Cameron Katri's Manual Page Server

INTEGERS

Boxed representations of 64-bit wide signed and unsigned integer types may be created with xpc_int64_create() and xpc_uint64_create() respectively. The boxed values may be retrieved using xpc_int64_get_value() and xpc_uint64_get_value().

FLOATING POINT

Boxed representations of double-precision floating point value representations may be created with the xpc_double_create() function and retrieved with the xpc_double_get_value() function.

DATES

Boxed representations of date and time values, expressed as an integral number of nanoseconds before or after the Unix epoch, can be created with the xpc_date_create() function and retrieved with the xpc_date_get_value() function. A date object representing the current date may be created with xpc_date_create_from_current() convenience function.

NULL AND BOOLEAN SINGLETONS

Boxed representations of null and Boolean values are expressed as XPC object singletons. The xpc_bool_create() function returns one of two constant singleton Boolean values:

• XPC_BOOL_TRUE
• XPC_BOOL_FALSE

The singleton values may be compared using direct pointer equality. Similarly, no type checking is required when retrieving these values from collections:

xpc_object_t xbool = xpc_dictionary_get_value(dictionary, "key");
if (xbool == XPC_BOOL_TRUE) {
	// Handle the true case.
} else if (xbool == XPC_BOOL_FALSE)
	// Handle the false case.
} else {
	// Handle the case where there was a type mismatch or where there was no
	// value for the key "key".
}

The xpc_null_create() function returns a constant singleton representation of a null value. There is currently no defined constant for this singleton.

It is safe to call xpc_retain(3) and xpc_release(3) on Boolean and null objects.

DATA

Boxed representations of arbitrary byte values may be created with the xpc_data_create() function which takes a pointer to a buffer and length. A pointer to the underlying storage of the data object may be obtained using xpc_data_get_bytes_ptr().

Important: This pointer is only valid for the lifetime of the data object. The underlying storage of the pointer value must not be modified by the caller. When ARC is enabled, care needs to be taken that the data object is not released prematurely, see xpc_object(3) for details.

The contents of a data object may be copied to an external buffer using the xpc_data_get_bytes() function. This function takes a pointer to a buffer of size length to which the data will be copied. The caller may also specify a non-zero offset into the source data at which to start the copy. The return value of this function is the number of bytes that were copied into the buffer. If the destination buffer is smaller than the size of the source data, as many bytes as possible will be copied and the return value will be equal to the number of bytes specified in length.

The underlying size of the data value may be determined using the xpc_data_get_length() function.

When creating a data object, the contents of the provided buffer are copied into internal storage. If the caller wishes to avoid a copy, the buffer may first be encapsulated in a dispatch_data_t object and passed to xpc_data_create_with_dispatch_data(). See dispatch_data_create(3) for more information.

Note: When the time comes to send a message, the XPC runtime will serialize the object graph, which will result in a copy of any data objects contained therein. This can be very costly for large amounts of data. To completely avoid any copying in the message-send path for large data objects (where "large" is defined by the system), you may create a data object using dispatch_data_create(3) with the DISPATCH_DATA_DESTRUCTOR_MUNMAP destructor specified. This will hint to the system that the data buffer may be safely shared copy-on-write with the recipient of the message.

Important: Data objects created with the intention of eliminating copies can only be safely created VM objects that the caller owns. Buffers returned by malloc(3) do NOT satisfy this condition as the caller does not own the underlying VM object associated with an allocation returned by malloc(3). Similarly, if the caller receives a buffer from an external subsystem across an API boundary, this buffer is not owned by the caller unless part of the API contract specifies how the buffer should have been created. Sending buffers not owned by the caller in this way can result in information leakage from elsewhere on the heap.

STRINGS

Boxed representations of C string values may be created using the xpc_string_create() function. The XPC framework assumes all strings are encoded as UTF-8 and does not support any other encodings. A pointer to the C string representation of a value may be obtained using xpc_string_get_string_ptr().

Important: This pointer is only valid for the lifetime of the string object. The underlying storage of the pointer value must not be modified by the caller. When ARC is enabled, care needs to be taken that the string object is not released prematurely, see xpc_object(3) for details.

The length of the C string value may be determined using the xpc_string_get_length() function. This length does not include the NUL terminator character, similar to strlen(3).

String objects may also be constructed from printf(3) -style format strings using the xpc_string_create_with_format() function. Additionally, the xpc_string_create_with_format_and_arguments() function allows the caller to pass an existing va_list argument with which to construct the formatted string.

UUIDs

Boxed representations of UUID byte values may be created using xpc_uuid_create(). See uuid(3) for more information. A pointer to storage for the underlying UUID value may be obtained using xpc_uuid_get_bytes(). The returned pointer may be safely passed to the relevant uuid(3) functions.

Important: This pointer is only valid for the lifetime of the UUID object. The underlying storage of the UUID value must not be modified by the caller. When ARC is enabled, care needs to be taken that the UUID object is not released prematurely, see xpc_object(3) for details.

The pattern of returning a pointer instead of copying the result into a uuid_t enables some convenient code simplification. For example:

if (uuid_compare(xpc_uuid_get_bytes(uuid_object), expected_uuid) == 0) {
	// They are the same.
}

FILE DESCRIPTORS

Boxed representations of file descriptors may be created using the xpc_fd_create() function. Once created, there is no way to retrieve the original file descriptor from the boxed representation. This function performs the equivalent of a dup(2) on the descriptor, and thus it is safe to call close(2) on the descriptor after boxing it.

The xpc_fd_dup() function can be used to create a new file descriptor from a boxed representation in a similar manner to dup(2). The caller is responsible for calling close(2) on the descriptor returned by xpc_fd_dup().

Multiple calls to xpc_fd_dup() will produce multiple unique file descriptor values. If a failure occurs (i.e. process file descriptor table is full), the invalid file descriptor value -1 will be returned.

Important: When file descriptors are copied with dup(2), they share some state, including flags and lseek(2) file offset. The same sharing obtains when these boxing and unboxing functions create copies of file descriptors, and when a boxed representation of a file descriptor is sent to another process using XPC.

For example, suppose one process opens a file and sends the file descriptor to another process. Then, the first process uses read(2) to read the first 100 bytes from the file. If the second process then calls read(2) on the descriptor that it received from the first process, the data it reads will start at the 101st byte of the file, not the 1st.

This is not usually the desired behavior.

Therefore, if you wish to send a file to another process, it is best not to box one shared file descriptor. Instead you should open(2) the file afresh for each other process you wish to share with.

SHARED MEMORY

Boxed representations of shared memory regions allocated using mmap(2) with the MAP_SHARED flag passed in the flags argument may be created using the xpc_shmem_create() function. Memory objects created using malloc(3) are not supported. The region argument is a pointer to the beginning of the shared region and the length argument specifies the length of the shared region.

The recipient of a shared memory object may map the underlying region into its address space using the xpc_shmem_map() function. As with file descriptor objects, each call to this function returns a distinct but equivalent mapping. On output, the region argument will point to the address of the new mapping, and the return value will be the size of that mapping. This size will always be an integral page size, as it is not possible to share memory regions at less than page granularity. The caller is responsible for unmapping the region with munmap(2). If the mapping operation failed, 0 will be returned.

New mappings will be created with the maximum permission as specified by the creator of the region. Currently, there is no direct way to modify the permissions that the recipient of a region will have. If the caller wishes to maintain read-write permissions to a region, for example, while giving others read-only access, it can create an equivalent mapping with the desired permissions using a combination of mach_make_memory_entry_64() and mach_vm_remap(). The details of this procedure are left as an exercise to the reader.

Certain operations that can operate on subranges of a region, such as vm_copy(), vm_read(), and vm_write(), may fragment the underlying representation of a memory region in order to avoid physical copies. After this fragmentation has occurred, it is not safe to create a shared memory object out of the region. For this reason, it is recommended that any such operations be delayed until after the shared memory object has been created, as the existence of the object will hint to the VM that the region's internal representation should be kept contiguous. Note that this will necessarily defeat these optimizations and force physical copies of subranges.