How Uses Interrupts and Exceptions

Under Linux the execution of a system call is invoked by a maskable interrupt or exception class transfer, caused by the instruction int 0x80. We use vector 0x80 to transfer control to the kernel. This interrupt vector is initialized during system startup, along with other important vectors like the system clock vector.

As of version 0.99.2 of , there are 116 system calls. Documentation for these can be found in the man (2) pages. When a user invokes a system call, execution flow is as follows:

• Each call is vectored through a stub in libc. Each call within the libc library is generally a syscallX() macro, where X is the number of parameters used by the actual routine. Some system calls are more complex then others because of variable length argument lists, but even these complex system calls must use the same entry point: they just have more parameter setup overhead. Examples of a complex system call include open() and ioctl().
• Each syscall macro expands to an assembly routine which sets up the calling stack frame and calls _system_call() through an interrupt, via the instruction int $0x80

  For example, the setuid system call is coded as

  

  Which will expand to:

  

  The macro definition for the syscallX() macros can be found in /usr/include/linux/unistd.h, and the user-space system call library code can be found in /usr/src/libc/syscall/

• At this point no system code for the call has been executed. Not until the int $0x80 is executed does the call transfer to the kernel entry point _system_call(). This entry point is the same for all system calls. It is responsible for saving all registers, checking to make sure a valid system call was invoked and then ultimately transfering control to the actual system call code via the offsets in the _sys_call_table. It is also responsible for calling _ret_from_sys_call() when the system call has been completed, but before returning to user space.

  Actual code for system_call entry point can be found in /usr/src/linux/kernel/sys_call.S Actual code for many of the system calls can be found in /usr/src/linux/kernel/sys.c, and the rest are found elsewhere. find is your friend.

• After the system call has executed, _ret_from_sys_call() is called. It checks to see if the scheduler should be run, and if so, calls it.
• Upon return from the system call, the syscallX() macro code checks for a negative return value, and if there is one, puts a positive copy of the return value in the global variable _errno, so that it can be accessed by code like perror().