ftruncatefd

ftruncatefd is a function in POSIX-compliant operating systems that allows a program to truncate or extend a file to a specified size. It takes two arguments: a file descriptor and a length. The file descriptor refers to an open file, and the length specifies the desired new size of the file in bytes.

If the specified length is less than the current size of the file, the file is truncated, and any data beyond the new length is discarded. If the specified length is greater than the current size of the file, the file is extended. In this case, the intervening bytes between the original end of the file and the new end are filled with zeros. This behavior is similar to lseek(2) with the SEEK_DATA or SEEK_HOLE flags, but ftruncatefd operates directly on the file descriptor.

ftruncatefd is useful for various file manipulation tasks, such as resizing log files, pre-allocating space for data, or ensuring a file has a specific size before writing to it. It is important to note that ftruncatefd can lead to data loss if the file is truncated to a size smaller than its original size. The function returns 0 on success and -1 on error, with errno being set to indicate the specific error. Common errors include EBADF if the file descriptor is invalid or an error during the underlying file system operation.