GNU/Linux application programming

chapter 06.Building Static Libraries

we use the ar command to build our library (libmyrand.a). The cru options are a standard set of options for creating or adding to an archive. The c option specifies to create the static library (unless it already exists, in which case the option is ignored). The r option tells ar to replace existing objects in the static library (if they already exist). Finally, the u option is a safety option to specify that objects are replaced in the archive only if the objects to be inserted are newer than existing objects in the archive (of the same name).

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In this example, we use nm to print the symbols within the shared library, but then only emit those with the tag " T " to stdout (those symbols that are part of the .text section, or code segments). We also use the -n option to sort the output numerically by address, rather than the default, which is alphabetically by symbol name. This gives us relative address information within the library; if we wanted to know the specific sizes of these .text sections, we could use the -S option, as:

$ nm -n -S /usr/local/lib/libmyrand.so | grep " T "
00000608 T _init
0000074c 00000036 T initRand
00000784 0000003a T getSRand
000007be 00000050 T getRand
00000844 T _fini
$

From this example, we can see that the initRand is located at relative offset 0x74c in the library and its size is 0¥36 (decimal 54) bytes. Many other options are available; the nm mainpage provides more detail on this.

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Dynamic Library APIs

    #include 
    void *dlopen( const char *filename, int flag );
    const char *dlerror( void );
    void *dlsym( void *handle, char *symbol );
    int dlclose( void *handle );