errno is a thread-local integer set by system calls and library functions when they fail. A return of -1 from a syscall is paired with errno holding the reason. Each value has a symbolic macro defined in errno.h, like ENOENT (2) or EACCES (13).

Are errno numbers the same on every system?

The symbolic names are standardised by POSIX, but the underlying numbers can differ between operating systems and architectures. On Linux the common numbers shown here apply to most architectures, though a few values differ on MIPS and SPARC. Always rely on the macro name, not the raw number, in portable code.

How do I see errno for a failing command?

Run the program under strace on Linux — each failing syscall line ends with the errno macro and message, for example openat(...) = -1 ENOENT (No such file or directory). In C, read errno right after the failing call and pass it to strerror or perror.

What is the difference between EAGAIN and EWOULDBLOCK?

They mean the same thing — an operation on a non-blocking resource would have to wait — and on Linux they share the same numeric value (11). POSIX allows them to differ, so portable code should check for both, but on Linux they are identical.

Is my lookup uploaded?

No. The errno table ships with the page and all matching is done in your browser.

Linux Error Codes (errno)

Email me this result

Get this tool's output sent to your inbox, plus one useful tool a week. No spam, unsubscribe any time.

Decode a Linux errno without leaving the page

When a system call fails it sets errno, and tools like strace, perror, and log files report the symbolic name (ENOENT, EACCES, ETIMEDOUT) or just the number. This tool turns either form into a clear meaning and a hint about the usual cause, using the standard POSIX/glibc definitions.

How it works

The page carries a table of common errno entries: the number, the symbolic macro (such as ENOENT), the standard description, and a practical hint. A numeric query matches the errno number exactly. A text query matches the macro name and the description case-insensitively, so permission surfaces EACCES and EPERM together. The numbers shown are the values used on most Linux architectures; because POSIX standardises the names rather than the numbers, the macro is the portable identifier and the one your code should compare against.

Tips and notes

In C, read errno immediately after a failing call — almost any later library call can overwrite it — and turn it into text with strerror(errno) or perror("context"). Under the shell, run a failing program with strace -e trace=... to see exactly which syscall failed and with which errno. Watch for the pairs that share a value on Linux: EAGAIN/EWOULDBLOCK (11) and, on some setups, ENOTSUP/EOPNOTSUPP. A few numbers differ on MIPS and SPARC, so never hard-code the integer; compare the macro. This table covers the most common codes; the full glibc errno.h and the errno(3) man page are authoritative.