_syscall - invoking a system call without library support (OBSOLETE)
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
- 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)
architectures, notably ia64, never provided the _syscall macros; on those
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
_syscall() macros will return the result r
of the system call when
is nonnegative, but will return -1 and set the variable errno
is negative. For the error codes, see
When defining a system call, the argument types must
be passed by-value
or by-pointer (for aggregates like structs).
#include <linux/unistd.h> /* for _syscallX macros/related stuff */
#include <linux/kernel.h> /* for struct sysinfo */
_syscall1(int, sysinfo, struct sysinfo *, info);
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
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