Loader in C/C++

The loader is the program of the operating system which loads the executable from the disk into the primary memory(RAM) for execution. It allocates the memory space to the executable module in the main memory and then transfers control to the beginning instruction of the program. 

The loader is an important part of the system, as it loads the program, places them in the memory, and prepares them for the final execution. Loading a program includes reading the content of the executable file that has program instructions from the memory, then it performs some other preparatory task to execute the task. 

Example: 

Compiling by xlC Compiler

akash @aix(/ u / akash) #xlC – o ak1.out./ ak1.cpp akash @aix(/ u / akash) #ls – lrt ak1 * -rw – rw – r– 1 akash dev 74 Nov 12 06 : 10 ak1.cpp – rwxrwxr – x 1 akash dev 8562 Nov 12 06 : 34 ak1.out akash @aix(/ u / akash) #

What Really Happens While Running the Executable

The strace command could be used for loading the executable file from RAM for the execution.

akash@aix(/u/akash)# truss ./ak1.out 

execve(“./ak1.out”, 0x2FF20A00, 0x200138A8) argc: 1 

read_sysconfig(0xF06F8278, 0x00000010, 0xFFFFFFF9, 0x10000000, 0x200007BC, 0x000000C0, 0x06010000, 0xF076A0F0) = 0x00000000 

sbrk(0x00000000) = 0x20000998 

vmgetinfo(0x2FF20350, 7, 16) = 0 

sbrk(0x00000000) = 0x20000998 

sbrk(0x00000008) = 0x20000998

 __libc_sbrk(0x00000000) = 0x200009A0 

loadquery(2, 0x200009C8, 0x00001000) = 0 

__loadx(0x0A040000, 0xF06F599C, 0x00000000, 0xF05BE208, 0x20001D20) = 0xF05BFD64 

loadbind(0, 0xF0760BBC, 0xF06D0E54) = 0 

kfcntl(0, F_GETFL, 0x00000000) = 67110914 

kfcntl(1, F_GETFL, 0x00000000) = 67110914 

kfcntl(2, F_GETFL, 0x00000000) = 67110914 

kfcntl(2, F_GETFL, 0x00000000) = 67110914 

kioctl(1, 22528, 0x00000000, 0x00000000) = 0 

Testing of Loader !kwrite(1, ” T e s t i n g o f L”.., 19) = 19 

kfcntl(1, F_GETFL, 0x00000070) = 67110914 

kfcntl(2, F_GETFL, 0x2FF22FFC) = 67110914 

_exit(0)

The loader is actually the first call that is displayed as ‘execve()‘. The loader is responsible for loading programs and libraries when the program started its execution. This loader loads the process that involves:

• Reading the file and creating an address space for the process.
• Page table entries for the instructions, data, and program stack are created and the register set is initialized.
• Then, Executes a jump instruction to the first instruction of the program which generally causes a page fault and the first page of your instructions is brought into memory.

Below are two points that are not related to the loader and are for just more information:

• Another thing we got is the kwrite call with the argument value which one passed to the printf function in our program. kwrite is a system call that actually gets called from the printf function with the value passed to it and this function is responsible to display the value to the console with the value passed to it.
• We also got the _exit(0) call at the last instruction which is the _exit system call with argument status as 0 which signifies to return back to the operating system with the successful signal. This _exit got called from the return(0) statement.