背景

最近收拾東西，從一堆雜物里翻出來塵封四年多的樹莓派 3B 主機來，打掃打掃灰塵，接上電源，居然還能通過之前設定好的 VNC 連上，欣慰之余，開始 clone 我的 git 專案，為它們拓展一個新的平臺，在執行 cnblogs 專案 (參考《博客園排名預測》) 對應的繪圖命令時，趨勢圖、預測圖是生成了，但沒有自動打開圖片，這個問題經過一番探索居然解決了，這篇文章就來分享一下解決問題的程序，

問題

第一眼看到的錯誤資訊：

$ open ./fit.png
無法獲取指向控制臺的檔案描述符

這里我設定了控制臺 locale 為中文，如果是英文的話，得到下面的結果：

Couldn't get a file descriptor referring to the console

果斷在網上搜索了這個錯誤，得到的結果比較少，根據解決方案主要分以下幾種：

setfont 命令：
- fedora - 錯誤：無法獲得指向控制臺的檔案描述符
- Error in linux console : Couldn't get a file descriptor referring to the console
loadkeys 命令：
- loadkeys says “Couldn't get a file descriptor referring to the console” in chroot on ChromeOS

這里我并沒有呼叫 setfont 或 loadkeys 命令，直接執行它倆也會報一樣的錯誤，難道需要在登錄腳本里執行一下？抱著試試看的態度，我在 ~/.bashrc 中加了這么一句：

loadkeys

重啟樹莓派，這回用 ssh 登錄，結果一上來就報錯：

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Sun Aug  8 23:07:46 2021 from 192.168.1.118
無法獲取指向控制臺的檔案描述符

看來網上的說法和我遇到的不一樣，只能自己探索了，好在遇事不決、量子力學，哦 no，strace，看看底層呼叫哪個環節出問題了：

$ strace open ./fit.png
execve("/bin/open", ["open", "fit.png"], [/* 36 vars */]) = 0
brk(0)                                  = 0x822000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f91000
access("/etc/ld.so.preload", R_OK)      = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76f90000
close(3)                                = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76f4f000
mprotect(0x76f54000, 61440, PROT_NONE)  = 0
mmap2(0x76f63000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76f63000
mprotect(0x7eb5f000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3)                                = 0
munmap(0x76f90000, 42)                  = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f3c000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f90000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76dfb000
mprotect(0x76f26000, 65536, PROT_NONE)  = 0
mmap2(0x76f36000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76f36000
mmap2(0x76f39000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76f39000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f8f000
set_tls(0x76f8f4c0, 0x76f8fba8, 0x76f94050, 0x76f8f4c0, 0x76f94050) = 0
mprotect(0x76f36000, 8192, PROT_READ)   = 0
mprotect(0x76f4f000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76f4f000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76f4f000, 0x76f54000, 0, 0x15, 0x7eb5f0b0) = 0
mprotect(0x22000, 4096, PROT_READ)      = 0
mprotect(0x76f93000, 4096, PROT_READ)   = 0
munmap(0x76f3c000, 76981)               = 0
brk(0)                                  = 0x822000
brk(0x843000)                           = 0x843000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76bfb000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x76a57000
close(3)                                = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3753, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3811, ...}) = 0
open("/proc/self/fd/0", O_RDWR)         = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7eb5f14b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty", O_RDWR)                = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7eb5f14b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty0", O_RDWR)               = -1 EACCES (Permission denied)
open("/dev/tty0", O_WRONLY)             = -1 EACCES (Permission denied)
open("/dev/tty0", O_RDONLY)             = -1 EACCES (Permission denied)
open("/dev/vc/0", O_RDWR)               = -1 ENOENT (No such file or directory)
open("/dev/vc/0", O_WRONLY)             = -1 ENOENT (No such file or directory)
open("/dev/vc/0", O_RDONLY)             = -1 ENOENT (No such file or directory)
open("/dev/systty", O_RDWR)             = -1 ENOENT (No such file or directory)
open("/dev/systty", O_WRONLY)           = -1 ENOENT (No such file or directory)
open("/dev/systty", O_RDONLY)           = -1 ENOENT (No such file or directory)
open("/dev/console", O_RDWR)            = -1 EACCES (Permission denied)
open("/dev/console", O_WRONLY)          = -1 EACCES (Permission denied)
open("/dev/console", O_RDONLY)          = -1 EACCES (Permission denied)
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(0, KDGKBTYPE, 0x7eb5f14b)         = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7eb5f0cc) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(2, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7eb5f0cc) = -1 ENOTTY (Inappropriate ioctl for device)
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fcntl64(3, F_GETFD)                     = 0x1 (flags FD_CLOEXEC)
fstat64(3, {st_mode=S_IFREG|0644, st_size=2492, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f8e000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2492
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x76f8e000, 4096)                = 0
open("/usr/share/locale/zh_CN.UTF-8/LC_MESSAGES/kbd.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/zh_CN.utf8/LC_MESSAGES/kbd.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/zh_CN/LC_MESSAGES/kbd.mo", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=30170, ...}) = 0
mmap2(NULL, 30170, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f87000
close(3)                                = 0
open("/usr/lib/arm-linux-gnueabihf/gconv/gconv-modules.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=26262, ...}) = 0
mmap2(NULL, 26262, PROT_READ, MAP_SHARED, 3, 0) = 0x76f48000
close(3)                                = 0
write(2, "\346\227\240\346\263\225\350\216\267\345\217\226\346\214\207\345\220\221\346\216\247\345\210\266\345\217\260\347\232\204\346\226"..., 46無法獲取指向控制臺的檔案描述符
) = 46
exit_group(1)                           = ?
+++ exited with 1 +++

brk (line 53) 之前的輸出都不用看，屬于程式初始化的邏輯；值得注意的報錯點在 line 70 與 79：

open("/dev/console", O_RDWR)            = -1 EACCES (Permission denied)

這兩個地方錯誤相似，都是打開 tty 設備時沒有權限，而且是試了三種權限都失敗了，分別為：讀寫 (O_RDWR)、只寫 (O_WRONLY)、只讀 (O_RDONLY)，看來為了打開這個設備，open 也是盡力了呀～

為什么好端端的會沒有權限呢？讓我們看下 ls 的輸出：

$ ls -lh /dev/console
crw------- 1 root root 5, 1 Aug  8 22:37 /dev/console

這個設備是 root 創建、root 擁有、且沒有給其它用戶開放任何權限，難怪會加載失敗，

解決方案

既然根因是權限導致的，那就從權限入手來解決，之前系統總結過 linux 檔案權限的規則 (參考《[apue] linux 檔案訪問權限那些事兒》)，要將 root 創建的檔案分享給當前用戶訪問，也不是什么難事，

console group

第一個冒出來的想法，就是創建一個新的用戶組 (例如 console)，把 root 和當前用戶 pi 都加入到這個組中，然后指定設備的所有組為 console，并開放適當的組權限，這樣就可以實作共享啦，有的人可能覺得麻煩，直接把出錯的 tty 設備 chown 到當前用戶不就行了？對于普通的資料檔案，我也經常這樣搞，但是對于公共的、系統相關的檔案，我勸大家還是不要這么自信，否則可能搞到開不了機 (個人慘痛遭遇就不展開了)，另外這樣做還有個額外的好處，就是當新用戶遇到了同樣問題時，只要把他加入這個組 (console) 就搞定了！下面是對應的腳本：

groups
sudo groupadd console
usermod -a -G console pi
groups
su
groups
usermod -a -G console root
groups
chgrp console /dev/console
chmod g+rw /dev/console
exit

夾在 usermod 前后的 groups 命令用來顯示當前用戶所屬的用戶組，用以驗證添加附加組是否成功 (實際上無效，見后面說明)，注意對 root 帳戶的操作需要使用 su 切換，切換后順便對它擁有的檔案 (/dev/console) 進行操作，以避免權限上的錯誤，最后執行完成后 exit 退回到當前用戶，這里有兩個點需要注意：

usermod -a 選項必需和 -G 配合使用，之前我單獨使用 -a 時 usemod 會直接列印 Usage，查了下 man 才弄明白；
將用戶添加到一個組后 groups 不能立即看到結果，必需重新登錄才會生效，特別對于 VNC 登錄，logout 選單似乎不起作用，需要重啟設備才行，

覺得敲命令麻煩的同學，也可以直接進入 /etc/group 檔案進行修改：

$ tail /etc/group
avahi:x:110:
ntp:x:111:
ssh:x:112:
bluetooth:x:113:
spi:x:999:pi
i2c:x:998:pi
gpio:x:997:pi
lightdm:x:114:
lpadmin:x:115:
console:x:1001:pi,root

最后一行就是了，下面是執行成功后新的 ls 的輸出：

# ls -lh /dev/console
crw-rw---- 1 root console 5, 1 Aug  8 22:37 /dev/console

此時再運行一次 strace open，發現針對 console 的訪問成功了！

open("/dev/console", O_RDWR)            = 3

雖然因為其它 tty 沒有修改仍然出錯了，但是至少證明方向是對的，

tty group

上面的方案是可行的，在查看 tty 設備時無意間得到了下面的輸出：

$ ls -lh /dev/tty[0-9]*
crw------- 1 root root 4,  0 Aug  8 23:51 /dev/tty0
crw------- 1 pi   tty  4,  1 Aug  8 23:52 /dev/tty1
crw------- 1 root tty  4, 10 Aug  8 23:51 /dev/tty10
crw------- 1 root tty  4, 11 Aug  8 23:51 /dev/tty11
crw------- 1 root tty  4, 12 Aug  8 23:51 /dev/tty12
crw------- 1 root tty  4, 13 Aug  8 23:51 /dev/tty13
crw------- 1 root tty  4, 14 Aug  8 23:51 /dev/tty14
crw------- 1 root tty  4, 15 Aug  8 23:51 /dev/tty15
crw------- 1 root tty  4, 16 Aug  8 23:51 /dev/tty16
crw------- 1 root tty  4, 17 Aug  8 23:51 /dev/tty17
crw------- 1 root tty  4, 18 Aug  8 23:51 /dev/tty18
crw------- 1 root tty  4, 19 Aug  8 23:51 /dev/tty19
crw------- 1 root root 4,  2 Aug  8 23:51 /dev/tty2
crw------- 1 root tty  4, 20 Aug  8 23:51 /dev/tty20
crw------- 1 root tty  4, 21 Aug  8 23:51 /dev/tty21
crw------- 1 root tty  4, 22 Aug  8 23:51 /dev/tty22
crw------- 1 root tty  4, 23 Aug  8 23:51 /dev/tty23
crw------- 1 root tty  4, 24 Aug  8 23:51 /dev/tty24
crw------- 1 root tty  4, 25 Aug  8 23:51 /dev/tty25
crw------- 1 root tty  4, 26 Aug  8 23:51 /dev/tty26
crw------- 1 root tty  4, 27 Aug  8 23:51 /dev/tty27
crw------- 1 root tty  4, 28 Aug  8 23:51 /dev/tty28
crw------- 1 root tty  4, 29 Aug  8 23:51 /dev/tty29
crw------- 1 root root 4,  3 Aug  8 23:51 /dev/tty3
crw------- 1 root tty  4, 30 Aug  8 23:51 /dev/tty30
crw------- 1 root tty  4, 31 Aug  8 23:51 /dev/tty31
crw------- 1 root tty  4, 32 Aug  8 23:51 /dev/tty32
crw------- 1 root tty  4, 33 Aug  8 23:51 /dev/tty33
crw------- 1 root tty  4, 34 Aug  8 23:51 /dev/tty34
crw------- 1 root tty  4, 35 Aug  8 23:51 /dev/tty35
crw------- 1 root tty  4, 36 Aug  8 23:51 /dev/tty36
crw------- 1 root tty  4, 37 Aug  8 23:51 /dev/tty37
crw------- 1 root tty  4, 38 Aug  8 23:51 /dev/tty38
crw------- 1 root tty  4, 39 Aug  8 23:51 /dev/tty39
crw------- 1 root tty  4,  4 Aug  8 23:51 /dev/tty4
crw------- 1 root tty  4, 40 Aug  8 23:51 /dev/tty40
crw------- 1 root tty  4, 41 Aug  8 23:51 /dev/tty41
crw------- 1 root tty  4, 42 Aug  8 23:51 /dev/tty42
crw------- 1 root tty  4, 43 Aug  8 23:51 /dev/tty43
crw------- 1 root tty  4, 44 Aug  8 23:51 /dev/tty44
crw------- 1 root tty  4, 45 Aug  8 23:51 /dev/tty45
crw------- 1 root tty  4, 46 Aug  8 23:51 /dev/tty46
crw------- 1 root tty  4, 47 Aug  8 23:51 /dev/tty47
crw------- 1 root tty  4, 48 Aug  8 23:51 /dev/tty48
crw------- 1 root tty  4, 49 Aug  8 23:51 /dev/tty49
crw------- 1 root tty  4,  5 Aug  8 23:51 /dev/tty5
crw------- 1 root tty  4, 50 Aug  8 23:51 /dev/tty50
crw------- 1 root tty  4, 51 Aug  8 23:51 /dev/tty51
crw------- 1 root tty  4, 52 Aug  8 23:51 /dev/tty52
crw------- 1 root tty  4, 53 Aug  8 23:51 /dev/tty53
crw------- 1 root tty  4, 54 Aug  8 23:51 /dev/tty54
crw------- 1 root tty  4, 55 Aug  8 23:51 /dev/tty55
crw------- 1 root tty  4, 56 Aug  8 23:51 /dev/tty56
crw------- 1 root tty  4, 57 Aug  8 23:51 /dev/tty57
crw------- 1 root tty  4, 58 Aug  8 23:51 /dev/tty58
crw------- 1 root tty  4, 59 Aug  8 23:51 /dev/tty59
crw------- 1 root tty  4,  6 Aug  8 23:51 /dev/tty6
crw------- 1 root tty  4, 60 Aug  8 23:51 /dev/tty60
crw------- 1 root tty  4, 61 Aug  8 23:51 /dev/tty61
crw------- 1 root tty  4, 62 Aug  8 23:51 /dev/tty62
crw------- 1 root tty  4, 63 Aug  8 23:51 /dev/tty63
crw------- 1 root tty  4,  7 Aug  8 23:51 /dev/tty7
crw------- 1 root tty  4,  8 Aug  8 23:51 /dev/tty8
crw------- 1 root tty  4,  9 Aug  8 23:51 /dev/tty9

除了 tty0 / tty2 / tty3 的用戶組屬于 root 外，其它 tty 設備都屬于 tty 用戶組，原來系統早為它們準備了現成的組，我在上面創建 console 組就顯得畫蛇添足了，而且觀察用戶 pi 所屬的附加組：

$ groups
pi adm tty dialout cdrom sudo audio video plugdev games users input netdev gpio i2c spi console

其中一個就是 tty，所以我從善如流的決定使用使用 tty 了，因此上節的腳本可以簡化為：

su
chgrp tty /dev/console /dev/tty[0-9]*
chmod g+rw /dev/console /dev/tty[0-9]*
exit

什么創建用戶組、為用戶添加附加組統統可以不要了，只需要修正 tty 設備檔案的用戶組就可以啦，

group permission

重啟設備讓修改生效后，再運行一次 strace open，發現還是有問題：

$ strace open ./fit.png
execve("/bin/open", ["open", "./fit.png"], [/* 36 vars */]) = 0
brk(0)                                  = 0xe6b000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f95000
access("/etc/ld.so.preload", R_OK)      = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76f94000
close(3)                                = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76f53000
mprotect(0x76f58000, 61440, PROT_NONE)  = 0
mmap2(0x76f67000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76f67000
mprotect(0x7e817000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3)                                = 0
munmap(0x76f94000, 42)                  = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f40000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f94000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76dff000
mprotect(0x76f2a000, 65536, PROT_NONE)  = 0
mmap2(0x76f3a000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76f3a000
mmap2(0x76f3d000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76f3d000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f93000
set_tls(0x76f934c0, 0x76f93ba8, 0x76f98050, 0x76f934c0, 0x76f98050) = 0
mprotect(0x76f3a000, 8192, PROT_READ)   = 0
mprotect(0x76f53000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76f53000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76f53000, 0x76f58000, 0, 0x15, 0x7e8170b0) = 0
mprotect(0x22000, 4096, PROT_READ)      = 0
mprotect(0x76f97000, 4096, PROT_READ)   = 0
munmap(0x76f40000, 76981)               = 0
brk(0)                                  = 0xe6b000
brk(0xe8c000)                           = 0xe8c000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76bff000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x76a5b000
close(3)                                = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3755, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3813, ...}) = 0
open("/proc/self/fd/0", O_RDWR)         = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty", O_RDWR)                = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty0", O_RDWR)               = -1 EACCES (Permission denied)
open("/dev/tty0", O_WRONLY)             = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 -opost -isig -icanon -echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b)         = 0
ioctl(3, VT_GETSTATE, 0x7e8171a8)       = 0
ioctl(3, VIDIOC_QUERYCAP or VT_OPENQRY, 0x7e8171a4) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x76f93068) = 1882
exit_group(0)                           = ?
+++ exited with 0 +++
open: Unable to open /dev/tty2: 權限不夠

line 70 有一次失敗，但是馬上 line 71 對同一個設備操作成功，末尾出現的權限不夠失敗對應的 /dev/tty2 檔案根本在前面沒有出現過，amazing～

用 ls 大法看一看：

$ ls -lh /dev/tty[0-9]*
crw--w---- 1 root tty 4,  0 Aug  9 00:13 /dev/tty0
crw------- 1 pi   tty 4,  1 Aug  9 00:14 /dev/tty1
crw--w---- 1 root tty 4, 10 Aug  9 00:13 /dev/tty10
crw--w---- 1 root tty 4, 11 Aug  9 00:13 /dev/tty11
crw--w---- 1 root tty 4, 12 Aug  9 00:13 /dev/tty12
crw--w---- 1 root tty 4, 13 Aug  9 00:13 /dev/tty13
crw--w---- 1 root tty 4, 14 Aug  9 00:13 /dev/tty14
crw--w---- 1 root tty 4, 15 Aug  9 00:13 /dev/tty15
crw--w---- 1 root tty 4, 16 Aug  9 00:13 /dev/tty16
crw--w---- 1 root tty 4, 17 Aug  9 00:13 /dev/tty17
crw--w---- 1 root tty 4, 18 Aug  9 00:13 /dev/tty18
crw--w---- 1 root tty 4, 19 Aug  9 00:13 /dev/tty19
crw--w---- 1 root tty 4,  2 Aug  9 00:13 /dev/tty2
crw--w---- 1 root tty 4, 20 Aug  9 00:13 /dev/tty20
crw--w---- 1 root tty 4, 21 Aug  9 00:13 /dev/tty21
crw--w---- 1 root tty 4, 22 Aug  9 00:13 /dev/tty22
crw--w---- 1 root tty 4, 23 Aug  9 00:13 /dev/tty23
crw--w---- 1 root tty 4, 24 Aug  9 00:13 /dev/tty24
crw--w---- 1 root tty 4, 25 Aug  9 00:13 /dev/tty25
crw--w---- 1 root tty 4, 26 Aug  9 00:13 /dev/tty26
crw--w---- 1 root tty 4, 27 Aug  9 00:13 /dev/tty27
crw--w---- 1 root tty 4, 28 Aug  9 00:13 /dev/tty28
crw--w---- 1 root tty 4, 29 Aug  9 00:13 /dev/tty29
crw--w---- 1 root tty 4,  3 Aug  9 00:13 /dev/tty3
crw--w---- 1 root tty 4, 30 Aug  9 00:13 /dev/tty30
crw--w---- 1 root tty 4, 31 Aug  9 00:13 /dev/tty31
crw--w---- 1 root tty 4, 32 Aug  9 00:13 /dev/tty32
crw--w---- 1 root tty 4, 33 Aug  9 00:13 /dev/tty33
crw--w---- 1 root tty 4, 34 Aug  9 00:13 /dev/tty34
crw--w---- 1 root tty 4, 35 Aug  9 00:13 /dev/tty35
crw--w---- 1 root tty 4, 36 Aug  9 00:13 /dev/tty36
crw--w---- 1 root tty 4, 37 Aug  9 00:13 /dev/tty37
crw--w---- 1 root tty 4, 38 Aug  9 00:13 /dev/tty38
crw--w---- 1 root tty 4, 39 Aug  9 00:13 /dev/tty39
crw--w---- 1 root tty 4,  4 Aug  9 00:13 /dev/tty4
crw--w---- 1 root tty 4, 40 Aug  9 00:13 /dev/tty40
crw--w---- 1 root tty 4, 41 Aug  9 00:13 /dev/tty41
crw--w---- 1 root tty 4, 42 Aug  9 00:13 /dev/tty42
crw--w---- 1 root tty 4, 43 Aug  9 00:13 /dev/tty43
crw--w---- 1 root tty 4, 44 Aug  9 00:13 /dev/tty44
crw--w---- 1 root tty 4, 45 Aug  9 00:13 /dev/tty45
crw--w---- 1 root tty 4, 46 Aug  9 00:13 /dev/tty46
crw--w---- 1 root tty 4, 47 Aug  9 00:13 /dev/tty47
crw--w---- 1 root tty 4, 48 Aug  9 00:13 /dev/tty48
crw--w---- 1 root tty 4, 49 Aug  9 00:13 /dev/tty49
crw--w---- 1 root tty 4,  5 Aug  9 00:13 /dev/tty5
crw--w---- 1 root tty 4, 50 Aug  9 00:13 /dev/tty50
crw--w---- 1 root tty 4, 51 Aug  9 00:13 /dev/tty51
crw--w---- 1 root tty 4, 52 Aug  9 00:13 /dev/tty52
crw--w---- 1 root tty 4, 53 Aug  9 00:13 /dev/tty53
crw--w---- 1 root tty 4, 54 Aug  9 00:13 /dev/tty54
crw--w---- 1 root tty 4, 55 Aug  9 00:13 /dev/tty55
crw--w---- 1 root tty 4, 56 Aug  9 00:13 /dev/tty56
crw--w---- 1 root tty 4, 57 Aug  9 00:13 /dev/tty57
crw--w---- 1 root tty 4, 58 Aug  9 00:13 /dev/tty58
crw--w---- 1 root tty 4, 59 Aug  9 00:13 /dev/tty59
crw--w---- 1 root tty 4,  6 Aug  9 00:13 /dev/tty6
crw--w---- 1 root tty 4, 60 Aug  9 00:13 /dev/tty60
crw--w---- 1 root tty 4, 61 Aug  9 00:13 /dev/tty61
crw--w---- 1 root tty 4, 62 Aug  9 00:13 /dev/tty62
crw--w---- 1 root tty 4, 63 Aug  9 00:13 /dev/tty63
crw--w---- 1 root tty 4,  7 Aug  9 00:13 /dev/tty7
crw--w---- 1 root tty 4,  8 Aug  9 00:13 /dev/tty8
crw--w---- 1 root tty 4,  9 Aug  9 00:13 /dev/tty9

所有 tty 設定的組權限變成了只寫？難道是上一步中重啟設備前我忘了設定各個檔案的權限? 用 root 帳號 chmod 一下，輸出這次正常了：

$ strace open ./fit.png
execve("/bin/open", ["open", "./fit.png"], [/* 36 vars */]) = 0
brk(0)                                  = 0x288000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efd000
access("/etc/ld.so.preload", R_OK)      = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76efc000
close(3)                                = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76ebb000
mprotect(0x76ec0000, 61440, PROT_NONE)  = 0
mmap2(0x76ecf000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76ecf000
mprotect(0x7ea80000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3)                                = 0
munmap(0x76efc000, 42)                  = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76ea8000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efc000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76d67000
mprotect(0x76e92000, 65536, PROT_NONE)  = 0
mmap2(0x76ea2000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76ea2000
mmap2(0x76ea5000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76ea5000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efb000
set_tls(0x76efb4c0, 0x76efbba8, 0x76f00050, 0x76efb4c0, 0x76f00050) = 0
mprotect(0x76ea2000, 8192, PROT_READ)   = 0
mprotect(0x76ebb000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76ebb000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76ebb000, 0x76ec0000, 0, 0x15, 0x7ea800b0) = 0
mprotect(0x22000, 4096, PROT_READ)      = 0
mprotect(0x76eff000, 4096, PROT_READ)   = 0
munmap(0x76ea8000, 76981)               = 0
brk(0)                                  = 0x288000
brk(0x2a9000)                           = 0x2a9000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76b67000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x769c3000
close(3)                                = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3755, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3813, ...}) = 0
open("/proc/self/fd/0", O_RDWR)         = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty", O_RDWR)                = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b)         = -1 ENOTTY (Inappropriate ioctl for device)
close(3)                                = 0
open("/dev/tty0", O_RDWR)               = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 -opost -isig -icanon -echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b)         = 0
ioctl(3, VT_GETSTATE, 0x7ea801a8)       = 0
ioctl(3, VIDIOC_QUERYCAP or VT_OPENQRY, 0x7ea801a4) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x76efb068) = 2013
exit_group(0)                           = ?
+++ exited with 0 +++

不過再次重啟后，這個檔案權限仍然會被重置，之前使用 console group 時，也發現重啟設備后 /dev/console 檔案的用戶組會自動重置為 root，組權限重置為空，好在這里只是將權限重置為只寫，難道每次登錄都要設定一遍 tty 檔案權限？

xdg-open

答案是 no，倒不是因為找到了解決方案，而是即使在權限正確的情況下、open 不再報錯了，圖片還是沒有自動打開 (汗)，于是我決定試試其它的命令，經過一番百度，找到了一個在 Ubuntu 下很常用的 xdg-open 命令，引數是要打開的圖片路徑，呼叫方式和 open 一樣，話說回來，open 是我在 mac 上使用的命令，不適用 raspberrypi 是情理之中的，至于這個上的 open 是干啥的，我沒找到對應的 man 記錄 (還是個三無命令)，另外一開始如果使用 xdg-open，是不是就不用設定設備檔案的用戶組和權限了？沒有試，不得而知，