imported>ThorstenStaerk |
imported>ThorstenStaerk |
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| − | = Example =
| + | #REDIRECT [[AuTofs]] |
| − | Let's say you want a library that defines a function help() that outputs "hello world". First create help.cpp
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| − | [[cat]] >help.cpp
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| − | #include <iostream>
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| − |
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| − | extern "C" void help()
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| − | {
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| − | std::cout << "hello world" << '\
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| − | ';
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| − | }
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| − | Then create main.cpp which will be the program calling the function help():
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| − | cat >main.cpp
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| − | #include <iostream>
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| − | #include <dlfcn.h>
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| − |
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| − | extern "C" void help();
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| − |
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| − | int main()
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| − | {
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| − | help();
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| − | return 0;
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| − | }
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| − | Now [[compile]] help.cpp, but do not make it an executable. An attempt to make it an executable will fail because it does not contain a main routine.
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| − | # gcc -c help.cpp
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| − | Verify your library help.o has been created:
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| − | # ls -ltr
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| − | total 12
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| − | -rw-r--r-- 1 root root 87 Dec 31 11:32 help.cpp
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| − | -rw-r--r-- 1 root root 105 Dec 31 11:57 main.cpp
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| − | -rw-r--r-- 1 root root 2712 Dec 31 11:57 help.o
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| − | Create an archive out of help.o. In a real case you would use more than one .o file:
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| − | # ar -r help.a help.o
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| − | # ls -ltr
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| − | total 16
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| − | -rw-r--r-- 1 root root 87 Dec 31 11:32 help.cpp
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| − | -rw-r--r-- 1 root root 105 Dec 31 11:57 main.cpp
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| − | -rw-r--r-- 1 root root 2712 Dec 31 11:57 help.o
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| − | -rw-r--r-- 1 root root 2854 Dec 31 12:00 help.a
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| − | Verify main.cpp has an unresolved [[dependency]] to a function help:
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| − | # g++ main.cpp
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| − | /tmp/ccvIeJnW.o: In function `main':
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| − | main.cpp:(.text+0x5): undefined reference to `help'
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| − | collect2: ld returned 1 exit status
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| − | Put the archive in a place so [[gcc]] can find it:
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| − | # cp help.a /usr/lib/libhelp.a
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| − | [[Compile]] and [[link]] your [[program]]:
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| − | # g++ -o myprogram main.cpp -lhelp
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| − | Test it:
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| − | # ./myprogram
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| − | hello world
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| − | Verify it does NOT have a dependency on help.so or help.o:
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| − | # ldd myprogram
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| − | linux-vdso.so.1 => (0x00007fff34dff000)
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| − | libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007f784ce16000)
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| − | libm.so.6 => /lib64/libm.so.6 (0x00007f784cbbf000)
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| − | libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f784c9a9000)
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| − | libc.so.6 => /lib64/libc.so.6 (0x00007f784c649000)
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| − | /lib64/ld-linux-x86-64.so.2 (0x00007f784d120000)
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| − | #
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| − | Verify it really contains code to output hello world in the executable file:
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| − | # [[strings]] myprogram
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| − | /lib64/ld-linux-x86-64.so.2
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| − | SuSESuSE
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| − | libstdc++.so.6
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| − | __gmon_start__
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| − | _Jv_RegisterClasses
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| − | _ZNSt8ios_base4InitD1Ev
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| − | _ZSt4cout
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| − | _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
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| − | _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_c
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| − | _ZNSt8ios_base4InitC1Ev
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| − | libm.so.6
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| − | libgcc_s.so.1
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| − | libc.so.6
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| − | __cxa_atexit
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| − | __libc_start_main
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| − | GLIBC_2.2.5
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| − | GLIBCXX_3.4
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| − | %z
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| − | %r
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| − | %j
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| − | %b
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| − | %Z
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| − | fff.
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| − | l$ L
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| − | t$(L
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| − | |$0H
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| − | hello world
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| − | Look at its size so you can compare it with the same example that has been linked dynamically:
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| − | # ll myprogram
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| − | -rwxr-xr-x 1 root root 12541 Dec 31 12:03 myprogram
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| − | You see it has a size of 12 541 bytes while the executable file that has been linked dynamically has a size of 12 173 bytes.
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| − | | |
| − | = See also =
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| − | * [[linking]]
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| − | * [[dynamic linking]]
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