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Subject[RFC][v6][PATCH 9/9]: Document clone_with_pids() syscall

Subject: [RFC][v6][PATCH 9/9]: Document clone_with_pids() syscall

This gives a brief overview of the clone_with_pids() system call. We should
eventually describe more details either in clone(2) or in a new man page.

Signed-off-by: Sukadev Bhattiprolu <sukadev@vnet.linux.ibm.com>
---
Documentation/clone-with-pids | 58 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 58 insertions(+)
Index: linux-2.6/Documentation/clone-with-pids
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/Documentation/clone-with-pids 2009-09-09 21:53:30.000000000 -0700
@@ -0,0 +1,58 @@
+
+struct pid_set {
+ unsigned int num_pids;
+ pid_t pids[];
+};
+
+clone_with_pids(int flags, void *child_stack_base, int *parent_tid_ptr,
+ int *child_tid_ptr, NULL, struct pid_set *pid_setp)
+
+ The clone_with_pids() system call is identical to clone(), except
+ that it allows the user to specify a pid for the child process
+ in each of the child processes' pid name spaces.
+
+ This system call is meant to be used when restarting an application
+ from an earlier checkpoint. When restarting the application, the
+ processes in the application must get the same pids they had at the
+ time of the checkpoint.
+
+ The 'pid_setp' parameter defines a set of pids to use, one for each
+ pid-namespace of the child process. The order pids in '->pids[]'
+ corresponds to the nesting order of pid-namespaces, with ->pids[0]
+ corresponding to the init_pid_ns.
+
+ If a pid in the ->pids list is 0, the kernel will assign the next
+ available pid in the pid namespace, for the process.
+
+ If a pid in the ->pids[] list is non-zero, the kernel tries to assign
+ the specified pid in that namespace. If that pid is already in use
+ by another process, the system call fails with -EBUSY.
+
+ On success, the system call returns the pid of the child process in
+ the parent's active pid namespace.
+
+ On failure, clone_with_pids() returns -1 and sets 'errno' to one of
+ following values (the child process is not created).
+
+ EPERM Caller does not have the SYS_ADMIN privilege needed to excute
+ this call.
+
+ EINVAL The number of pids specified in 'pid_set.num_pids' exceeds
+ the current nesting level of parent process
+
+ EBUSY A requested 'pid' is in use by another process in that name
+ space.
+
+Example:
+
+ struct pid_set pid_set { 3, {0, 99, 177} };
+ void *child_stack = malloc(STACKSIZE);
+
+ /* set up child_stack, like with clone() */
+ rc = clone_with_pids(clone_flags, child_stack, NULL, NULL, &pid_set);
+
+ if (rc < 0) {
+ perror("clone_with_pids()");
+ exit(1);
+ }
+

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