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Linux kernel and C library user-space interface documentation


_syscall - invoking a system call without library support (OBSOLETE)


#include <linux/unistd.h>
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 seven macros that make the actual call into the system easier. They have the form:
X is 0-6, 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.




The use of these macros is Linux-specific, and deprecated.


Starting around kernel 2.6.18, the _syscall macros were removed from header files supplied to user space. Use syscall(2) instead. (Some architectures, notably ia64, never provided the _syscall macros; on those architectures, syscall(2) was always required.)
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, for example, -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).
When defining a system call, the argument types must be passed by-value or by-pointer (for aggregates like structs).


#include <stdio.h> #include <stdlib.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);
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,

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


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