open和creat
注意,c语言没有函数重载,这里是通过可变参数实现的,mode不指定的话会用umask去默认创建,这里creat功能可以直接在open中指定O_CREAT的flag实现,不必使用creat
open和creat调用时返回文件描述符,失败返回-1
在open时flags必须包含以下任意一个
access modes: O_RDONLY, O_WRONLY, or O_RDWR
创建时会通过umask抹去一些权限
/*
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags, mode_t mode);
参数:
- pathname:要创建的文件的路径
- flags:对文件的操作权限和其他的设置
- 必选项:O_RDONLY, O_WRONLY, O_RDWR 这三个之间是互斥的
- 可选项:O_CREAT 文件不存在,创建新文件
- mode:八进制的数,表示创建出的新的文件的操作权限,比如:0775
最终的权限是:mode & ~umask
0777 -> 111111111
& 0775 -> 111111101
----------------------------
111111101
按位与:0和任何数都为0
umask的作用就是抹去某些权限。
flags参数是一个int类型的数据,占4个字节,32位。
flags 32个位,每一位就是一个标志位。
*/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
int main() {
// 创建一个新的文件
int fd = open("create.txt", O_RDWR | O_CREAT, 0777);
if(fd == -1) {
perror("open");
}
// 关闭
close(fd);
return 0;
}
还有一些可添加的flag
In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags. The file creation flags are O_CLOEXEC, O_CREAT, O_DIREC‐
TORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TRUNC, and O_TTY_INIT. The file status flags are all of the remaining flags listed below. The distinction between
these two groups of flags is that the file status flags can be retrieved and (in some cases) modified using fcntl(2). The full list of file creation flags
and file status flags is as follows:
O_APPEND
The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2). O_APPEND may lead
to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a
file, so the client kernel has to simulate it, which can't be done without a race condition.
O_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file
descriptor. This feature is available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further
details.
O_CLOEXEC (Since Linux 2.6.23)
Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl(2) F_SETFD operations to
set the FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate fcntl(2) F_SETFD operation
to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor at the same time as another thread does a
fork(2) plus execve(2).
O_DIRECT (Since Linux 2.4.10)
Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such
as when applications do their own caching. File I/O is done directly to/from user-space buffers. The O_DIRECT flag on its own makes an effort to
transfer data synchronously, but does not give the guarantees of the O_SYNC flag that data and necessary metadata are transferred. To guarantee syn‐
chronous I/O, O_SYNC must be used in addition to O_DIRECT. See NOTES below for further discussion.
A semantically similar (but deprecated) interface for block devices is described in raw(8).
O_DIRECTORY
If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and was added in kernel version 2.1.126, to avoid denial-of-ser‐
vice problems if opendir(3) is called on a FIFO or tape device.
O_EXCL Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open() will fail.
When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then open() fails regardless of where the symbolic
link points to.
In general, the behavior of O_EXCL is undefined if it is used without O_CREAT. There is one exception: on Linux 2.6 and later, O_EXCL can be used
without O_CREAT if pathname refers to a block device. If the block device is in use by the system (e.g., mounted), open() fails with the error EBUSY.
On NFS, O_EXCL is supported only when using NFSv3 or later on kernel 2.6 or later. In NFS environments where O_EXCL support is not provided, programs
that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lock‐
file, and need to avoid reliance on NFS support for O_EXCL, can create a unique file on the same file system (e.g., incorporating hostname and PID),
and use link(2) to make a link to the lockfile. If link(2) returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if
its link count has increased to 2, in which case the lock is also successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. The _LARGEFILE64_SOURCE macro
must be defined (before including any header files) in order to obtain this definition. Setting the _FILE_OFFSET_BITS feature test macro to 64 (rather
than using O_LARGEFILE) is the preferred method of accessing large files on 32-bit systems (see feature_test_macros(7)).
O_NOATIME (Since Linux 2.6.8)
Do not update the file last access time (st_atime in the inode) when the file is read(2). This flag is intended for use by indexing or backup pro‐
grams, where its use can significantly reduce the amount of disk activity. This flag may not be effective on all file systems. One example is NFS,
where the server maintains the access time
O_NOCTTY
If pathname refers to a terminal device—see tty(4)—it will not become the process's controlling terminal even if the process does not have one.
O_NOFOLLOW
If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added to Linux in version 2.1.126. Symbolic links in ear‐
lier components of the pathname will still be followed. See also O_NOPATH below.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode. Neither the open() nor any subsequent operations on the file descriptor which is returned will
cause the calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in con‐
junction with mandatory file locks and with file leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two purposes: to indicate a location in the file-system tree and to perform operations that act purely at
the file descriptor level. The file itself is not opened, and other file operations (e.g., read(2), write(2), fchmod(2), fchown(2), fgetxattr(2)) fail
with the error EBADF.
O_SYNC The file is opened for synchronous I/O. Any write(2)s on the resulting file descriptor will block the calling process until the data has been physi‐
cally written to the underlying hardware. But see NOTES below.
O_TRUNC
If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If
the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.
Some of these optional flags can be altered using fcntl(2) after the file has been opened.
creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
read和write
注意,非阻塞io在数据没准备好时会返回EAGAIN,所以这时候不能认为这是个错误
/*
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
参数:
- fd:文件描述符,open得到的,通过这个文件描述符操作某个文件
- buf:需要读取数据存放的地方,数组的地址(传出参数)
- count:指定的数组的大小
返回值:
- 成功:
>0: 返回实际的读取到的字节数
=0:文件已经读取完了
- 失败:-1 ,并且设置errno
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);
参数:
- fd:文件描述符,open得到的,通过这个文件描述符操作某个文件
- buf:要往磁盘写入的数据,数据
- count:要写的数据的实际的大小
返回值:
成功:实际写入的字节数
失败:返回-1,并设置errno
*/
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
// 1.通过open打开english.txt文件
int srcfd = open("english.txt", O_RDONLY);
if(srcfd == -1) {
perror("open");
return -1;
}
// 2.创建一个新的文件(拷贝文件)
int destfd = open("cpy.txt", O_WRONLY | O_CREAT, 0664);
if(destfd == -1) {
perror("open");
return -1;
}
// 3.频繁的读写操作
char buf[1024] = {0};
int len = 0;
while((len = read(srcfd, buf, sizeof(buf))) > 0) {
write(destfd, buf, len);
}
// 4.关闭文件描述符
close(destfd);
close(srcfd);
return 0;
}
lseek
用来操作文件描述符对应的偏移量,也就是去移动文件指针
whence指定策略
/*
标准C库的函数
#include <stdio.h>
int fseek(FILE *stream, long offset, int whence);
Linux系统函数
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
参数:
- fd:文件描述符,通过open得到的,通过这个fd操作某个文件
- offset:偏移量
- whence:
SEEK_SET
设置文件指针的偏移量
SEEK_CUR
设置偏移量:当前位置 + 第二个参数offset的值
SEEK_END
设置偏移量:文件大小 + 第二个参数offset的值
返回值:返回文件指针的位置
作用:
1.移动文件指针到文件头
lseek(fd, 0, SEEK_SET);
2.获取当前文件指针的位置
lseek(fd, 0, SEEK_CUR);
3.获取文件长度
lseek(fd, 0, SEEK_END);
4.拓展文件的长度,当前文件10b, 110b, 增加了100个字节
lseek(fd, 100, SEEK_END)
注意:需要写一次数据
*/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
int main() {
int fd = open("hello.txt", O_RDWR);
if(fd == -1) {
perror("open");
return -1;
}
// 扩展文件的长度
int ret = lseek(fd, 100, SEEK_END);
if(ret == -1) {
perror("lseek");
return -1;
}
// 写入一个空数据
write(fd, " ", 1);
// 关闭文件
close(fd);
return 0;
}
stat
用于获取文件信息
stat结构体
/*
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int stat(const char *pathname, struct stat *statbuf);
作用:获取一个文件相关的一些信息
参数:
- pathname:操作的文件的路径
- statbuf:结构体变量,传出参数,用于保存获取到的文件的信息
返回值:
成功:返回0
失败:返回-1 设置errno
int lstat(const char *pathname, struct stat *statbuf);
参数:
- pathname:操作的文件的路径
- statbuf:结构体变量,传出参数,用于保存获取到的文件的信息
返回值:
成功:返回0
失败:返回-1 设置errno
*/
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
int main() {
struct stat statbuf;
int ret = stat("a.txt", &statbuf);
if(ret == -1) {
perror("stat");
return -1;
}
printf("size: %ld\n", statbuf.st_size);
return 0;
}
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <pwd.h>
#include <grp.h>
#include <time.h>
#include <string.h>
// 模拟实现 ls -l 指令
// -rw-rw-r-- 1 nowcoder nowcoder 12 12月 3 15:48 a.txt
int main(int argc, char * argv[]) {
// 判断输入的参数是否正确
if(argc < 2) {
printf("%s filename\n", argv[0]);
return -1;
}
// 通过stat函数获取用户传入的文件的信息
struct stat st;
int ret = stat(argv[1], &st);
if(ret == -1) {
perror("stat");
return -1;
}
// 获取文件类型和文件权限
char perms[11] = {0}; // 用于保存文件类型和文件权限的字符串
switch(st.st_mode & S_IFMT) {
case S_IFLNK:
perms[0] = 'l';
break;
case S_IFDIR:
perms[0] = 'd';
break;
case S_IFREG:
perms[0] = '-';
break;
case S_IFBLK:
perms[0] = 'b';
break;
case S_IFCHR:
perms[0] = 'c';
break;
case S_IFSOCK:
perms[0] = 's';
break;
case S_IFIFO:
perms[0] = 'p';
break;
default:
perms[0] = '?';
break;
}
// 判断文件的访问权限
// 文件所有者
perms[1] = (st.st_mode & S_IRUSR) ? 'r' : '-';
perms[2] = (st.st_mode & S_IWUSR) ? 'w' : '-';
perms[3] = (st.st_mode & S_IXUSR) ? 'x' : '-';
// 文件所在组
perms[4] = (st.st_mode & S_IRGRP) ? 'r' : '-';
perms[5] = (st.st_mode & S_IWGRP) ? 'w' : '-';
perms[6] = (st.st_mode & S_IXGRP) ? 'x' : '-';
// 其他人
perms[7] = (st.st_mode & S_IROTH) ? 'r' : '-';
perms[8] = (st.st_mode & S_IWOTH) ? 'w' : '-';
perms[9] = (st.st_mode & S_IXOTH) ? 'x' : '-';
// 硬连接数
int linkNum = st.st_nlink;
// 文件所有者
char * fileUser = getpwuid(st.st_uid)->pw_name;
// 文件所在组
char * fileGrp = getgrgid(st.st_gid)->gr_name;
// 文件大小
long int fileSize = st.st_size;
// 获取修改的时间
char * time = ctime(&st.st_mtime);
char mtime[512] = {0};
strncpy(mtime, time, strlen(time) - 1);
char buf[1024];
sprintf(buf, "%s %d %s %s %ld %s %s", perms, linkNum, fileUser, fileGrp, fileSize, mtime, argv[1]);
printf("%s\n", buf);
return 0;
}
access
判断某个文件是否有某个权限,或者判断文件是否存在
The mode specifies the accessibility check(s) to be performed, and is either the value F_OK, or a mask consisting of the bitwise OR of one or more of R_OK,W_OK, and X_OK. F_OK tests for the existence of the file. R_OK, W_OK, and X_OK test whether the file exists and grants read, write, and execute permissions,respectively
/*
#include <unistd.h>
int access(const char *pathname, int mode);
作用:判断某个文件是否有某个权限,或者判断文件是否存在
参数:
- pathname: 判断的文件路径
- mode:
R_OK: 判断是否有读权限
W_OK: 判断是否有写权限
X_OK: 判断是否有执行权限
F_OK: 判断文件是否存在
返回值:成功返回0, 失败返回-1
*/
#include <unistd.h>
#include <stdio.h>
int main() {
int ret = access("a.txt", F_OK);
if(ret == -1) {
perror("access");
}
printf("文件存在!!!\n");
return 0;
}
chmod
修改文件权限,两个函数不同是一个根据文件路径,一个根据传入文件描述符去修改
/*
#include <sys/stat.h>
int chmod(const char *pathname, mode_t mode);
修改文件的权限
参数:
- pathname: 需要修改的文件的路径
- mode:需要修改的权限值,八进制的数
返回值:成功返回0,失败返回-1
*/
#include <sys/stat.h>
#include <stdio.h>
int main() {
int ret = chmod("a.txt", 0777);
if(ret == -1) {
perror("chmod");
return -1;
}
return 0;
}
chown
更改文件所有者
truncate
缩减或者扩展文件的尺寸至指定的大小
注意,偏移量不会变
/*
#include <unistd.h>
#include <sys/types.h>
int truncate(const char *path, off_t length);
作用:缩减或者扩展文件的尺寸至指定的大小
参数:
- path: 需要修改的文件的路径
- length: 需要最终文件变成的大小
返回值:
成功返回0, 失败返回-1
*/
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
int main() {
int ret = truncate("b.txt", 5);
if(ret == -1) {
perror("truncate");
return -1;
}
return 0;
}
mkdir
注意,一个目录必须有可执行权限才能进入
/*
#include <sys/stat.h>
#include <sys/types.h>
int mkdir(const char *pathname, mode_t mode);
作用:创建一个目录
参数:
pathname: 创建的目录的路径
mode: 权限,八进制的数
返回值:
成功返回0, 失败返回-1
*/
#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
int main() {
int ret = mkdir("aaa", 0777);
if(ret == -1) {
perror("mkdir");
return -1;
}
return 0;
}
chdir
修改工作目录
/*
#include <unistd.h>
int chdir(const char *path);
作用:修改进程的工作目录
比如在/home/nowcoder 启动了一个可执行程序a.out, 进程的工作目录 /home/nowcoder
参数:
path : 需要修改的工作目录
#include <unistd.h>
char *getcwd(char *buf, size_t size);
作用:获取当前工作目录
参数:
- buf : 存储的路径,指向的是一个数组(传出参数)
- size: 数组的大小
返回值:
返回的指向的一块内存,这个数据就是第一个参数
*/
#include <unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
int main() {
// 获取当前的工作目录
char buf[128];
getcwd(buf, sizeof(buf));
printf("当前的工作目录是:%s\n", buf);
// 修改工作目录
int ret = chdir("/home/nowcoder/Linux/lesson13");
if(ret == -1) {
perror("chdir");
return -1;
}
// 创建一个新的文件
int fd = open("chdir.txt", O_CREAT | O_RDWR, 0664);
if(fd == -1) {
perror("open");
return -1;
}
close(fd);
// 获取当前的工作目录
char buf1[128];
getcwd(buf1, sizeof(buf1));
printf("当前的工作目录是:%s\n", buf1);
return 0;
}
rename
重命名文件
/*
#include <stdio.h>
int rename(const char *oldpath, const char *newpath);
*/
#include <stdio.h>
int main() {
int ret = rename("aaa", "bbb");
if(ret == -1) {
perror("rename");
return -1;
}
return 0;
}
opendir和readdir
通过目录名打开一个目录流,读取目录数据
/*
// 打开一个目录
#include <sys/types.h>
#include <dirent.h>
DIR *opendir(const char *name);
参数:
- name: 需要打开的目录的名称
返回值:
DIR * 类型,理解为目录流
错误返回NULL
// 读取目录中的数据
#include <dirent.h>
struct dirent *readdir(DIR *dirp);
- 参数:dirp是opendir返回的结果
- 返回值:
struct dirent,代表读取到的文件的信息
读取到了末尾或者失败了,返回NULL
// 关闭目录
#include <sys/types.h>
#include <dirent.h>
int closedir(DIR *dirp);
*/
#include <sys/types.h>
#include <dirent.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int getFileNum(const char * path);
// 读取某个目录下所有的普通文件的个数
int main(int argc, char * argv[]) {
if(argc < 2) {
printf("%s path\n", argv[0]);
return -1;
}
int num = getFileNum(argv[1]);
printf("普通文件的个数为:%d\n", num);
return 0;
}
// 用于获取目录下所有普通文件的个数
int getFileNum(const char * path) {
// 1.打开目录
DIR * dir = opendir(path);
if(dir == NULL) {
perror("opendir");
exit(0);
}
struct dirent *ptr;
// 记录普通文件的个数
int total = 0;
while((ptr = readdir(dir)) != NULL) {
// 获取名称
char * dname = ptr->d_name;
// 忽略掉. 和..
if(strcmp(dname, ".") == 0 || strcmp(dname, "..") == 0) {
continue;
}
// 判断是否是普通文件还是目录
if(ptr->d_type == DT_DIR) {
// 目录,需要继续读取这个目录
char newpath[256];
sprintf(newpath, "%s/%s", path, dname);
total += getFileNum(newpath);
}
if(ptr->d_type == DT_REG) {
// 普通文件
total++;
}
}
// 关闭目录
closedir(dir);
return total;
}
dup
重定向文件描述符
/*
#include <unistd.h>
int dup(int oldfd);
作用:复制一个新的文件描述符
fd=3, int fd1 = dup(fd),
fd指向的是a.txt, fd1也是指向a.txt
从空闲的文件描述符表中找一个最小的,作为新的拷贝的文件描述符
*/
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
int main() {
int fd = open("a.txt", O_RDWR | O_CREAT, 0664);
int fd1 = dup(fd);
if(fd1 == -1) {
perror("dup");
return -1;
}
printf("fd : %d , fd1 : %d\n", fd, fd1);
close(fd);
char * str = "hello,world";
int ret = write(fd1, str, strlen(str));
if(ret == -1) {
perror("write");
return -1;
}
close(fd1);
return 0;
}
/*
#include <unistd.h>
int dup2(int oldfd, int newfd);
作用:重定向文件描述符
oldfd 指向 a.txt, newfd 指向 b.txt
调用函数成功后:newfd 和 b.txt 做close, newfd 指向了 a.txt
oldfd 必须是一个有效的文件描述符
oldfd和newfd值相同,相当于什么都没有做
*/
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
int main() {
int fd = open("1.txt", O_RDWR | O_CREAT, 0664);
if(fd == -1) {
perror("open");
return -1;
}
int fd1 = open("2.txt", O_RDWR | O_CREAT, 0664);
if(fd1 == -1) {
perror("open");
return -1;
}
printf("fd : %d, fd1 : %d\n", fd, fd1);
int fd2 = dup2(fd, fd1);
if(fd2 == -1) {
perror("dup2");
return -1;
}
// 通过fd1去写数据,实际操作的是1.txt,而不是2.txt
char * str = "hello, dup2";
int len = write(fd1, str, strlen(str));
if(len == -1) {
perror("write");
return -1;
}
printf("fd : %d, fd1 : %d, fd2 : %d\n", fd, fd1, fd2);
close(fd);
close(fd1);
return 0;
}
fcntl
针对文件描述符执行操作,主要是操作文件描述符的flag,如O_APPEND,NONBLOK等
注意在设置时应该先将flag取出来,然后设置对应位再设置进去
/*
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ...);
参数:
fd : 表示需要操作的文件描述符
cmd: 表示对文件描述符进行如何操作
- F_DUPFD : 复制文件描述符,复制的是第一个参数fd,得到一个新的文件描述符(返回值)
int ret = fcntl(fd, F_DUPFD);
- F_GETFL : 获取指定的文件描述符文件状态flag
获取的flag和我们通过open函数传递的flag是一个东西。
- F_SETFL : 设置文件描述符文件状态flag
必选项:O_RDONLY, O_WRONLY, O_RDWR 不可以被修改
可选性:O_APPEND, O)NONBLOCK
O_APPEND 表示追加数据
NONBLOK 设置成非阻塞
阻塞和非阻塞:描述的是函数调用的行为。
*/
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
int main() {
// 1.复制文件描述符
// int fd = open("1.txt", O_RDONLY);
// int ret = fcntl(fd, F_DUPFD);
// 2.修改或者获取文件状态flag
int fd = open("1.txt", O_RDWR);
if(fd == -1) {
perror("open");
return -1;
}
// 获取文件描述符状态flag
int flag = fcntl(fd, F_GETFL);
if(flag == -1) {
perror("fcntl");
return -1;
}
flag |= O_APPEND; // flag = flag | O_APPEND
// 修改文件描述符状态的flag,给flag加入O_APPEND这个标记
int ret = fcntl(fd, F_SETFL, flag);
if(ret == -1) {
perror("fcntl");
return -1;
}
char * str = "nihao";
write(fd, str, strlen(str));
close(fd);
return 0;
}