Diagnosing Common TeX Compilation Errors: Missing Packages, Font Issues, Capacity Limits, and Macro Loops

Recognizable Symptoms

When a TeX document fails to compile, the log file often contains one of the following patterns:

• ! LaTeX Error: File `.sty` not found.
• Font shape ... undefined or Using ... instead.
• ! TeX capacity exceeded, sorry [main memory size=...].
• ! TeX capacity exceeded, sorry [input stack size=...] with repetitive lines.

Cause and Diagnostic Table

Symptom	Likely Cause	Quick Diagnostic
Missing package error	Package not installed or name misspelled	Search log for the missing file; run kpsewhich package.sty or tlmgr info package
Font substitution warnings	Requested font unavailable for the current engine	Check engine (pdfTeX, XeTeX, LuaTeX); list available fonts with fc-list (XeTeX/LuaTeX) or inspect map files for pdfTeX
Main memory exceeded	Deep macro nesting, large tables, excessive token use	Look at memory usage numbers in the log; simplify macros or reduce table size
Input stack exceeded with repeating lines	Macro calls itself without termination	Observe repeated patterns in the log; enable \tracingmacros=1 to see expansion

Ordered Checks

1. Examine the .log file for the exact error message and note any surrounding context.
2. Identify which symptom matches the table above.
3. Run the corresponding diagnostic command:
   • For missing packages: kpsewhich pkg.sty (replace pkg with the reported name).
   • For font issues: determine engine with pdftex --version, xetex --version, or lualatex --version; then list fonts.
   • For capacity errors: note the memory numbers shown in the log.
   • For recursion: look for repeated lines; optionally add \tracingmacros=1 to the preamble.
4. Interpret the output: if the file is not found, the package is missing; if the font list lacks the requested name, the font is unavailable; if memory usage is near the limit, capacity is exceeded; if the trace shows endless expansion, recursion is present.

Fixes Tied to Findings

• Missing package: Install via your TeX distribution manager. Example (TeX Live): sudo tlmgr install pkg. For MiKTeX use the admin console or mpm --install=pkg. Verify by re-running kpsewhich pkg.sty.
• Font substitution: Switch to an engine that supports the font (XeTeX or LuaTeX) or install the font in the system font directory. For pdfTeX, ensure the font map entry exists; run updmap-sys after adding a map file. Confirm by checking that the log no longer contains “Font shape … undefined”.
• Main memory exceeded: Simplify the document: break large tables into smaller parts, replace deep macro recursion with iterative loops, or increase the memory limit. To raise the limit, edit texmf.cnf (locate via kpsewhich texmf.cnf) and increase main_memory (e.g., from 3000000 to 5000000). Then run fmtutil-sys --all to rebuild formats. Note: this requires write access to the TeX tree; on multi-user systems ask an administrator.
• Input stack exceeded (recursion): Locate the macro definition causing the loop. Add a terminating condition or rewrite using \def with a counter. Test with \tracingmacros=1 to see each expansion; comment out the macro temporarily to confirm the error disappears. After fixing, remove the tracing line.

Escalation Criteria

If after applying the appropriate fix the error persists, consider the following steps:

• Create a minimal working example (MWE) that isolates the suspected cause; if the MWE still fails, the issue may be deeper in the installation.
• Check for version mismatches: run tex --version (or engine-specific) and compare with the distribution’s release notes; outdated formats can cause strange errors.
• Reinstall the TeX distribution or update all packages with tlmgr update --self --all (TeX Live) or the MiKTeX console.
• Seek help from the TeX Stack Exchange community, providing the full log excerpt, the MWE, and the output of the diagnostic commands.

Limitations and Verification

The guidance assumes a standard TeX Live or MiKTeX installation on a Unix-like system; Windows paths differ but the commands are analogous. Engine-specific primitives (e.g., \XeTeXinterchartoks) behave only in XeTeX/LuaTeX, so font-related fixes may not apply to pdfTeX. To verify that a fix succeeded, recompile the original document (or the MWE) and confirm that the specific error message no longer appears in the log and that the output PDF matches expectations.