ftruncate

ftruncate is a POSIX function that truncates or extends the size of the file associated with a given open file descriptor. It operates on the file described by the descriptor and is declared in unistd.h. The function returns 0 on success and -1 on error, with errno set to indicate the failure.

If the new length is smaller than the current size, the file is truncated to that length. If the new length is larger, the file is extended to that size and the added region is filled with zero bytes. The operation affects the file’s contents up to the specified length; bytes beyond that length are discarded. The file offset is not required to be modified by ftruncate.

The file descriptor passed to ftruncate must refer to a file that supports resizing. It typically cannot be a directory, pipe, socket, or symbolic link opened in a non-resizing mode, and the caller must have appropriate write permissions for the file. On error, ftruncate returns -1 and sets errno to indicate the cause, with common values including EBADF (bad file descriptor), EINVAL (invalid length or unsupported operation), EISDIR (attempt to truncate a directory), EACCES (permissions denied), and EFBIG or ENOSPC (size constraints or space issues).

Prototype and usage: int ftruncate(int fd, off_t length); length is an off_t value representing the new size. ftruncate is widely supported on Unix-like systems; a related function truncate can modify a file by path rather than by descriptor.

See also: truncate, file descriptor manipulation, and related filesystem resizing operations.