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RedHat 9 (Linux i386) - man page for getsid (redhat section 2)

GETSID(2)						     Linux Programmer's Manual							 GETSID(2)

NAME
getsid - get session ID
SYNOPSIS
#include <unistd.h> pid_t getsid(pid_t pid);
DESCRIPTION
getsid(0) returns the session ID of the calling process. getsid(p) returns the session ID of the process with process ID p. (The session ID of a process is the process group ID of the session leader.) On error, (pid_t) -1 will be returned, and errno is set appropriately.
ERRORS
EPERM A process with process ID p exists, but it is not in the same session as the current process, and the implementation considers this an error. ESRCH No process with process ID p was found.
CONFORMING TO
SVr4, POSIX 1003.1-2001.
NOTES
Linux does not return EPERM. Linux has this system call since Linux 1.3.44. There is libc support since libc 5.2.19. To get the prototype under glibc, define both _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED, or use "#define _XOPEN_SOURCE n" for some integer n larger than or equal to 500.
SEE ALSO
getpgid(2), setsid(2) Linux 2.5.0 2001-12-17 GETSID(2)