Why is C/C++ considered low-level languages??? Post: 302622637

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Top Forums Programming Why is C/C++ considered low-level languages??? Post 302622637 by Corona688 on Thursday 12th of April 2012 11:26:50 AM

04-12-2012

Registered User

fopen, fclose, et al are function calls -- they jump your program to a location in memory and execute the code there. If you ran the program in a debugger, you could potentially trace inside these functions and see what they do. They are built for reasons of convenience and portability -- you could build a function which works the same everywhere, for instance, even when the system calls might be slightly different, or add simple functionality like buffers, which is indeed what stdio calls like fwrite are for. It's faster to call putc() 10,000 times to write single chars than to call write() 10,000 times for single chars because putc will just dump it in memory for later.

They certainly can't replace system calls. To write to a file, fwrite() must ultimately call write().

System calls on the other hand are not libraries. They don't jump to the start of an instruction in memory, they pass a message to the operating system, then wait. If you try and trace inside a system call, there's nothing to trace, because your program literally stops running while the system call happens -- the action happens inside the kernel itself, where you can't see.

C is able to freely use raw system calls because it can understand the same data structures the kernel uses for system calls -- the kernel is also C. It compiles it down into raw assembly language like everything else; it becomes the setting of a few registers then something like INT 0x80 to make a software interrupt to inform the kernel you want a system call.

This is difficult for other languages to do natively. Surely they can do INT 0x80, but without the C language itself, it's very difficult to get the data structures right. I've seen some people hardcode system calls in perl and have their code stop working when they move it to a different system, because the data structures changed, but their code didn't. In C, you'd just use the native data structures for wherever you were, and your code wouldn't need to change.

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LEARN ABOUT SUNOS

kmem

mem(7D) 							      Devices								   mem(7D)

NAME

       mem, kmem, allkmem - physical or virtual memory access

SYNOPSIS

       /dev/mem

       /dev/kmem

       /dev/allkmem

DESCRIPTION

       The file /dev/mem is a special file that provides access to the physical memory of the computer.

       The  file  /dev/kmem  is  a special file that provides access to the virtual address space of the operating system kernel, excluding memory
       that is associated with an I/O device.

       The file /dev/allkmem is a special file that provides access to the virtual address space of the operating system kernel, including  memory
       that is associated with an I/O device.  You can use any of these devices to examine and modify the system.

       Byte  addresses	in  /dev/mem are interpreted as physical memory addresses. Byte addresses in /dev/kmem and /dev/allkmem are interpreted as
       kernel virtual memory addresses.  A reference to a non-existent location returns an error. See ERRORS for more information.

       The file /dev/mem accesses physical memory; the size of the file is equal to the amount of physical memory in the computer.  This size  may
       be  larger  than 4GB on a system running the 32-bit operating environment. In this case, you can access memory beyond 4GB using a series of
       read(2) and write(2) calls, a pread64() or pwrite64() call, or a combination of llseek(2) and read(2) or write(2).

ERRORS

       EFAULT	       Occurs when trying to  write(2) a read-only location (allkmem),	read(2) a write-only location  (allkmem),  or  read(2)	or
		       write(2) a non-existent or unimplemented location (mem, kmem, allkmem).

       EIO	       Occurs  when trying to read(2) or write(2) a memory location that is associated with an I/O device using the /dev/kmem spe-
		       cial file.

       ENXIO	       Results from attempting to mmap(2) a non-existent physical (mem) or virtual (kmem, allkmem) memory address.

FILES

       /dev/mem        Provides access to the computer's physical memory.

       /dev/kmem       Provides access to the virtual address space of the operating system kernel, excluding memory that is  associated  with	an
		       I/O device.

       /dev/allkmem    Provides  access  to  the virtual address space of the operating system kernel, including memory that is associated with an
		       I/O device.

SEE ALSO

       llseek(2), mmap(2), read(2), write(2)

WARNINGS

       Using these devices to modify (that is, write to) the address space of a live running operating system or to modify  the  state	of	 a
       hardware  device  is  extremely	dangerous  and	may  result  in a system panic if kernel data structures are damaged or if device state is
       changed.

SunOS 5.10							    18 Feb 2002 							   mem(7D)