read函数: #include <unistd.h> ssize_t read(int feledes,void *buff,size_t nbytes); 返回: 读到的字节数，若已到文件末尾则为0，若出错为-1
write函数: #include <unistd.h> ssize_t write(int filedes,const void *buff,size_t nbytes); 返回: 若成功为已写的字节数，若出错为-1 对于普通文件，写操作从文件的当前位移量处开始。如果打开时制定了O_APPEND选择项，则在每次写操作之
前将文件位移量设置在文件的当前结尾处。在一次成功写之后，该文件位移量增加实际写的字节数 Example: write.c
/*write.c*/ #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <stdlib.h> #include <stdio.h> #include <string.h> #define MAXSIZE int main(void) { int i,fd,size,len; char *buf="Hello! I'm writing to this file!"; char buf_r[10]; len = strlen(buf); buf_r[10] = '\0'; if((fd = open("/tmp/hello.c", O_CREAT | O_TRUNC | O_RDWR,0666 ))<0) { perror("open:"); exit(1); } else { printf("open file:hello.c %d\n",fd); } if((size = write( fd, buf, len)) < 0) { perror("write:"); exit(1); } else { printf("Write:%s\n",buf); } lseek( fd, 0, SEEK_SET ); if((size = read( fd, buf_r, 10))<0) { perror("read:"); exit(1); } else { printf("read form file:%s\n",buf_r); } if( close(fd) < 0 ) { perror("close:"); exit(1); } else { printf("Close hello.c\n"); } exit(0);