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Manpages INTROSection: Linux Programmer's Manual (2)Updated: 1996-05-22 Index Return to Main Contents NAMEintro - Introduction to system callsDESCRIPTIONThis chapter describes the Linux system calls. For a list of the 164 syscalls present in Linux 2.0, see syscalls(2).Calling DirectlyIn 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.Synopsis#include <linux/unistd.h>A _syscall macro desired system call SetupThe 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:
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) { 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 Outputcode 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 NOTESThe _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). CONFORMING TOCertain codes are used to indicate Unix variants and standards to which calls in the section conform. These are:
FILES/usr/include/linux/unistd.hSEE ALSOerrno(3)
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