Low-level system calls with the GNU C library

Last issue we used assembly language to access Linux kernel services. Now we’re going to use the C run-time library, glibc, instead of calling the kernel services directly. The glibc functions are in many cases thin wrappers around the Linux kernel services. This is the preferred way to access Linux kernel services.

Kernel system calls are limited to six arguments, but that’s not enough for the C library. We use almost the same six registers that we used for kernel system calls: RDI, RSI, RDX, RCX (instead of R10), R8 and R9, but any number of additional arguments can be passed to C library functions on the stack. We populate the registers listed above with the arguments to the function. We then PUSH the remaining arguments onto the stack and remove them from the stack after the C library function returns. You’ll see this in environment.asm.

When using the kernel system calls we called a common location using the software interrupt instruction SYSCALL and passed the ID of the specific service in the RAX register. When using the C library, we link to and call the specific function we want by name – though RAX still returns success or failure status to the caller.

ARRRGs!

Our next programs are and . When a main function is invoked it has a few parameters that the user types on the command line. If you type ./cmdline alpha beta goldfish at the command prompt, Linux will execute the program cmdline . The program will receive as parameters, argc , which is the total number of string arguments (four in this case) followed by an array of pointers to the strings on the command line which