EPERM

EPERM, short for operation not permitted, is an errno value used by POSIX-compliant operating systems to indicate that a requested operation is not allowed by the system policy or by the privileges currently held by the process. It signals that the operation itself is disallowed, even if the target resource could be accessed under different circumstances.

Common situations include attempting to change a file’s ownership or permissions without sufficient privileges, or trying to perform privileged actions such as setting a process’s user ID, mounting a device, or modifying kernel-level settings when not running with appropriate capabilities. EPERM is reported when the kernel blocks an operation for policy reasons rather than simply due to insufficient access to a specific resource. Security modules, such as SELinux or AppArmor, or capability-based restrictions can also cause EPERM.

EPERM is distinct from EACCES, which indicates that access to a resource is denied due to its mode or ACLs. EPERM means the operation itself is not permitted by policy regardless of file permissions. In practice, a program may receive EPERM when attempting an action that is not allowed by the process’s privileges or by system policy.

In the C language and many POSIX interfaces, EPERM is defined in errno.h and is part of the standard errno set. On many systems it has the numeric value 1, though the exact value is system-specific; the symbolic constant EPERM should be used for portability.

Related errno values include EACCES, ENOENT, EINVAL, and EISDIR. The meaning of EPERM can vary with the platform and security configuration, so consulting system-specific documentation is advisable when debugging permission-related errors.