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本篇內(nèi)容介紹了“Linux操作文件系統(tǒng)怎么調(diào)用”的有關(guān)知識(shí),在實(shí)際案例的操作過(guò)程中,不少人都會(huì)遇到這樣的困境,接下來(lái)就讓小編帶領(lǐng)大家學(xué)習(xí)一下如何處理這些情況吧!希望大家仔細(xì)閱讀,能夠?qū)W有所成!
需要引入的頭文件:
#inlcude<unistd.h>
打開(kāi)一個(gè)已存在的文件
int open(const char *pathname, int flags);
新建一個(gè)文件并創(chuàng)建權(quán)限
int open(const char *pathname, int flags, mode_t mode);
pathname:將要打開(kāi)的文件路徑和名稱
flags:打開(kāi)標(biāo)志
標(biāo)志介紹:
The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read- only, write-only, or read/write, respectively.
O_RDONLY 只讀打開(kāi)
O_RDWR 讀寫打開(kāi)
O_CREAT 文件不存在則創(chuàng)建
O_APPEND 文件末尾追加
O_TRUNC 清空文件,重新寫入 mode
The following symbolic constants are provided for mode: S_IRWXU 00700 user (file owner) has read, write, and execute permission S_IRUSR 00400 user has read permission S_IWUSR 00200 user has write permission S_IXUSR 00100 user has execute permission S_IRWXG 00070 group has read, write, and execute permission S_IRGRP 00040 group has read permission S_IWGRP 00020 group has write permission S_IXGRP 00010 group has execute permission S_IRWXO 00007 others have read, write, and execute permission S_IROTH 00004 others have read permission S_IWOTH 00002 others have write permission S_IXOTH 00001 others have execute permission
返回值:文件描述符
ssize_t read(int fd, void *buf, size_t count);
參數(shù)介紹
fd:對(duì)應(yīng)打開(kāi)的文件描述符buf : 存放數(shù)據(jù)的空間count: 計(jì)劃一次從文件中讀多少字節(jié)數(shù)據(jù)返回值: 實(shí)際讀到的字節(jié)數(shù)
ssize_t write(int fd, const void *buf, size_t count);
參數(shù)介紹:
fd :對(duì)應(yīng)打開(kāi)的文件描述符buf:存放待寫入的數(shù)據(jù)count:計(jì)劃一次向文件中寫入多少數(shù)據(jù)
int close(int fd);
fd :對(duì)應(yīng)的文件描述符
如果父進(jìn)程先打開(kāi)一個(gè)文件,fork 后子進(jìn)程是否可以共享使用?
文件內(nèi)容
代碼
#include<stdio.h> #include<unistd.h> #include<assert.h> #include<fcntl.h> #include<stdlib.h> int main() { char buff[128] = {0}; int fd = open("myfile.txt", O_RDONLY); pid_t pid = fork(); assert(pid != -1); if (pid == 0) { read(fd, buff, 1); printf("child buff = %s\n", buff); sleep(1); read(fd, buff, 1); printf("child buff = %s\n", buff); } else { read(fd, buff, 1); printf("parent buff = %s\n", buff); sleep(1); read(fd, buff, 1); printf("parent buff = %s\n", buff); } close(fd); exit(0); }
運(yùn)行結(jié)果:
結(jié)論:
由于 fork 創(chuàng)建的子進(jìn)程的 PCB 是拷貝父進(jìn)程的,子進(jìn)程的 PCB 中的文件表指向打開(kāi)文件的指針只是拷貝了父進(jìn)程 PCB 中的值,所以父子進(jìn)程共享父進(jìn)程 fork 之前打開(kāi)的所有文件描述符。
完成對(duì)一個(gè)文件的復(fù)制(類似命令:cp)
原文件內(nèi)容為:
代碼:
#include<stdio.h> #include<unistd.h> #include<fcntl.h> #include<stdlib.h> #include<assert.h> int main(void) { char buff[128] = {0}; int fdr = open("myfile.txt", O_RDONLY); assert(fdr != -1); int fdw = open("newfile.txt", O_WRONLY | O_CREAT, 0600); assert(fdw != -1); int n = 0; while (n = read(fdr, buff, 128) > 0) { write(fdw, buff, n); } close(fdr); close(fdw); exit(0); }
運(yùn)行示例:
可以看到newfile.txt創(chuàng)建成功
區(qū)別: 系統(tǒng)調(diào)用的實(shí)現(xiàn)在內(nèi)核中,屬于內(nèi)核空間,庫(kù)函數(shù)的實(shí)現(xiàn)在函數(shù)庫(kù)中,屬于用戶空間。
系統(tǒng)調(diào)用執(zhí)行過(guò)程:
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