So you want us to Google for you

I've used a debugger to step through some of the printf() code on MS-Windows and found that eventually it calls win32 api WriteFile(). From that experience I would imagine other functions also eventually call win32 api functions. But those are the details that are compiler-implementation specific. There is nothing in the c or c++ standards that tell compiler writers how to implement the standard functions; they are free to do it any way they want to.

( confused here on how the mapping is done between read ,write and scanf ,printf. because read/write takes file descriptors, buffers and size of data where as printf and scanf takes the variable lenght args: format strings and arguments)

similarly all the dma functions(malloc,calloc, realloc, memset,memcpy, memmove) also uses some built in system calls.
i just want to know is there any source that describes the library functions and whose corresponding system calls on different OS's.