EEXIST

EEXIST is an error code used in POSIX-compliant systems to indicate that a target already exists when an operation requires it not to exist. It is exposed as errno.EEXIST in many programming environments, such as C and languages that mirror POSIX errno values.

The numeric value of EEXIST is typically 17 on most Unix-like systems, but the exact value is implementation-defined and may vary across platforms.

Common contexts where EEXIST is returned include attempts to create something at a path that already exists with an exclusive creation flag. For example, opening a file with O_CREAT|O_EXCL when the file already exists often yields EEXIST. Creating a directory with mkdir when a directory already exists at the path can also return EEXIST on many systems. Similarly, attempting to create a hard link or a symbolic link where the destination path is already taken can yield EEXIST, depending on the operation.

EEXIST is distinct from EISDIR, which signals that the existing target is a directory and cannot be used for a given operation. It is also considered in the broader category of I/O errors returned by system calls and libraries.

In practice, programs use EEXIST to implement exclusive or idempotent behavior, to detect conflicts when creating new resources, and to decide whether to reuse an existing resource or report a controlled error to the caller. Cross-language code often checks for EEXIST via the language’s errno or error-handling facilities.