Result Handling

Eventually, the system call returns to the calling process, either successfully or unsuccessfully. On the HP300 architecture, success or failure is returned as the carry bit in the user process’s program status longword: If it is zero, the return was successful; otherwise, it was unsuccessful. On the HP300 and many other machines, return values of C functions are passed back through a general-purpose register (for the HP300, data register 0). The routines in the kernel that implement system calls return the values that are normally associated with the global variable errno. After a system call, the kernel system-call handler leaves this value in the register. If the system call failed, a C library routine moves that value into errno, and sets the return register to -1. The calling process is expected to notice the value of the return register, and then to examine errno. The mechanism involving the carry bit and the global variable errno exists for historical reasons derived from the PDP-11.

There are two kinds of unsuccessful returns from a system call: those where kernel routines discover an error, and those where a system call is interrupted. The most common case is a system call that is interrupted when it has relinquished the processor to wait for an event that may not occur for a long time (such as terminal input), and a signal arrives in the interim. When signal handlers are initialized by a process, they specify whether system calls that they interrupt should be restarted, or whether the system call should return with an interrupted system call (EINTR) error.