open(pathname,flag,mode) - open file

creat(pathname,mode) - create file

close(filedes) - close an open file

read(filedes,buffer,bytes) - read data from an open file

write(filedes,buffer,bytes) - write data to an open file

lseek(filedes,offset,from) - position an open file

dup(filedes) - duplicate an existing file descriptor

dup2(oldfd,newfd) - duplicate to a desired file descriptor

fcntl(filedes,cmd,arg) - change properties of an open file

ioctl(filedes,request,arg) - change the behaviour of an open file

The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.