INTRO(2) Linux Programmer's Manual INTRO(2)
NAME
intro - Introduction to system calls
DESCRIPTION
This chapter describes the Linux system calls.
Calling Directly
In most cases, it is unnecessary to invoke a system call
directly, but there are times where the Standard C library
does not implement a nice function call for you.
Synopsis
#include <linux/unistd.h>
A _syscall macro
desired system call
Setup
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:
_syscallX(type,name,type1,arg1,type2,arg2,...)
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 argu-
ments you specify. Once you include the _syscall() in
your source file, you call the system call by name.
EXAMPLE
#include <stdio.h>
#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. */
int main(void)
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INTRO(2) Linux Programmer's Manual INTRO(2)
{
struct sysinfo s_info;
int error;
error = sysinfo(&s_info);
printf("code error = %d\n", error);
printf("Uptime = %ds\nLoad: 1 min %d / 5 min %d / 15 min %d\n"
"RAM: total %d / free %d / shared %d\n"
"Memory in buffers = %d\nSwap: total %d / free %d\n"
"Number of processes = %d\n",
s_info.uptime, s_info.loads[0],
s_info.loads[1], s_info.loads[2],
s_info.totalram, s_info.freeram,
s_info.sharedram, s_info.bufferram,
s_info.totalswap, s_info.freeswap,
s_info.procs);
return(0);
}
Sample Output
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
NOTES
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 nega-
tive 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.
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).
FILES
/usr/include/linux/unistd.h
AUTHORS
Look at the header of the manual page for the author(s)
and copyright conditions. Note that these can be
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different from page to page!
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