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Full Discussion: System calls
Top Forums Programming System calls Post 302176832 by Perderabo on Wednesday 19th of March 2008 08:57:41 AM
Old 03-19-2008
A system call usually involves a single machine language instruction that behaves very much like an interrupt. Anyone can invoke it. But it jumps to a specific routine in the kernel called syscall. syscall can only do system calls. The jump to syscall also switch into kernel mode. There is not a way to to switch to kernel mode and jump to arbitrary point in the kernel. You must jump to syscall. (At least this is what happens on a reasonably designed computer.) Also, there is no way to switch kernel mode and continue executing user code even if you are the kernel. In kernel mode you must be running code in kernel address space.
 

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GETUID(2)						      BSD System Calls Manual							 GETUID(2)

NAME
getuid, geteuid -- get user identification LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> #include <sys/types.h> uid_t getuid(void); uid_t geteuid(void); DESCRIPTION
The getuid() system call returns the real user ID of the calling process. The geteuid() system call returns the effective user ID of the calling process. The real user ID is that of the user who has invoked the program. As the effective user ID gives the process additional permissions during execution of ``set-user-ID'' mode processes, getuid() is used to determine the real-user-id of the calling process. ERRORS
The getuid() and geteuid() system calls are always successful, and no return value is reserved to indicate an error. SEE ALSO
getgid(2), issetugid(2), setgid(2), setreuid(2), setuid(2) STANDARDS
The geteuid() and getuid() system calls are expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
The getuid() and geteuid() functions appeared in Version 7 AT&T UNIX. BSD
June 4, 1993 BSD
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