linux 系统I/O 1

I/O 是主存和外部设备之间（终端，磁盘驱动器，网络）复制数据的过程，输入是从 从I/O设备复制数据到主存，输出是主存复制数据到I/O设备。

1.读写文件

ssize_t Read(int fd, void *buf, size_t count);//返回： 若成功则返回读的字节数，若失败 则为-1，EOF则为0//read函数从描述符为fd的当前文件为准复制最多n个字节到内存位置buf，一般返回值是实际传送的字节数量ssize_t Write(int fd, const void *buf, size_t count);//返回： 若成功则返回写的字节数，若失败 则为-1//write函数从内存位置buf为准复制最多n个字节到描述符为fd的当前文件位置。 ssize_t Read(int fd, void *buf, size_t count) //size_t 是unsigned_long型。{    ssize_t rc;//有符号大小的long型，因为有可能返回负值，所以类型是ssize_t.    if ((rc = read(fd, buf, count)) < 0)     unix_error("Read error");    return rc;}ssize_t Write(int fd, const void *buf, size_t count) {    ssize_t rc;    if ((rc = write(fd, buf, count)) < 0)    unix_error("Write error");    return rc;}

下面是示例程序：

#include"csapp.h"int main(void){  char c;  while(Read(STDIN_FILENO,&c,1)!=0) //把标准输入复制到标准输出。    Write(STDOUT_FILENO,&c,1);      exit(0);}

zsj@zsj-virtual-machine:~/ccode$ gcc -o iotest2 iotest2.c -lpthreadzsj@zsj-virtual-machine:~/ccode$ ./iotest2w  wwertwertdgfdhfhfghdgfdhfhfgh

用RIO健壮的读写

RIO会自动的为您处理不足值，在像网络程序这种容易出现不足值的应用中，RIO提供了方便、健壮、高效的I/O.

RIO函数分类：

（1）RIO无缓冲的输入输出函数

rio_readn  //函数从描述符为fd的当前文件为准复制最多n个字节到内存位置buf，遇到EOF时返回一个不足的值 rio_writen //函数从内存位置buf为准复制最多n个字节到描述符为fd的当前文件位置

/* $begin rio_readn */  ssize_t rio_readn(int fd, void *usrbuf, size_t n) //rio_readn源码{    size_t nleft = n;    ssize_t nread;    char *bufp = (char *)usrbuf;    while (nleft > 0) {    if ((nread = read(fd, bufp, nleft)) < 0) {        if (errno == EINTR) /* interrupted by sig handler return */        nread = 0;      /* and call read() again */        else        return -1;      /* errno set by read() */     }     else if (nread == 0)        break;              /* EOF */    nleft -= nread;    bufp += nread;    }    return (n - nleft);         /* return >= 0 */}/* $end rio_readn */* * rio_writen - robustly write n bytes (unbuffered) *//* $begin rio_writen */ssize_t rio_writen(int fd, void *usrbuf, size_t n)// rio_writen 源码{    size_t nleft = n;    ssize_t nwritten;    char *bufp = (char *)usrbuf;    while (nleft > 0) {    if ((nwritten = write(fd, bufp, nleft)) <= 0) {        if (errno == EINTR)  /* interrupted by sig handler return */        nwritten = 0;    /* and call write() again */        else        return -1;       /* errno set by write() */    }    nleft -= nwritten;    bufp += nwritten;    }    return n;}/* $end rio_writen */

（2）RIO带缓冲的输入输出函数

/* $begin rio_read */static ssize_t rio_read(rio_t *rp, char *usrbuf, size_t n){    int cnt;    while (rp->rio_cnt <= 0) {  /* refill if buf is empty */    rp->rio_cnt = read(rp->rio_fd, rp->rio_buf,                sizeof(rp->rio_buf));    if (rp->rio_cnt < 0) {        if (errno != EINTR) /* interrupted by sig handler return */        return -1;    }    else if (rp->rio_cnt == 0)  /* EOF */        return 0;    else         rp->rio_bufptr = rp->rio_buf; /* reset buffer ptr */    }    /* Copy min(n, rp->rio_cnt) bytes from internal buf to user buf */    cnt = n;              if (rp->rio_cnt < n)       cnt = rp->rio_cnt;    memcpy(usrbuf, rp->rio_bufptr, cnt);    rp->rio_bufptr += cnt;    rp->rio_cnt -= cnt;    return cnt;} /* $end rio_read *//*/*  * rio_readlineb - robustly read a text line (buffered) *//* $begin rio_readlineb */ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen) {    int n, rc;    char c, *bufp = (char *)usrbuf;    for (n = 1; n < maxlen; n++) {     if ((rc = rio_read(rp, &c, 1)) == 1) {        *bufp++ = c;        if (c == '\n')        break;    } else if (rc == 0) {        if (n == 1)        return 0; /* EOF, no data read */        else        break;    /* EOF, some data was read */    } else        return -1;    /* error */    }    *bufp = 0;    return n;}/* * rio_readinitb - Associate a descriptor with a read buffer and reset buffer *//* $begin rio_readinitb */void rio_readinitb(rio_t *rp, int fd) {    rp->rio_fd = fd;      rp->rio_cnt = 0;      rp->rio_bufptr = rp->rio_buf;}/* $end rio_readinitb *//* * rio_readnb - Robustly read n bytes (buffered) *//* $begin rio_readnb */ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n) {    size_t nleft = n;    ssize_t nread;    char *bufp = (char *)usrbuf;    while (nleft > 0) {    if ((nread = rio_read(rp, bufp, nleft)) < 0) {        if (errno == EINTR) /* interrupted by sig handler return */        nread = 0;      /* call read() again */        else        return -1;      /* errno set by read() */     }     else if (nread == 0)        break;              /* EOF */    nleft -= nread;    bufp += nread;    }    return (n - nleft);         /* return >= 0 */}/* $end rio_readnb *//* 

（3）读取元数据

应用程序能够通过调用 stat 和fastat 函数，检索关于文件的信息。

void Stat(const char *filename, struct stat *buf) {    if (stat(filename, buf) < 0)    unix_error("Stat error");}void Fstat(int fd, struct stat *buf) {    if (fstat(fd, buf) < 0)    unix_error("Fstat error");}

应用程序可以用readdir系列函数来读取目录的内容
函数 closedir关闭流释放其所有资源。

（4）共享文件

（5）I/O重定向

（6）标准I/O