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Unformatted text preview: Chapter 10. System Calls Operating systems offer processes running in User Mode a set of interfaces to interact with hardware devices such as the CPU, disks, and printers. Putting an extra layer between the application and the hardware has several advantages. First, it makes programming easier by freeing users from studying low-level programming characteristics of hardware devices. Second, it greatly increases system security, because the kernel can check the accuracy of the request at the interface level before attempting to satisfy it. Last but not least, these interfaces make programs more portable, because they can be compiled and executed correctly on every kernel that offers the same set of interfaces. Unix systems implement most interfaces between User Mode processes and hardware devices by means of system calls issued to the kernel. This chapter examines in detail how Linux implements system calls that User Mode programs issue to the kernel. 10.1. POSIX APIs and System Calls Let's start by stressing the difference between an application programmer interface (API) and a system call. The former is a function definition that specifies how to obtain a given service, while the latter is an explicit request to the kernel made via a software interrupt. Unix systems include several libraries of functions that provide APIs to programmers. Some of the APIs defined by the libc standard C library refer to wrapper routines (routines whose only purpose is to issue a system call). Usually, each system call has a corresponding wrapper routine, which defines the API that application programs should employ. The converse is not true, by the wayan API does not necessarily correspond to a specific system call. First of all, the API could offer its services directly in User Mode. (For something abstract such as math functions, there may be no reason to make system calls.) Second, a single API function could make several system calls. Moreover, several API functions could make the same system call, but wrap extra functionality around it. For instance, in Linux, the malloc( ) , calloc( ) , and free( ) APIs are implemented in the libc library. The code in this library keeps track of the allocation and deallocation requests and uses the brk( ) system call to enlarge or shrink the process heap (see the section " Managing the Heap " in Chapter 9 ). The POSIX standard refers to APIs and not to system calls. A system can be certified as POSIX-compliant if it offers the proper set of APIs to the application programs, no matter how the corresponding functions are implemented. As a matter of fact, several non-Unix systems have been certified as POSIX-compliant, because they offer all traditional Unix services in User Mode libraries....
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- Spring '12