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) { long _ _res; asm("int $0x80" : "=a" (_ _res) : "0" (_ _NR_write), "b" ((long)fd), "c" ((long)buf), "d" ((long)count)); if ((unsigned long)_ _res >= (unsigned long)-125) { errno = -_ _res; _ _res = -1; } return (int) _ _res; }
The _ _NR_write
macro is derived from the second
parameter of _syscall3
; it expands into the system
call number of write( )
. When compiling the
preceding function, the following assembly language code is produced:
write: pushl %ebx ; push ebx into stack movl 8(%esp), %ebx ; put first parameter in ebx movl 12(%esp), %ecx ; put second parameter in ecx movl 16(%esp), %edx ; put third parameter in edx movl $4, %eax ; put _ _NR_write in eax int $0x80 ; invoke system call cmpl $-126, %eax ; check return code jbe .L1 ; if no error, jump negl %eax ; complement the value of eax movl %eax, errno ; put result in errno movl $-1, %eax ; set eax to -1 .L1: popl %ebx ; pop ebx from stack ret ; return to calling program
Notice how the parameters of the write( )
function
are loaded into the CPU registers before the int
$0x80
instruction is executed. The value returned
in eax
must be interpreted as an error code if it
lies between -1 and -125 (the kernel assumes that the largest error
code defined in include/asm-i386/errno.h is
125). If this is the case, the wrapper routine stores the value of
-eax
in errno
and returns the
value -1; otherwise, it returns the value of
eax
.
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