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For modern C programming, defining functions is a two step process. First, you declare a function and its signature using a prototype. Then you define the function by providing a function body that defines what the function does.
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cap_enter(2) [freebsd man page]

CAP_ENTER(2)						      BSD System Calls Manual						      CAP_ENTER(2)

NAME
cap_enter, cap_getmode -- Capability mode system calls LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/capsicum.h> int cap_enter(void); int cap_getmode(u_int *modep); DESCRIPTION
cap_enter() places the current process into capability mode, a mode of execution in which processes may only issue system calls operating on file descriptors or reading limited global system state. Access to global name spaces, such as file system or IPC name spaces, is prevented. If the process is already in a capability mode sandbox, the system call is a no-op. Future process descendants created with fork(2) or pdfork(2) will be placed in capability mode from inception. When combined with cap_rights_limit(2), cap_ioctls_limit(2), cap_fcntls_limit(2), cap_enter() may be used to create kernel-enforced sandboxes in which appropriately-crafted applications or application components may be run. cap_getmode() returns a flag indicating whether or not the process is in a capability mode sandbox. CAVEAT
Creating effective process sandboxes is a tricky process that involves identifying the least possible rights required by the process and then passing those rights into the process in a safe manner. Consumers of cap_enter() should also be aware of other inherited rights, such as access to VM resources, memory contents, and other process properties that should be considered. It is advisable to use fexecve(2) to create a runtime environment inside the sandbox that has as few implicitly acquired rights as possible. RETURN VALUES
The cap_enter() and cap_getmode() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The cap_enter() and cap_getmode() system calls will fail if: [ENOSYS] The kernel is compiled without: options CAPABILITY_MODE The cap_getmode() system call may also return the following error: [EFAULT] Pointer modep points outside the process's allocated address space. SEE ALSO
cap_fcntls_limit(2), cap_ioctls_limit(2), cap_rights_limit(2), fexecve(2), cap_sandboxed(3), capsicum(4) HISTORY
Support for capabilities and capabilities mode was developed as part of the TrustedBSD Project. AUTHORS
These functions and the capability facility were created by Robert N. M. Watson at the University of Cambridge Computer Laboratory with sup- port from a grant from Google, Inc. BSD
March 27, 2014 BSD

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CAP_FCNTLS_LIMIT(2)					      BSD System Calls Manual					       CAP_FCNTLS_LIMIT(2)

NAME
cap_fcntls_limit, cap_fcntls_get -- manage allowed fcntl commands LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/capsicum.h> int cap_fcntls_limit(int fd, uint32_t fcntlrights); int cap_fcntls_get(int fd, uint32_t *fcntlrightsp); DESCRIPTION
If a file descriptor is granted the CAP_FCNTL capability right, the list of allowed fcntl(2) commands can be selectively reduced (but never expanded) with the cap_fcntls_limit() system call. A bitmask of allowed fcntls commands for a given file descriptor can be obtained with the cap_fcntls_get() system call. FLAGS
The following flags may be specified in the fcntlrights argument or returned in the fcntlrightsp argument: CAP_FCNTL_GETFL Permit F_GETFL command. CAP_FCNTL_SETFL Permit F_SETFL command. CAP_FCNTL_GETOWN Permit F_GETOWN command. CAP_FCNTL_SETOWN Permit F_SETOWN command. RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
cap_fcntls_limit() succeeds unless: [EBADF] The fd argument is not a valid descriptor. [EINVAL] An invalid flag has been passed in fcntlrights. [ENOTCAPABLE] fcntlrights would expand the list of allowed fcntl(2) commands. cap_fcntls_get() succeeds unless: [EBADF] The fd argument is not a valid descriptor. [EFAULT] The fcntlrightsp argument points at an invalid address. SEE ALSO
cap_ioctls_limit(2), cap_rights_limit(2), fcntl(2) HISTORY
Support for capabilities and capabilities mode was developed as part of the TrustedBSD Project. AUTHORS
This function was created by Pawel Jakub Dawidek <pawel@dawidek.net> under sponsorship of the FreeBSD Foundation. BSD
March 27, 2014 BSD

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