UNIX环境高级编程：实用指南与源代码

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closedir(dp);

exit(0);

}

Working Directory

Every process has a working directory, sometimes called the current working directory. This is the directory

from which all relative pathnames are interpreted. A process can change its working directory with the

chdir

function.

For example, the relative pathname

doc/memo/joe

refers to the file or directory

joe

, in the directory

memo

, in

the directory

doc

, which must be a directory within the working directory. From looking just at this pathname,

we know that both

doc

and

memo

have to be directories, but we can't tell whether

joe

is a file or a directory. The

pathname

/usr/lib/lint

is an absolute pathname that refers to the file or directory

lint

in the directory

lib

in the directory

usr

, which is in the root directory.

Home Directory

When we log in, the working directory is set to our home directory. Our home directory is obtained from our

entry in the password file (Section 1.3).

1.5. Input and Output

File Descriptors

File descriptors are normally small non-negative integers that the kernel uses to identify the files being accessed

by a particular process. Whenever it opens an existing file or creates a new file, the kernel returns a file

descriptor that we use when we want to read or write the file.

Standard Input, Standard Output, and Standard Error

By convention, all shells open three descriptors whenever a new program is run: standard input, standard output,

and standard error. If nothing special is done, as in the simple command

then all three are connected to the terminal. Most shells provide a way to redirect any or all of these three

descriptors to any file. For example,

ls > file.list

executes the

command with its standard output redirected to the file named

file.list

Unbuffered I/O

Unbuffered I/O is provided by the functions

open

read

, write,

lseek

, and

. These functions all work

with file descriptors.

Example

If we're willing to read from the standard input and write to the standard output, then the program in Figure 1.4

copies any regular file on a UNIX system.

The

<unistd.h>

header, included by

apue.h

, and the two constants

STDIN_FILENO

and

STDOUT_FILENO

are

part of the POSIX standard (about which we'll have a lot more to say in the next chapter). In this header are

function prototypes for many of the UNIX system services, such as the

read

and write functions that we call.

The constants

STDIN_FILENO

and

STDOUT_FILENO

are defined in

<unistd.h>

and specify the file descriptors

for standard input and standard output. These values are typically 0 and 1, respectively, but we'll use the new

names for portability.

In Section 3.9, we'll examine the

BUFFSIZE

constant in detail, seeing how various values affect the efficiency of

the program. Regardless of the value of this constant, however, this program still copies any regular file.

The

read

function returns the number of bytes that are read, and this value is used as the number of bytes to

write. When the end of the input file is encountered,

read

returns 0 and the program stops. If a read error

occurs,

read

returns -1. Most of the system functions return –1 when an error occurs.

If we compile the program into the standard name (

a.out

) and execute it as

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UNIX环境高级编程：实用指南与源代码

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