The creat system call on UN*X creates a new file (or truncates an existing old file), opens it for writing, and returns a file descriptor to the file. The file is created with the given mode as modified by the current umask.
int creat(const char *path, mode_t mode);
Nowadays, creat is completely equivalent to opening the file with
O_WRONLY | O_CREAT | O_TRUNC; as such, it is rarely used. (But the interesting name lives on in the flag
The behaviour of creat is atomic. Nothing can occur between the creation of the file and its opening. This can be very useful.
The spelling is quaint. Ken Thompson is said to have said after 20 years of UNIX that, if allowed to change one thing in it, he'd "add an e".