Creating and Using Libraries

Caution When Mixing Shared and Archive Libraries

 

Example 3: Hidden Definitions

 

This example shows how mixing archive libraries and shared libraries

 

can lead to multiple definitions in the application and unexpected

 

results. If one of the definitions happens to be a data symbol, the results

 

can be catastrophic. If any of the definitions are code symbols, different

 

versions of the same routine could end up being used in the application.

 

This could lead to incompatibilities.

 

Duplicate definitions can occur when a dependent shared library is

 

updated to refer to a symbol contained in the program file but not visible

 

to the shared library. The new symbol import must be satisfied somehow

 

by either adding the symbol to the library or by updating the shared

 

library dependency list. Otherwise the application must be relinked.

 

Using an archive version of libc in an application using shared libraries

 

is the most common cause of duplicate definitions. Remember that

 

symbols not referenced by a shared library at link time will not be

 

exported by default.

 

 

Duplicate definitions can be avoided if any or all symbols that may be

NOTE

 

referenced by a shared library are exported from the application at link

 

time. Shared libraries always reference the first occurrence of a

 

definition. In the following example the first definition is in the

 

executable file, a.out. See the -Eoption and +e symbol option described

 

in ld(1) and “Exporting Symbols from main with -E” on page 81,

 

“Exporting Symbols with +ee” on page 81, and “Exporting Symbols with

 

+e” on page 79.

 

 

The following example illustrates this situation. Suppose you have a

 

 

main program, main(), and three functions, f1(), f2(), and f3() each

 

in a separate source file. main() calls f1(), f2(), and f3().

 

$ cc -c main.c

Compile to relocatable code.

 

$ cc -c +z f1.c f2.c f3.c

Compile to position-independent code.

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Kenwood HP 9000 manual Example 3 Hidden Definitions
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