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.
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 six macros that make the actual call into the system easier. They have the form:
where X is 0-5, 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
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 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.
Certain codes are used to indicate Unix variants and standards to which calls in the section conform. These are:
System V Release 4 Unix, as described in the
System V Interface Definition, as described in
IEEE 1003.1-1990 part 1, aka ISO/IEC 9945-1:1990s, aka
IEEE Std 1003.1b-1993 (POSIX.1b standard) describing real-time facilities for portable operating systems, aka ISO/IEC 9945-1:1996, as elucidated in
Single Unix Specification. (Developed by X/Open and The Open Group. See also http://www.UNIX-systems.org/version2/ .)
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD was upward-compatible from 4.3.