TLPI-Chapter 4文件IO

所有执行I/O操作的系统调用都以文件描述符（一个非负整数）来指代打开的文件。文件描述符用以表示所有类型的已打开文件，包括管道、FIFO、socket、终端、设备、普通文件。它是一个索引值，指向内核为每一个进程所维护的该进程打开文件的记录表。当程序打开一个现有文件或者创建一个新文件时，内核向进程返回一个文件描述符。每个进程，文件描述符都自成一套。
三种标准的文件描述符：
0 标准输入 1 标准输出 2 标准错误
然后介绍执行文件IO操作的4个主要系统调用：
fd = open(pathname,flags,mode) 打开或创建一个新文件.

numread = read(fd,buffer,count) 读取fd所指代的文件中之多count字节的数据，并存储到buffer中.

numwritten = write(fd,buffer,count)调用从buffer中读取多达count字节的数据写入由fd指代的已打开的文件中.

status = close(fd)
释放文件描述符fd以及与之相关的内核资源.

改变文件偏移量：lseek()
off_t lseek(int fd, off_t offset, int whence)
offset参数指定了一个以字节为单位的数值
whence参数则表明赢参照哪个基点来解释offset参数，应为下列其中之一：
SEEK_SET：文件头部开始
SEEK_CUR：当前文件偏移量处
SEEK_END：文件结尾

.通用I/O模型以外的操作：ioctl()、fcntl()

ioctl()
ioctl()系统调用又为执行文件和设备操作提供了一种多用途机制。
int ioctl(int fd, int request,…);
request指定了将在fd上执行的控制操作
第三个参数…（argp）可以是任意数据类型，根据request的参数值来确定argp所期望的类型。通常情况，argp指向整数或结构的指针
fcntl()
fcntl()系统调用对一个打开的文件描述符执行一些列控制操作
int fcntl(intn fd, int cmd, …)
cmd参数所支持的操作范围很广
主要代码有两个：

#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"

#ifndef BUF_SIZE        /* Allow "cc -D" to override definition */
#define BUF_SIZE 1024
#endif

int
main(int argc, char *argv[])
{
    int inputFd, outputFd, openFlags;
    mode_t filePerms;
    ssize_t numRead;
    char buf[BUF_SIZE];

    if (argc != 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s old-file new-file\n", argv[0]);

    /* Open input and output files */

    inputFd = open(argv[1], O_RDONLY);
    if (inputFd == -1)
        errExit("opening file %s", argv[1]);

    openFlags = O_CREAT | O_WRONLY | O_TRUNC;
    filePerms = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
                S_IROTH | S_IWOTH;      /* rw-rw-rw- */
    outputFd = open(argv[2], openFlags, filePerms);
    if (outputFd == -1)
        errExit("opening file %s", argv[2]);

    /* Transfer data until we encounter end of input or an error */

    while ((numRead = read(inputFd, buf, BUF_SIZE)) > 0)
        if (write(outputFd, buf, numRead) != numRead)
            fatal("couldn't write whole buffer");
    if (numRead == -1)
        errExit("read");

    if (close(inputFd) == -1)
        errExit("close input");
    if (close(outputFd) == -1)
        errExit("close output");

    exit(EXIT_SUCCESS);
}

#include <sys/stat.h>
#include <fcntl.h>
#include <ctype.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    size_t len;
    off_t offset;
    int fd, ap, j;
    char *buf;
    ssize_t numRead, numWritten;

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file {r<length>|R<length>|w<string>|s<offset>}...\n",
                 argv[0]);

    fd = open(argv[1], O_RDWR | O_CREAT,
                S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
                S_IROTH | S_IWOTH);                     /* rw-rw-rw- */
    if (fd == -1)
        errExit("open");

    for (ap = 2; ap < argc; ap++) {
        switch (argv[ap][0]) {
        case 'r':   /* Display bytes at current offset, as text */
        case 'R':   /* Display bytes at current offset, in hex */
            len = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);

            buf = malloc(len);
            if (buf == NULL)
                errExit("malloc");

            numRead = read(fd, buf, len);
            if (numRead == -1)
                errExit("read");

            if (numRead == 0) {
                printf("%s: end-of-file\n", argv[ap]);
            } else {
                printf("%s: ", argv[ap]);
                for (j = 0; j < numRead; j++) {
                    if (argv[ap][0] == 'r')
                        printf("%c", isprint((unsigned char) buf[j]) ?
                                                buf[j] : '?');
                    else
                        printf("%02x ", (unsigned int) buf[j]);
                }
                printf("\n");
            }

            free(buf);
            break;

        case 'w':   /* Write string at current offset */
            numWritten = write(fd, &argv[ap][1], strlen(&argv[ap][1]));
            if (numWritten == -1)
                errExit("write");
            printf("%s: wrote %ld bytes\n", argv[ap], (long) numWritten);
            break;

        case 's':   /* Change file offset */
            offset = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
            if (lseek(fd, offset, SEEK_SET) == -1)
                errExit("lseek");
            printf("%s: seek succeeded\n", argv[ap]);
            break;

        default:
            cmdLineErr("Argument must start with [rRws]: %s\n", argv[ap]);
        }
    }

    if (close(fd) == -1)
        errExit("close");

    exit(EXIT_SUCCESS);
}