最近需要在Cygwin平台下的Emacs Org mode中生成LaTeX數學公式的預覽圖片，從而得到圖文並貌的筆記與任務管理文檔。但當我執行org-toggle-latex-fragment命令后卻出現了如下錯誤：
org-compile-file: File "/tmp/orgtex5473MjV.dvi" wasn’t produced. Please adjust ’dvipng’ part of ‘org-preview-latex-process-alist’.
根據提示判斷，應該是變量org-preview-latex-process-alist中dvipng部分的配置出了問題。於是按C-h v查看該變量的值，並參照man手冊核對了dvipng的命令行參數，並未發現什么明顯的異常。之后，在網上搜索看到有人說是未安裝texlive-latex-extra包的問題。然后，便轉到/tmp目錄下查看LaTeX編譯日志文件orgtex5473MjV.log里是否有找不到相關宏包的提示。可奇怪的是，該文件根本不見蹤影。這就說明很可能連latex命令的啟動都未成功，以至於根本沒有進入到實質的編譯階段。接下來，我便手動創建了一個簡單的tex文件用latex編譯，結果直接報出下面的錯誤：
Running `LaTeX' on `test' with ``pdflatex -file-line-error -interaction=nonstopmode "\input" test.tex''
C:/cygwin64/bin/pdftex.exe: error while loading shared libraries: ?: cannot open shared object file: No such file or directory
TeX Output exited abnormally with code 127 at Wed Apr 2 20:58:49
錯誤信息中的error while loading shared libraries給出了明顯的提示，那就是缺少了某些動態鏈接庫。然后通過上網搜索，得知可以使用cygcheck命令檢查可執行文件對於動態鏈接庫的依賴關系。將其用於latex.exe則得到如下信息：
$ cygcheck latex.exe
Found: C:\cygwin64\bin\latex.exe
Found: D:\texlive\2016\bin\win32\latex.exe
Found: C:\cygwin64\bin\latex.exe
C:\cygwin64\bin\latex.exe
C:\cygwin64\bin\cygkpathsea-6.dll
C:\cygwin64\bin\cygwin1.dll
C:\Windows\system32\KERNEL32.dll
C:\Windows\system32\API-MS-Win-Core-RtlSupport-L1-1-0.dll
C:\Windows\system32\ntdll.dll
C:\Windows\system32\KERNELBASE.dll
C:\Windows\system32\API-MS-Win-Core-ProcessThreads-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Heap-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Memory-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Handle-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Synch-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-File-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-IO-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-ThreadPool-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-LibraryLoader-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-NamedPipe-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Misc-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-SysInfo-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Localization-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-ProcessEnvironment-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-String-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Debug-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-ErrorHandling-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Fibers-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Util-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Core-Profile-L1-1-0.dll
C:\Windows\system32\API-MS-Win-Security-Base-L1-1-0.dll
C:\cygwin64\bin\cygpng16-16.dll
C:\cygwin64\bin\cygz.dll
C:\cygwin64\bin\cygpoppler-49.dll
C:\cygwin64\bin\cygcurl-4.dll
C:\cygwin64\bin\cyggssapi_krb5-2.dll
C:\cygwin64\bin\cygk5crypto-3.dll
C:\cygwin64\bin\cygkrb5support-0.dll
C:\cygwin64\bin\cygintl-8.dll
C:\cygwin64\bin\cygiconv-2.dll
C:\cygwin64\bin\cygkrb5-3.dll
C:\cygwin64\bin\cygcom_err-2.dll
C:\cygwin64\bin\cyggcc_s-seh-1.dll
C:\cygwin64\bin\cygidn2-0.dll
C:\cygwin64\bin\cygunistring-2.dll
C:\cygwin64\bin\cyglber-2-4-2.dll
C:\cygwin64\bin\cygldap-2-4-2.dll
C:\cygwin64\bin\cygsasl2-3.dll
C:\cygwin64\bin\cygnghttp2-14.dll
C:\cygwin64\bin\cygpsl-5.dll
C:\cygwin64\bin\cygssh2-1.dll
C:\cygwin64\bin\cygfontconfig-1.dll
C:\cygwin64\bin\cygexpat-1.dll
C:\cygwin64\bin\cygfreetype-6.dll
C:\cygwin64\bin\cygbz2-1.dll
C:\cygwin64\bin\cygjpeg-8.dll
C:\cygwin64\bin\cyglcms2-2.dll
C:\cygwin64\bin\cygtiff-6.dll
C:\cygwin64\bin\cygjbig-2.dll
C:\cygwin64\bin\cyglzma-5.dll
C:\cygwin64\bin\cygstdc++-6.dll
cygcheck: track_down: could not find cygcrypto-1.0.0.dll
cygcheck: track_down: could not find cygcrypto-1.0.0.dll
cygcheck: track_down: could not find cygssl-1.0.0.dll
cygcheck: track_down: could not find cygcrypto-1.0.0.dll
cygcheck: track_down: could not find cygssl-1.0.0.dll
可以看出，當前的Cygwin系統缺少了cygcrypto-1.0.0.dll與cygssl-1.0.0.dll兩個文件，而且在目錄/bin下也確實沒有這兩個文件：
$ ls /bin | grep "\(cygcrypto\)\|\(cygssl\)"
cygcrypto-1.1.dll*
cygssl-1.1.dll*
cygssl3.dll*
至此，Emacs Org mode無法生成LaTeX公式預覽圖片的原因真相大白：並非Org mode或dvipng本身的問題，而是由於Cygwin缺少動態鏈接庫導致的。要想解決這個問題，就需要找到它們。我們使用cygcheck的-p選項，可以查詢與動態鏈接庫相關的軟件包：
$ cygcheck --help
-p, --package-query search for REGEXP in the entire cygwin.com package
repository (requires internet connectivity)
$ cygcheck -p cygssl-1.0.0.dll
Found 6 matches for cygssl-1.0.0
libopenssl100-1.0.2p-1 - libopenssl100: A general purpose cryptography toolkit with TLS implementation (runtime)
libopenssl100-1.0.2r-1 - libopenssl100: A general purpose cryptography toolkit with TLS implementation (runtime)
libssl1.0-1.0.2r-2 - libssl1.0: A general purpose cryptography toolkit with TLS implementation (runtime)
openssl-debuginfo-1.0.2p-1 - openssl-debuginfo: Debug info for openssl
openssl-debuginfo-1.0.2r-1 - openssl-debuginfo: Debug info for openssl
openssl10-debuginfo-1.0.2r-2 - openssl10-debuginfo: Debug info for openssl10
$ cygcheck -p cygcrypto-1.0.0.dll
libopenssl100-1.0.2p-1 - libopenssl100: A general purpose cryptography toolkit with TLS implementation (runtime)
libopenssl100-1.0.2r-1 - libopenssl100: A general purpose cryptography toolkit with TLS implementation (runtime)
libssl1.0-1.0.2r-2 - libssl1.0: A general purpose cryptography toolkit with TLS implementation (runtime)
openssl-debuginfo-1.0.2p-1 - openssl-debuginfo: Debug info for openssl
openssl-debuginfo-1.0.2r-1 - openssl-debuginfo: Debug info for openssl
openssl10-debuginfo-1.0.2r-2 - openssl10-debuginfo: Debug info for openssl10
可以看出，這兩個庫均與openssl軟件包有關。幸好，我之前從鏡象服務器下載的緩存文件里有libopenssl100-1.0.2r-1.tar.xz，其中正好包含了上述兩個動態鏈接庫。將它們拷貝到C:\cygwin64\bin下，一切就恢復正常了。
從以上問題解決的過程中我學到了如下兩條經驗：
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軟件給出的錯誤信息並不一定反映問題的真相，而有可能是由另一個潛在的或者更深層次的問題導致的。這個時候不能拘泥於表象，要設計新的測試環境來予以探索、驗證。
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當遇到了異常與錯誤，需要使用合適的工具來搜集線索、定位問題的源頭以至最終將其解決。