TeX auxiliary file corruption during concurrent compilation

File Access and Atomic Updates in TeX

The TeX engine does not prioritize file access through a queuing or locking system; it relies on the underlying operating system's file handling. Because TeX lacks native file-locking mechanisms, it does not synchronize access when multiple processes attempt to write to the same auxiliary file (such as .aux, .toc, or .out). Consequently, there is no documented method within the engine core to implement atomic updates for these files.

Likely Cause of Corruption

When two or more TeX processes execute simultaneously on the same document, they enter a race condition. Since the engine typically overwrites or appends to auxiliary files without checking for external locks, the resulting file often contains interleaved data or truncated commands. This corruption usually manifests in subsequent runs as Undefined control sequence or Missing item errors, as the engine can no longer parse the malformed auxiliary data.

Resolution: Process Isolation

To prevent corruption during parallel execution, you must isolate the working environment for each process. The most reliable method is to assign a unique output directory to every concurrent build instance.

1. Assign Unique Directories: Create a temporary directory for each process (e.g., build_1/, build_2/).
2. Redirect Output: Use the -output-directory flag to ensure auxiliary files are written to these isolated paths.

# Example for Process A
pdflatex -output-directory=build_a main.tex

# Example for Process B
pdflatex -output-directory=build_b main.tex

Verification

To verify that isolation is working, check the contents of the specified output directories. Each should contain its own independent set of .aux and .log files. If you see these files appearing in the root source directory during a parallel run, the isolation is failing.

Diagnostic Detail Required: Are you using a build automation tool (like latexmk) or a custom CI/CD script to launch these processes? This determines if the output directory needs to be managed via a configuration file or a shell environment variable.