What is ELF? ELF (Executable and Linking Format) is file formatthat defines how an object file is composed and organized. With thisinformation, your kernel and the binary loader know how to loadthe file, where to look for the code, where to look the initializeddata, which shared library that needs to be loaded and so on.First of all, you should know about different kind of ELF object:
Relocatable file: an object file that holds code and data suitable for linking with other object files to create an executable or a shared object file. In other word, you can say that relocatable file is a foundation for creating executables and libraries.
This is kind of file you get if you compile a source code like this:
$ gcc -c test.c
That will produce test.o, which is a relocatable file.
Kernel module (either suffixed with .o or .ko) is also a form of relocatable file.
Executable file: object file that holds a program suitable for execution. Yes, that means, your XMMS mp3 player, your vcd software player, even your text editor are all ELF executable files.
This is also a familiar file if you compile a program:
$ gcc -o test test.c
After you make sure the executable bit of "test" is enabled, you can execute it. The question is, what about shell script? Shell script is NOT ELF executable, but the interpreter IS.
Shared object file: This file holds code and data suitable for linking in two contexts:
The link editor may process it with other relocatable and shared shared object file to create another object file.
The dynamic linker combines it with an executable file and other shared objects to create a process image.
Is there any other way to detect the ELF type? Yes there is. In every ELF object, there is a file header that explains what kind file it is. Assuming you have installed binutils package, you can use readelf to read this header. For example (command results are shortened to show related fields only):
$ readelf -h /bin/ls
Type: EXEC (Executable file)
$ readelf -h /usr/lib/crt1.o
Type: REL (Relocatable file)
$ readelf -h /lib/libc-2.3.2.so
Type: DYN (Shared object file)
Lets study ELF in next section
Relocatable file: an object file that holds code and data suitable for linking with other object files to create an executable or a shared object file. In other word, you can say that relocatable file is a foundation for creating executables and libraries.
This is kind of file you get if you compile a source code like this:
$ gcc -c test.c
That will produce test.o, which is a relocatable file.
Kernel module (either suffixed with .o or .ko) is also a form of relocatable file.
Executable file: object file that holds a program suitable for execution. Yes, that means, your XMMS mp3 player, your vcd software player, even your text editor are all ELF executable files.
This is also a familiar file if you compile a program:
$ gcc -o test test.c
After you make sure the executable bit of "test" is enabled, you can execute it. The question is, what about shell script? Shell script is NOT ELF executable, but the interpreter IS.
Shared object file: This file holds code and data suitable for linking in two contexts:
The link editor may process it with other relocatable and shared shared object file to create another object file.
The dynamic linker combines it with an executable file and other shared objects to create a process image.
Is there any other way to detect the ELF type? Yes there is. In every ELF object, there is a file header that explains what kind file it is. Assuming you have installed binutils package, you can use readelf to read this header. For example (command results are shortened to show related fields only):
$ readelf -h /bin/ls
Type: EXEC (Executable file)
$ readelf -h /usr/lib/crt1.o
Type: REL (Relocatable file)
$ readelf -h /lib/libc-2.3.2.so
Type: DYN (Shared object file)
Lets study ELF in next section
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