ESPIPE

ESPIPE is an errno value used by POSIX and other Unix-like systems to indicate an illegal seek operation. It is raised when a process attempts to reposition the file offset on a non-seekable data stream, such as a pipe, FIFO, or network socket, using functions like lseek or fseeko. Regular files and other seekable objects support repositioning, so lseek on those descriptors succeeds.

In POSIX, ESPIPE is defined as part of the set of error numbers (errno values) and is described by the message "Illegal seek." The exact numeric value varies by system; for example, Linux commonly defines ESPIPE as 29. Some systems may map similar conditions to different codes, but the semantic meaning remains the same: the operation is not supported for the underlying object.

Programmatically, encountering ESPIPE typically means the caller is using a non-seekable descriptor in a context that requires seeking. One cannot reliably move to an arbitrary offset in a pipe or socket, so programs must either avoid seeking, switch to sequential processing, or use alternative mechanisms to manage data position.

See also: ENOTTY, ENXIO, EINVAL, and the general errno mechanism in C and POSIX.