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# get_robust_list() - Unix, Linux System Call

# NAME

get_robust_list, set_robust_list - get/set the list of robust futexes

# SYNOPSIS

    #include <linux/futex.h>
    #include <syscall.h>

    long get_robust_list(int pid, struct robust_list_head **head_ptr,
                              size_t *
    long set_robust_list(struct robust_list_head *head, size_t len);

# DESCRIPTION

The robust futex implementation needs to maintain per-thread lists of robust futexes which are unlocked when the thread exits. These lists are managed in user space, the kernel is only notified about the location of the head of the list.

get_robust_list returns the head of the robust futex list of the thread with TID defined by the pid argument. If pid is 0, the returned head belongs to the current thread. head_ptr is the pointer to the head of the list of robust futexes. The get_robust_list function stores the address of the head of the list here. len_ptr is the pointer to the length variable. get_robust_list stores sizeof(**head_ptr) here.

set_robust_list sets the head of the list of robust futexes owned by the current thread to head. len is the size of *head.

# RETURN VALUE

The set_robust_list and get_robust_list functions return zero when the operation is successful, an error code otherwise.

# ERRORS

The set_robust_list function fails with EINVAL if the len value does not match the size of structure struct robust_list_head expected by kernel.

The get_robust_list function fails with EPERM if the current process does not have permission to see the robust futex list of the thread with the TID pid, ESRCH if a thread with the TID pid does not exist, or EFAULT if the head of the robust futex list can’t be stored in the space specified by the head argument.

# APPLICATION USAGE

A thread can have only one robust futex list; therefore applications that wish to use this functionality should use robust mutexes provided by glibc.

The system call is only available for debugging purposes and is not needed for normal operations.

Both system calls are not available to application programs as functions; they can be called using the syscall(3) function.

# SEE ALSO

- futex (2)

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