ranlib

ranlib archive_file...

ranlib is a utility used to create or update an index in an archive (static library) file. Static libraries, typically with a .a extension, are collections of object files (.o) that the linker can use to resolve external symbol references. The index generated by ranlib is a table of contents within the archive that lists the global symbols defined by the object files it contains. When the linker (ld) needs to find a symbol, it can consult this index to quickly locate the relevant object file within the library, significantly speeding up the linking process for large libraries. Without an index, the linker would have to scan every object file in the archive sequentially, which can be inefficient. On most Linux systems, ranlib is commonly a hard link to the ar (archiver) command, specifically executing the equivalent of ar -s (create a symbol table) or ar -ts (list and create symbol table). Therefore, its primary function is to ensure that a static library is properly indexed for efficient linking.

ranlib's behavior is highly dependent on the underlying ar implementation. On GNU/Linux systems, it's typically a hard link to ar and behaves as ar -s. This means it accepts archive files as arguments but no specific ranlib options.

The ranlib command does not modify the object files themselves; it only adds or updates a symbol table within the archive file.

For archives that contain object files compiled for different architectures (e.g., in cross-compilation environments), it's crucial to use the ranlib (or ar) from the correct toolchain to ensure the index is generated correctly for the target architecture.

An indexed static library significantly speeds up the linking process. Without an index, the linker would need to perform a linear scan of all object files within the archive to resolve symbol dependencies. For large libraries, this can be extremely time-consuming. ranlib creates a lookup table, allowing the linker to directly jump to the relevant object file.

Running ranlib multiple times on the same archive is generally safe and idempotent. If an index already exists, ranlib will update it; if it doesn't, it will create it. It does not corrupt existing data or unnecessarily bloat the archive.

The concept of indexing archive files for faster linking has existed since the early days of Unix. ranlib emerged as a dedicated utility to perform this indexing operation, simplifying the command for users. While its core function remains consistent, its implementation has evolved. On many modern systems, particularly those using GNU binutils, ranlib is no longer a separate executable but rather a symbolic or hard link to the ar command, which then executes its internal s (symbol table) function when invoked as ranlib. This consolidation streamlines maintenance and ensures consistency between ar and ranlib's indexing capabilities.

ar(1), ld(1), nm(1)