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intro, _syscall - Introduction to system calls
This chapter describes the Linux system calls. For a list of the 164
syscalls present in Linux 2.0, see syscalls(2).
In most cases, it is unnecessary to invoke a system call directly, but
there are times when the Standard C library does not implement a nice
function call for you.
A _syscall macro
desired system call
The important thing to know about a system call is its prototype. You
need to know how many arguments, their types, and the function return
type. There are six macros that make the actual call into the system
easier. They have the form:
where X is 0–5, which are the number of arguments taken
by the system call
type is the return type of the system call
name is the name of the system call
typeN is the Nth argument’s type
argN is the name of the Nth argument
These macros create a function called name with the arguments you
specify. Once you include the _syscall() in your source file, you call
the system call by name.
#include <linux/unistd.h> /* for _syscallX macros/related stuff */
#include <linux/kernel.h> /* for struct sysinfo */
_syscall1(int, sysinfo, struct sysinfo *, info);
/* Note: if you copy directly from the nroff source, remember to
REMOVE the extra backslashes in the printf statement. */
struct sysinfo s_info;
error = sysinfo(&s_info);
printf("code error = %d\n", error);
printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
"RAM: total %lu / free %lu / shared %lu\n"
"Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
"Number of processes = %d\n",
code error = 0
uptime = 502034s
Load: 1 min 13376 / 5 min 5504 / 15 min 1152
RAM: total 15343616 / free 827392 / shared 8237056
Memory in buffers = 5066752
Swap: total 27881472 / free 24698880
Number of processes = 40
The _syscall() macros DO NOT produce a prototype. You may have to
create one, especially for C++ users.
System calls are not required to return only positive or negative error
codes. You need to read the source to be sure how it will return
errors. Usually, it is the negative of a standard error code, e.g.,
-EPERM. The _syscall() macros will return the result r of the system
call when r is nonnegative, but will return -1 and set the variable
errno to -r when r is negative. For the error codes, see errno(3).
Some system calls, such as mmap(), require more than five arguments.
These are handled by pushing the arguments on the stack and passing a
pointer to the block of arguments.
When defining a system call, the argument types MUST be passed by-value
or by-pointer (for aggregates like structs).
The preferred way to invoke system calls that glibc does not know about
yet, is via syscall(2).
Certain codes are used to indicate Unix variants and standards to which
calls in the section conform. These are:
SVr4 System V Release 4 Unix, as described in the "Programmer’s
Reference Manual: Operating System API (Intel processors)"
(Prentice-Hall 1992, ISBN 0-13-951294-2)
SVID System V Interface Definition, as described in "The System V
Interface Definition, Fourth Edition".
IEEE 1003.1-1990 part 1, aka ISO/IEC 9945-1:1990s, aka "IEEE
Portable Operating System Interface for Computing Environments",
as elucidated in Donald Lewine’s "POSIX Programmer’s Guide"
(O’Reilly & Associates, Inc., 1991, ISBN 0-937175-73-0.
IEEE Std 1003.1b-1993 (POSIX.1b standard) describing real-time
facilities for portable operating systems, aka ISO/IEC
9945-1:1996, as elucidated in "Programming for the real world -
POSIX.4" by Bill O. Gallmeister (O’Reilly & Associates, Inc.
Single Unix Specification. (Developed by X/Open and The Open
Group. See also http://www.UNIX-systems.org/version2/ .)
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD was
upward-compatible from 4.3.
V7 Version 7, the ancestral Unix from Bell Labs.