Kernel Wrapper Routines
Although system calls are used mainly
by User Mode processes, they can also be invoked by kernel threads,
which cannot use library functions. To simplify the declarations of
the corresponding wrapper routines, Linux defines a set of seven
macros called _syscall0
through
_syscall6
.
In the name of each macro, the numbers 0 through 6 correspond to the number of parameters used by the system call (excluding the system call number). The macros are used to declare wrapper routines that are not already included in the libc standard library (for instance, because the Linux system call is not yet supported by the library); however, they cannot be used to define wrapper routines for system calls that have more than six parameters (excluding the system call number) or for system calls that yield nonstandard return values.
Each macro requires exactly 2+2×n
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) { ...
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