Org mode无法生成LaTeX公式预览图片

最近需要在Cygwin平台下的Emacs Org mode中生成LaTeX数学公式的预览图片，从而得到图文并貌的笔记与任务管理文档。但当我执行org-toggle-latex-fragment命令后却出现了如下错误：

 org-compile-file: File "/tmp/orgtex5473MjV.dvi" wasn’t produced. Please adjust ’dvipng’ part of ‘org-preview-latex-process-alist’.

根据提示判断，应该是变量org-preview-latex-process-alist中dvipng部分的配置出了问题。于是按C-h v查看该变量的值，并参照man手册核对了dvipng的命令行参数，并未发现什么明显的异常。之后，在网上搜索看到有人说是未安装texlive-latex-extra包的问题。然后，便转到/tmp目录下查看LaTeX编译日志文件orgtex5473MjV.log里是否有找不到相关宏包的提示。可奇怪的是，该文件根本不见踪影。这就说明很可能连latex命令的启动都未成功，以至于根本没有进入到实质的编译阶段。接下来，我便手动创建了一个简单的tex文件用latex编译，结果直接报出下面的错误：

 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

错误信息中的error while loading shared libraries给出了明显的提示，那就是缺少了某些动态链接库。然后通过上网搜索，得知可以使用cygcheck命令检查可执行文件对于动态链接库的依赖关系。将其用于latex.exe则得到如下信息：

 $ 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，当前的Cygwin系统缺少了cygcrypto-1.0.0.dll与cygssl-1.0.0.dll两个文件，而且在目录/bin下也确实没有这两个文件：

 $ ls /bin | grep "\(cygcrypto\)\|\(cygssl\)"
 cygcrypto-1.1.dll*
 cygssl-1.1.dll*
 cygssl3.dll*

至此，Emacs Org mode无法生成LaTeX公式预览图片的原因真相大白：并非Org mode或dvipng本身的问题，而是由于Cygwin缺少动态链接库导致的。要想解决这个问题，就需要找到它们。我们使用cygcheck的-p选项，可以查询与动态链接库相关的软件包：

 $ 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，这两个库均与openssl软件包有关。幸好，我之前从镜象服务器下载的缓存文件里有libopenssl100-1.0.2r-1.tar.xz，其中正好包含了上述两个动态链接库。将它们拷贝到C:\cygwin64\bin下，一切就恢复正常了。

从以上问题解决的过程中我学到了如下两条经验：

1. 软件给出的错误信息并不一定反映问题的真相，而有可能是由另一个潜在的或者更深层次的问题导致的。这个时候不能拘泥于表象，要设计新的测试环境来予以探索、验证。

2. 当遇到了异常与错误，需要使用合适的工具来搜集线索、定位问题的源头以至最终将其解决。