open_append man page
open_append — open a file for appending
int open_append(const char *filename);
open_append opens the file filename for appending write-only use and returns the file handle. If it does not exist, it will be created with mode 0600. If there was an error opening or creating the file, open_append returns -1 and sets errno accordingly.
All write operations will append after the last byte, regardless of previous calls to lseek(2) or other processes also appending to the same file.