8、UNIX系统编程：从底层I/O到标准I/O库

UNIX系统编程：从底层I/O到标准I/O库

底层I/O的局限与操作

底层I/O是UNIX操作系统提供的基础输入输出接口，不具备格式化和数据转换能力。以一个程序为例，它通过以下代码实现字符串的查找和输出：

/* 
 * Prompt for the string number. 
 */ 
write(1, "Which string (0 to quit)? ", 26); 
n = read(0, answer, sizeof(answer)); 
answer[n-1] = '\0'; 
n = atoi(answer); 

if (n == 0) { 
    close(fd); 
    exit(0); 
} 

if (n < 0 || n > NSTRINGS) { 
    write(2, "Out of range.\n", 14); 
    continue; 
} 

/* 
 * Find the string and read it. 
 */ 
lseek(fd, (n-1) * STRSIZE, SEEK_SET); 
read(fd, buf, STRSIZE); 

/* 
 * Print it out. 
 */ 
write(1, "String ", 7); 
write(1, answer, strlen(answer)); 
write(1, " = ", 3); 
write(1, buf, STRSIZE); 
write(1, "\n\n", 2); 

这个程序的运行示例如下：

% seeker 
Whi