1IO的共用和效率
read與write其中資料緩衝的大小建設設定為:getpagesize (一頁的大小)
或者4092
2 定位與讀取資料(隨機讀取)
read和write時自動移動讀取位置
lseek改變讀取位置
pread/pwrite在指定位置讀寫
2.1lseek函數說明
off_t lseek(int fd, //檔案描述符
off_t offset,//位移
int whence);//定位參數 開始SEEK_SET, 當前位置SEEK_CUR , 結束SEEK_END
傳回值:當前讀取在檔案中的絕地位置
3 例子
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h>#include <unistd.h>#include <stdlib.h>#include <stdio.h>//注意這裡 為了以後讀取方便 請人為對齊 不要讓電腦來對齊struct stu{int no;char name[16];float score;};int openfile(const char* filename){int fd = open(filename,O_WRONLY|O_CREAT|O_EXCL,0666);if(fd < 0){perror("open error!");}return fd;}void input(struct stu *record){printf("請輸入學生ID:");scanf("%d",&(record->no));printf("請輸入學生姓名:");scanf("%s",record->name);printf("請輸入學產生績:");scanf("%f",&(record->score));}void save(int fd,struct stu* record){write(fd,record,sizeof(struct stu));}int iscontinue(){char c;printf("是否繼續輸入:y/n\n");scanf("%c",&c);scanf("%c",&c);if(c == 'y'){return 1;}elsereturn 0;}int main(){int fd =openfile("stu.dat");if(fd < 0){return 1;}struct stu record;while(1){input(&record);save(fd,&record);if(! iscontinue() ){break;}}close(fd);}
/*讀取檔案中的姓名檔案以結構體的形式寫入struct stu{int no;char name[16];float score;};*/#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/types.h>int main(){int fd = open("stu.dat",O_RDONLY);if(fd < 0){perror("open");return 1;}int i = 0;char buf[4092];lseek(fd,i*24+4,SEEK_SET); while(read(fd,buf,16)){printf("%s ",buf);i++;lseek(fd,i*24+4,SEEK_SET);}printf("\n");close(fd);}
4 lseek的定位位置超出檔案大小時會發生什嗎?1 lseek只要位置合法(絕對位置>=0),傳回值都是當前位置
位置可以超出檔案範圍,只要不寫入檔案大小不會變化
#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/types.h>int main(){int fd = open("dat",O_RDWR|O_CREAT,0666);if(fd < 0){perror("open");return 1;}int r = lseek(fd,2000,SEEK_SET);printf("%d\n",r);close(fd);}
zhao@ubuntu:~/unix/4$ ./lseek2
2000
zhao@ubuntu:~/unix/4$ ls -l dat
-rw-r--r-- 1 zhao zhao 0 2013-06-01 04:59 dat
lseek超出檔案範圍,寫入後檔案大小會多出很多
#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/types.h>int main(){int fd = open("dat",O_RDWR|O_CREAT,0666);if(fd < 0){perror("open");return 1;}int r = lseek(fd,2000,SEEK_SET);printf("%d\n",r);write(fd,"hello"5);close(fd);}
zhao@ubuntu:~/unix/4$ ls -l dat
-rw-r--r-- 1 zhao zhao 2005 2013-06-01 05:02 d
2 若是位置不合法 ,比如負數,返回-15 pread pwrite
函數描述
#include <unistd.h>
ssize_t pread(int fd, void *buf, size_t count, off_t offset); //注意offset是絕對位置
ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);
pread=lseek+read
pwrite=lseek+write
傳回值
讀寫的資料大小,出錯返回-1
//讀取檔案中的姓名#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/types.h>int main(){int fd = open("stu.dat",O_RDONLY);if(fd < 0){perror("open");return 1;}int i = 0;char buf[4092];while(pread(fd,buf,16,i*24+4)){printf("%s ",buf);i++;}printf("\n");close(fd);}
pread pwrite會改變讀寫位置嗎?
pread() reads up to count bytes from file descriptor fd at offset off‐
set (from the start of the file) into the buffer starting at buf. The
file offset is not changed.
pwrite() writes up to count bytes from the buffer starting at buf to
the file descriptor fd at offset offset. The file offset is not
changed.
記住 都不改變讀寫位置 , 這就是它們和lseek+read/write的區別
案例 讀取/proc/$(pid)/mem 檔案(虛擬記憶體 )
#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/types.h>#include <string.h>int a = 91119;int main(){char filename[20];memset(filename,0,20);sprintf(filename,"/proc/%d/mem",getpid());//mem檔案 虛擬記憶體空間映射到mem上了int fd = open(filename,O_RDWR);if(fd < 0){perror("open error");return 1;}//讀取&a這個位置的地址int t;pread(fd,&t,4,(off_t)&a);printf("t:%d\n",t); //91119t = 88888;pwrite(fd,&t,4,(off_t)&a);//往所在a的地方寫資料printf("a=%d\n",a); //沒有改變 因為系統沒有給寫的許可權}