What does errno 56 (EDEADLOCK) mean?

EDEADLOCK

File locking deadlock compatibility code

On Solaris, EDEADLOCK is errno value 56. The base message is file locking deadlock error, but the useful diagnostic context is narrower than the short message: Solaris reports a file-locking deadlock using this older compatibility name.

Use the Solaris-specific errno table first, then compare with POSIX only for portable behavior. Some Solaris values share names with other UNIX systems but have different numbers or more specific historical meanings.

What to check

• Treat it like a lock-ordering failure, not as a storage or permission problem.
• Capture the file, byte range, process ids, and lock acquisition order.
• Prefer portable handling that also recognizes EDEADLK.

References

• Oracle Solaris intro(2): error numbers
• illumos errno.h definitions
• POSIX errno.h

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