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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 82
SEC. 1.6
SYSTEM CALLS
51
mechanics of issuing a system call are highly machine dependent and often must
be expressed in assembly code, a procedure library is provided to make it possible
to make system calls from C programs and often from other languages as well.
It is useful to keep the following in mind. Any single-CPU computer can ex-
ecute only one instruction at a time.
If a process is running a user program in user
mode and needs a system service, such as reading data from a file, it has to execute
a trap instruction to transfer control to the operating system. The operating system
then figures out what the calling process wants by inspecting the parameters. Then
it carries out the system call and returns control to the instruction following the
system call.
In a sense, making a system call is like making a special kind of pro-
cedure call, only system calls enter the kernel and procedure calls do not.
To make the system-call mechanism clearer, let us take a quick look at the
read
system call.
As mentioned above, it has three parameters: the first one specifying
the file, the second one pointing to the buffer, and the third one giving the number
of bytes to read. Like nearly all system calls, it is invoked from C programs by cal-
ling a library procedure with the same name as the system call:
read
.
A call from a
C program might look like this:
count = read(fd, buffer, nbytes);
The system call (and the library procedure) return the number of bytes actually
read in
count
.
This value is normally the same as
nbytes
, but may be smaller, if,
for example, end-of-file is encountered while reading.
If the system call cannot be carried out owing to an invalid parameter or a disk
error,
count
is set to
−
1, and the error number is put in a global variable,
errno
.
Programs should always check the results of a system call to see if an error oc-
curred.
System calls are performed in a series of steps.
To make this concept clearer,
let us examine the
read
call discussed above.
In preparation for calling the
read
li-
brary procedure, which actually makes the
read
system call, the calling program
first pushes the parameters onto the stack, as shown in steps 1–3 in Fig. 1-17.
C and C++ compilers push the parameters onto the stack in reverse order for
historical reasons (having to do with making the first parameter to
printf
, the for-
mat string, appear on top of the stack).
The first and third parameters are called by
value, but the second parameter is passed by reference, meaning that the address of
the buffer (indicated by &) is passed, not the contents of the buffer. Then comes the
actual call to the library procedure (step 4).
This instruction is the normal proce-
dure-call instruction used to call all procedures.
The library procedure, possibly written in assembly language, typically puts
the system-call number in a place where the operating system expects it, such as a
register (step 5).
Then it executes a
TRAP
instruction to switch from user mode to
kernel mode and start execution at a fixed address within the kernel (step 6).
The
TRAP
instruction is actually fairly similar to the procedure-call instruction in the
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