Low level system call [closed]

Question

C provides library functions such as write(),read().. for system calls.How to make a system call without using any library in linux in C ?

Answer 1

Ambition or a overly severe urge for purity can lead you to do in-line assembly. For example, on x86_64 systems, you can do an open(2) system call like this:

#include <sys/syscall.h>
int
linux_open(const char *pathname, unsigned long flags, unsigned long mode)
{
    long ret;
    asm volatile ("syscall" : "=a" (ret) : "a" (__NR_open),
              "D" (pathname), "S" (flags), "d" (mode) :
              "cc", "memory", "rcx",
              "r8", "r9", "r10", "r11" );
    if (ret < 0)
    {
        errno = -ret;
        ret = -1;
    }
    return (int) ret;
}

You can look at the source of more comprehensible libc's (like Musl) to find out how system calls get implemented, too.

Answer 2

In order to do system calls you normally have to execute some functions on the CPU that are not part of the C language specification. The system calls are either written in assembly for the CPU, and linked against, or some CPU specific inline assembly within a C function is used.

Within the Linux kernel there are various macros defined to support this, because all system calls require a very similar setup. The result is that it is often quite unclear where the CPU specific assembler is actually coming from, but if you dig deep enough, you will get at it.