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# NpyString API

New in version 2.0.

This API allows access to the UTF-8 string data stored in NumPy StringDType arrays. See NEP-55 for more in-depth details into the design of StringDType.

## Examples

### Loading a String

Say we are writing a ufunc implementation for `StringDType`. If we are given `const char *buf` pointer to the beginning of a `StringDType` array entry, and a `PyArray_Descr *` pointer to the array descriptor, one can access the underlying string data like so:

    npy_string_allocator *allocator = NpyString_acquire_allocator(
            (PyArray_StringDTypeObject *)descr);

    npy_static_string sdata = {0, NULL};
    npy_packed_static_string *packed_string = (npy_packed_static_string *)buf;
    int is_null = 0;

    is_null = NpyString_load(allocator, packed_string, &sdata);

    if (is_null == -1) {
        // failed to load string, set error
        return -1;
    }
    else if (is_null) {
        // handle missing string
        // sdata->buf is NULL
        // sdata->size is 0
    }
    else {
        // sdata->buf is a pointer to the beginning of a string
        // sdata->size is the size of the string
    }
    NpyString_release_allocator(allocator);

### Packing a String

This example shows how to pack a new string entry into an array:

    char *str = "Hello world";
    size_t size = 11;
    npy_packed_static_string *packed_string = (npy_packed_static_string *)buf;

    npy_string_allocator *allocator = NpyString_acquire_allocator(
            (PyArray_StringDTypeObject *)descr);

    // copy contents of str into packed_string
    if (NpyString_pack(allocator, packed_string, str, size) == -1) {
        // string packing failed, set error
        return -1;
    }

    // packed_string contains a copy of "Hello world"

    NpyString_release_allocator(allocator);

## Types

type npy_packed_static_string

  An opaque struct that represents “packed” encoded strings. Individual entries in array buffers are instances of this struct. Direct access to the data in the struct is undefined and future version of the library may change the packed representation of strings.

type npy_static_string

  An unpacked string allowing access to the UTF-8 string data.

      typedef struct npy_unpacked_static_string {
          size_t size;
          const char *buf;
      } npy_static_string;

  size_t size

    The size of the string, in bytes.

  const char *buf

    The string buffer. Holds UTF-8-encoded bytes. Does not currently end in a null string but we may decide to add null termination in the future, so do not rely on the presence or absence of null-termination.

    Note that this is a `const` buffer. If you want to alter an entry in an array, you should create a new string and pack it into the array entry.

type npy_string_allocator

  An opaque pointer to an object that handles string allocation. Before using the allocator, you must acquire the allocator lock and release the lock after you are done interacting with strings managed by the allocator.

type PyArray_StringDTypeObject

  The C struct backing instances of StringDType in Python. Attributes store the settings the object was created with, an instance of `npy_string_allocator` that manages string allocations for arrays associated with the DType instance, and several attributes caching information about the missing string object that is commonly needed in cast and ufunc loop implementations.

      typedef struct {
          PyArray_Descr base;
          PyObject *na_object;
          char coerce;
          char has_nan_na;
          char has_string_na;
          char array_owned;
          npy_static_string default_string;
          npy_static_string na_name;
          npy_string_allocator *allocator;
      } PyArray_StringDTypeObject;

  PyArray_Descr base

    The base object. Use this member to access fields common to all descriptor objects.

  PyObject *na_object

    A reference to the object representing the null value. If there is no null value (the default) this will be NULL.

  char coerce

    1 if string coercion is enabled, 0 otherwise.

  char has_nan_na

    1 if the missing string object (if any) is NaN-like, 0 otherwise.

  char has_string_na

    1 if the missing string object (if any) is a string, 0 otherwise.

  char array_owned

    1 if an array owns the StringDType instance, 0 otherwise.

  npy_static_string default_string

    The default string to use in operations. If the missing string object is a string, this will contain the string data for the missing string.

  npy_static_string na_name

    The name of the missing string object, if any. An empty string otherwise.

  npy_string_allocator allocator

    The allocator instance associated with the array that owns this descriptor instance. The allocator should only be directly accessed after acquiring the allocator_lock and the lock should be released immediately after the allocator is no longer needed

## Functions

npy_string_allocator *NpyString_acquire_allocator(const PyArray_StringDTypeObject *descr)

  Acquire the mutex locking the allocator attached to `descr`. `NpyString_release_allocator` must be called on the allocator returned by this function exactly once. Note that functions requiring the GIL should not be called while the allocator mutex is held, as doing so may cause deadlocks.

void NpyString_acquire_allocators(size_t n_descriptors, PyArray_Descr *const descrs[], npy_string_allocator *allocators[])

  Simultaneously acquire the mutexes locking the allocators attached to multiple descriptors. Writes a pointer to the associated allocator in the allocators array for each StringDType descriptor in the array. If any of the descriptors are not StringDType instances, write NULL to the allocators array for that entry.

  `n_descriptors` is the number of descriptors in the descrs array that should be examined. Any descriptor after `n_descriptors` elements is ignored. A buffer overflow will happen if the `descrs` array does not contain n_descriptors elements.

  If pointers to the same descriptor are passed multiple times, only acquires the allocator mutex once but sets identical allocator pointers appropriately. The allocator mutexes must be released after this function returns, see `NpyString_release_allocators`.

  Note that functions requiring the GIL should not be called while the allocator mutex is held, as doing so may cause deadlocks.

void NpyString_release_allocator(npy_string_allocator *allocator)

  Release the mutex locking an allocator. This must be called exactly once after acquiring the allocator mutex and all operations requiring the allocator are done.

  If you need to release multiple allocators, see NpyString_release_allocators, which can correctly handle releasing the allocator once when given several references to the same allocator.

void NpyString_release_allocators(size_t length, npy_string_allocator *allocators[])

  Release the mutexes locking N allocators. `length` is the length of the allocators array. NULL entries are ignored.

  If pointers to the same allocator are passed multiple times, only releases the allocator mutex once.

int NpyString_load(npy_string_allocator *allocator, const npy_packed_static_string *packed_string, npy_static_string *unpacked_string)

  Extract the packed contents of `packed_string` into `unpacked_string`.

  The `unpacked_string` is a read-only view onto the `packed_string` data and should not be used to modify the string data. If `packed_string` is the null string, sets `unpacked_string.buf` to the NULL pointer. Returns -1 if unpacking the string fails, returns 1 if `packed_string` is the null string, and returns 0 otherwise.

  A useful pattern is to define a stack-allocated npy_static_string instance initialized to `{0, NULL}` and pass a pointer to the stack-allocated unpacked string to this function. This function can be used to simultaneously unpack a string and determine if it is a null string.

int NpyString_pack_null(npy_string_allocator *allocator, npy_packed_static_string *packed_string)

  Pack the null string into `packed_string`. Returns 0 on success and -1 on failure.

int NpyString_pack(npy_string_allocator *allocator, npy_packed_static_string *packed_string, const char *buf, size_t size)

  Copy and pack the first `size` entries of the buffer pointed to by `buf` into the `packed_string`. Returns 0 on success and -1 on failure.