Understanding the Linux Kernel

8.3. Wrapper Routines

Although system calls are mainly used by User Mode processes, they can also be invoked by kernel threads, which cannot make use of library functions. In order to simplify the declarations of the corresponding wrapper routines, Linux defines a set of six macros called _syscall0 through _syscall5.

The numbers through 5 in the name of each macro correspond to the number of parameters used by the system call (excluding the system call number). The macros may also be used to simplify the declarations of the wrapper routines in the libc standard library; however, they cannot be used to define wrapper routines for system calls having more than five parameters (excluding the system call number) or for system calls that yield nonstandard return values.

Each macro requires exactly 2+2xn parameters, with n being the number of parameters of the system call. The first two parameters specify the return type and the name of the system call; each additional pair of parameters specifies the type and the name of the corresponding system call parameter. Thus, for instance, the wrapper routine of the fork( ) system call may be generated by:

_syscall0(int,fork)

while the wrapper routine of the write( ) system call may be generated by:

_syscall3(int,write,int,fd,const char *,buf,unsigned int,count)

In the latter case, the macro yields the following code:

int write(int fd,const char * buf,unsigned int count) { long __res; asm("int $0x80" : "=a" (__res) : "0" (__NR_write), "b" ...