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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.
In this case, the programmer must manually invoke the
system call using either one of the _syscall macros, or
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:
These macros create a function called Certain codes are used to indicate Unix variants and standards to which calls in the section conform. See standards(7). 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 Some system calls, such as mmap(2), 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). However,
this mechanism can only be used if using a libc (such as
glibc) that supports syscall(2), and if the
Some architectures, notably ia64, do not provide the _syscall macros. On these architectures, syscall(2) must be used.
#include <stdio.h>
#include <errno.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)
{
struct sysinfo s_info;
int error;
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",
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);
exit(EXIT_SUCCESS);
}
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