamically, and the extra time taken to load the libraries.
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Of course, if the libraries are not available, the binary will be unable
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to execute.
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Calls across shared libraries are also slightly slower and cannot be
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inlined, not even with link time optimization.
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The C compiler,
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.Xr cc 1 ,
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can be instructed to link dynamically by specifying the
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.Fl shared
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flag.
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.Pp
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Shared libraries, as well as static libraries on architectures which produce
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position-independent executables
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.Pq PIEs
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by default, contain position-independent code
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.Pq PIC .
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Normally, compilers produce relocatable code.
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Relocatable code needs to be modified at run-time, depending on where in
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memory it is to be run.
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The C compiler,
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.Xr cc 1 ,
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can be instructed to generate PIC code by specifying the
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.Fl fPIC
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flag.
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.Pp
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Static libraries are generated using the
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.Xr ar 1
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utility.
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The libraries contain an index to the contents of the library,
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stored within the library itself.
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The index lists each symbol defined by a member of a library that is a
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relocatable object file.
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This speeds up linking to the library, and allows routines in the library
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to call each other regardless of their placement within the library.
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.Sh SEE ALSO
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.Xr ar 1 ,
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.Xr cc 1 ,
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.Xr ld 1 ,
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.Xr nm 1 ,
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.Xr intro 2 ,
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.Xr math 3 ,
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.Xr stdio 3
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.Xr stdio 3 ,
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.Xr make.conf 5 ,
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.Xr src.conf 5
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.Sh HISTORY
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An
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.Nm
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