嵌入式Linux学习疑惑随手记

2022-04-22

字数统计: 1.1k字 | 阅读时长≈ 5分

1. 嵌入式Linux中App开发人员是如何得知可以操控哪些底层硬件的？

系统会自动挂载，如果没有自动挂载就需要去手动执行挂载命令；我们怎么直到是否自动挂载呢？在终端执行：

1	cat /proc/mounts

查看系统设备：

1	ls /dev/* -l # 查看所有的设备，包括其主设备号和次设备号

在Linux内核里面，有两类驱动程序：字符(char)设备驱动程序和块(block)设备驱动程序。

图中，c代表字符设备驱动程序， 89就是设备的主设备号，0/1/2就是设备的次设备号。设备节点对应哪一个设备驱动程序通过主设备号来确定，而次设备号则是决定该设备节点对应的是设备驱动程序中对应的哪一个硬件。

2. Linux中对文件的操作

打开文件：open

需要包含的头文件：

1
2
3

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

可以调用的API：

1 2	int open(const char pathname, int flags); int open(const char pathname, int flags, mode_t mode);

当成功打开一个文件后，open的返回值会是这个文件的描述符句柄，如果打开失败，那么它的返回值就是-1。

参数flags的范围：

flags	涵义
O_RDONLY	只读
O_WRONLY	只写
O_RDWR	可读可写
O_APPEND	以附件的方式打开文件，在对该文件进行写的时候，会偏移到文件的末尾开始写
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.
O_CLOEXEC	Enable the close-on-exec flag for the new file descriptor
O_CREAT	If pathname does not exist, create it as a regular file.这个Flags会用到mode
O_DIRECT	Try to minimize cache effects of the I/O to and from this file.
O_DIRECTORY	If pathname is not a directory, cause the open to fail
O_DSYNC	Write operations on the file will complete according to the requirements of synchronized I/O data integrity completion.
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() fails with the error EEXIST.
O_LARGEFILE	Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened.
O_NOATIME	Do not update the file last access time (st_atime in the inode) when the file is read(2).
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, with the error ELOOP.
O_NONBLOCK/O_NDELAY	When possible, the file is opened in nonblocking mode.
O_PATH	Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree and to perform operations that act purely at the file descriptor level.
O_SYNC	Write operations on the file will complete according to the requirements of synchronized I/O file integrity completion(by contrast with the synchronized I/O data integrity completion provided by O_DSYNC.)
O_TMPFILE	Create an unnamed temporary regular file.
O_TRUNC	If the file already exists and is a regular file and the access 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.

当flags支持O_CREAT时，可以选择的mode有：

mode	作用
S_IRWXU	user (file owner) has read, write, and execute permission
S_IRUSR	user has read permission
S_IWUSR	user has write permission
S_IXUSR	user has execute permission
S_IRWXG	group has read, write, and execute permission
S_IRGRP	group has read permission
S_IWGRP	group has write permission
S_IXGRP	group has execute permission
S_IRWXO	others have read, write, and execute permission
S_IROTH	others have read permission
S_IWOTH	others have write permission
S_IXOTH	others have execute permission
S_ISUID	set-user-ID bit
S_ISGID	set-group-ID bit
S_ISVTX	sticky bit

其中，user/group/others指代的什么？

示例：打开一个“只读”文件

测试代码：

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

/**
 * @brief 
 * ./open hello.txt
 * @param argc 
 * @param argv 
 * @return int 
 */

int main(int argc, char const *argv[])
{
    char buf[1024] = {0};
    int len = 0;
    printf("Open a file by read only.\r\n");
    int fd = open(argv[1], O_RDONLY);
    if(fd == -1)
        printf("Open file failed.\r\n");
    
    printf("Read file: \r\n");
    while( (len = read(fd, buf, 1024)) > 0 )
    {
        printf("%s", buf);
    }

    printf("\r\n");

    printf("Try to write: ABCD\r\n");

    len = write(fd, "ABCD", 4);
    if(len == -1)
        printf("Write to file failed.\r\n");

    close(fd);

    return 0;
}

输出结果：

book@100ask:~/workspace/FileOption$ ./open_ro hello.txt 
Open a file by read only.
Read file: 
Hello World!

Try to write: ABCD
Write to file failed.

本文作者： 摘星星的小朋友
本文链接： http://slhking.github.io/2022/04/22/LinuxLearnNote/
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