.\" .\" Copyright (c) 1993 Michael Haardt (u31b3hs@pool.informatik.rwth-aachen.de), .\" Fri Apr 2 11:32:09 MET DST 1993 .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, .\" USA. .\" .\" Tue Jul 6 12:42:46 MDT 1993 (dminer@nyx.cs.du.edu) .\" Added "Calling Directly" and supporting paragraphs .\" .\" Modified Sat Jul 24 15:19:12 1993 by Rik Faith (faith@cs.unc.edu) .\" .\" Modified 21 Aug 1994 by Michael Chastain (mec@shell.portal.com): .\" Added explanation of arg stacking when 6 or more args. .\" .\" Modified 10 June 1995 by Andries Brouwer (aeb@cwi.nl) .\" .TH INTRO 2 "22 May 1996" "Linux 1.2.13" "Linux Programmer's Manual" .SH NAME intro \- Introduction to system calls .SH DESCRIPTION This chapter describes the Linux system calls. .SS "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. .SS "Synopsis" .B #include A _syscall macro desired system call .SS 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: .sp .RS .RI _syscall X ( type , name , type1 , arg1 , type2 , arg2 ,...) .RS .HP where \fIX\fP is 0\(en5, which are the number of arguments taken by the system call .HP \fItype\fP is the return type of the system call .HP \fIname\fP is the name of the system call .HP \fItypeN\fP is the Nth argument's type .HP \fIargN\fP is the name of the Nth argument .RE .RE .sp These macros create a function called \fIname\fP with the arguments you specify. Once you include the _syscall() in your source file, you call the system call by \fIname\fP. .SH EXAMPLE .nf .sp #include #include /* for _syscallX macros/related stuff */ #include /* 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); } .fi .SH "Sample Output" .nf 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 .fi .SH NOTES The _syscall() macros DO NOT produce a prototype. You may have to create one, especially for C++ users. .sp 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., \-\fBEPERM\fP. The _syscall() macros will return the result \fIr\fP of the system call when \fIr\fP is nonnegative, but will return \-1 and set the variable .I errno to \-\fIr\fP when \fIr\fP is negative. .sp Some system calls, such as .BR 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. .sp When defining a system call, the argument types MUST be passed by-value or by-pointer (for aggregates like structs). .SH FILES .I /usr/include/linux/unistd.h .SH AUTHORS Look at the header of the manual page for the author(s) and copyright conditions. Note that these can be different from page to page!