High-Level C API

The libzfp C API provides functionality for sequentially compressing and decompressing whole integer and floating-point arrays or single blocks. It is broken down into a high-level API and a low-level API. The high-level API handles compression of entire arrays and supports a variety of back-ends (e.g., serial, OpenMP). The low-level API exists for processing individual, possibly partial blocks as well as reduced-precision integer data less than 32 bits wide. Both C APIs are declared in zfp.h.

The following sections are available:



Macros identifying the zfp library version (major.minor.patch.tweak). ZFP_VERSION_TWEAK is new as of zfp 1.0.0 and is used to mark intermediate develop versions (unofficial releases).


Macro signifying that the current version is an intermediate version that differs from the last official release. This macro is undefined for official releases; when defined, its value equals 1. Note that this macro may be defined even if the four version identifiers have not changed. Available as of zfp 1.0.0.


A single integer constructed from the four version identifiers. This integer can be generated by ZFP_MAKE_VERSION or ZFP_MAKE_FULLVERSION. Its value equals the global constant zfp_library_version.


Although ZFP_VERSION increases monotonically with release date and with the four version identifiers it depends on, the mapping to ZFP_VERSION changed with the introduction of ZFP_VERSION_TWEAK in zfp 1.0.0.

Going forward, we recommend using ZFP_MAKE_VERSION or ZFP_MAKE_FULLVERSION in conditional code that depends on ZFP_VERSION, e.g., #if ZFP_VERSION >= ZFP_MAKE_VERSION(1, 0, 0). Note that such constructions should not be used with older versions of zfp, e.g., if (zfp_library_version == ZFP_MAKE_VERSION(0, 5, 5)) will not give the expected result with binary versions of libzfp before version 1.0.0.


ZFP_VERSION_STRING is a string literal composed of the four version identifiers. It is a component of zfp_version_string.

ZFP_MAKE_VERSION(major, minor, patch)
ZFP_MAKE_VERSION_STRING(major, minor, patch)

Utility macros for constructing ZFP_VERSION and ZFP_VERSION_STRING, respectively. Available as of zfp 1.0.0, these macros may be used by applications to test for a certain zfp version number, e.g., #if ZFP_VERSION >= ZFP_MAKE_VERSION(1, 0, 0).

ZFP_MAKE_FULLVERSION(major, minor, patch, tweak)
ZFP_MAKE_FULLVERSION_STRING(major, minor, patch, tweak)

Utility macros for constructing ZFP_VERSION and ZFP_VERSION_STRING, respectively. Includes tweak version used by intermediate develop versions. Available as of zfp 1.0.0, these macros may be used by applications to test for a certain zfp version number, e.g., #if ZFP_VERSION >= ZFP_MAKE_FULLVERSION(1, 0, 0, 2).


Macro identifying the version of the compression CODEC. See also zfp_codec_version.


Default compression parameter settings that impose no constraints. The largest possible compressed block size, corresponding to 4D blocks of doubles, is given by ZFP_MAX_BITS. See also zfp_stream.


Null representation of the 52-bit encoding of field metadata. This value is returned by zfp_field_metadata() when the field metadata cannot be encoded in 64 bits, such as when the array dimensions are too large (see Limitations). In addition to signaling error, this value is guaranteed not to represent valid metadata.

The ZFP_HEADER bit mask specifies which portions of a header to output (if any). The constants below should be bitwise ORed together. Use ZFP_HEADER_FULL to output all header information available. The compressor and decompressor must agree on which parts of the header to read/write. See zfp_read_header() and zfp_write_header() for how to read and write header information.


Magic constant that identifies the data as a zfp stream compressed using a particular CODEC version.


Array size and scalar type information stored in the zfp_field struct.


Compression mode and parameters stored in the zfp_stream struct.


Full header information (bitwise OR of all ZFP_HEADER constants).


Number of bits used by each portion of the header. These macros are primarily informational and should not be accessed by the user through the high-level API. For most common compression parameter settings, only ZFP_MODE_SHORT_BITS bits of header information are stored to encode the mode (see zfp_stream_mode()).

The ZFP_DATA bit mask specifies which portions of array data structures to compute total storage size for. These constants should be bitwise ORed together. Use ZFP_DATA_ALL to count all storage used.


Allocated but unused data.


Padding for alignment purposes.


Class members and other fixed-size storage.


Miscellaneous uncategorized storage.


Compressed data encoding array elements.


Block index information.


Uncompressed cached data.


Header information.


All storage (bitwise OR of all ZFP_DATA constants).


Available rounding modes for ZFP_ROUNDING_MODE, which specifies at build time how zfp performs rounding in lossy compression mode.


type zfp_stream

The zfp_stream struct encapsulates all information about the compressed stream for a single block or a collection of blocks that represent an array. See the section on compression modes for a description of the members of this struct.

typedef struct {
  uint minbits;       // minimum number of bits to store per block
  uint maxbits;       // maximum number of bits to store per block
  uint maxprec;       // maximum number of bit planes to store
  int minexp;         // minimum floating point bit plane number to store
  bitstream* stream;  // compressed bit stream
  zfp_execution exec; // execution policy and parameters
} zfp_stream;

type zfp_execution

The zfp_stream also stores information about how to execute compression, e.g., sequentially or in parallel. The execution is determined by the policy and any policy-specific parameters such as number of threads.

typedef struct {
  zfp_exec_policy policy; // execution policy (serial, omp, cuda, ...)
  void* params;           // execution parameters
} zfp_execution;


As of zfp 1.0.0 zfp_execution replaces the former zfp_exec_params with a void* to the associated zfp_exec_params type (e.g., zfp_exec_params_omp) to limit ABI-breaking changes due to future extensions to zfp execution policies.

type zfp_exec_policy

Currently three execution policies are available: serial, OpenMP parallel, and CUDA parallel.

typedef enum {
  zfp_exec_serial = 0, // serial execution (default)
  zfp_exec_omp    = 1, // OpenMP multi-threaded execution
  zfp_exec_cuda   = 2  // CUDA parallel execution
} zfp_exec_policy;

type zfp_exec_params_omp

Execution parameters for OpenMP parallel compression. These are initialized to default values. When nonzero, they indicate the number of threads to request for parallel compression and the number of consecutive blocks to assign to each thread.

typedef struct {
  uint threads;    // number of requested threads
  uint chunk_size; // number of blocks per chunk
} zfp_exec_params_omp;

type zfp_mode

Enumerates the compression modes.

typedef enum {
  zfp_mode_null            = 0, // an invalid configuration of the 4 params
  zfp_mode_expert          = 1, // expert mode (4 params set manually)
  zfp_mode_fixed_rate      = 2, // fixed rate mode
  zfp_mode_fixed_precision = 3, // fixed precision mode
  zfp_mode_fixed_accuracy  = 4, // fixed accuracy mode
  zfp_mode_reversible      = 5  // reversible (lossless) mode
} zfp_mode;

type zfp_config

Encapsulates compression mode and parameters (if any).

typedef struct {
  zfp_mode mode;      // compression mode */
  union {
    double rate;      // compressed bits/value (negative for word alignment)
    uint precision;   // uncompressed bits/value
    double tolerance; // absolute error tolerance
    struct {
      uint minbits;   // min number of compressed bits/block
      uint maxbits;   // max number of compressed bits/block
      uint maxprec;   // max number of uncompressed bits/value
      int minexp;     // min floating point bit plane number to store
    } expert;         // expert mode arguments
  } arg;              // arguments corresponding to compression mode
} zfp_config;

type zfp_type

Enumerates the scalar types supported by the compressor and describes the uncompressed array. The compressor and decompressor must use the same zfp_type, e.g., one cannot compress doubles and decompress to floats or integers.

typedef enum {
  zfp_type_none   = 0, // unspecified type
  zfp_type_int32  = 1, // 32-bit signed integer
  zfp_type_int64  = 2, // 64-bit signed integer
  zfp_type_float  = 3, // single precision floating point
  zfp_type_double = 4  // double precision floating point
} zfp_type;

type zfp_field

The uncompressed array is described by the zfp_field struct, which encodes the array’s scalar type, dimensions, and memory layout.

typedef struct {
  zfp_type type;            // scalar type (e.g., int32, double)
  size_t nx, ny, nz, nw;    // sizes (zero for unused dimensions)
  ptrdiff_t sx, sy, sz, sw; // strides (zero for contiguous array a[nw][nz][ny][nx])
  void* data;               // pointer to array data
} zfp_field;

For example, a static multidimensional C array declared as

double array[n1][n2][n3][n4];

would be described by a zfp_field with members

type = zfp_type_double;
nx = n4; ny = n3; nz = n2; nw = n1;
sx = 1; sy = n4; sz = n3 * n4; sw = n2 * n3 * n4;
data = &array[0][0][0][0];

The strides, when nonzero, specify how the array is laid out in memory. Strides can be used in case multiple fields are stored interleaved via “array of struct” (AoS) rather than “struct of array” (SoA) storage, or if the dimensions should be transposed during (de)compression. Strides may even be negative, allowing one or more dimensions to be traversed in reverse order. Given 4D array indices (x, y, z, w), the corresponding array element is stored at

data[x * sx + y * sy + z * sz + w * sw]

where data is a pointer to the first array element.


The zfp_field struct was modified in zfp 1.0.0 to use size_t and ptrdiff_t for array dimensions and strides, respectively, to support 64-bit addressing of very large arrays (previously, uint and int were used). This ABI incompatible change may require rebuilding applications that use zfp and may in some cases also require code changes to handle pointers to size_t instead of pointers to uint (see zfp_field_size(), for instance).


It is paramount that the field dimensions, nx, ny, nz, and nw, and strides, sx, sy, sz, and sw, be correctly mapped to how the uncompressed array is laid out in memory. Although compression will still succeed if array dimensions are accidentally transposed, compression ratio and/or accuracy may suffer greatly. Since the leftmost index, x, is assumed to vary fastest, zfp can be thought of as assuming Fortran ordering. For C ordered arrays, the user should transpose the dimensions or specify strides to properly describe the memory layout. See this FAQ for further details.

type zfp_bool

zfp_bool is new as of zfp 1.0.0. Although merely an alias for int, this type serves to document that a return value or function parameter should be treated as Boolean. Two enumerated constants are available:

enum {
  zfp_false = 0,
  zfp_true = !zfp_false

The reason why zfp_bool is not an enumerated type itself is that in C++ this would require an explicit cast between the bool type resulting from logical expressions, e.g., zfp_bool done = static_cast<zfp_bool>(queue.empty() && work == 0). Such casts from bool to a non-enumerated int are not necessary.

The zfp 1.0.0 API has changed to use zfp_bool in place of int where appropriate; this change should not affect existing code.


const uint zfp_codec_version

The version of the compression CODEC implemented by this version of the zfp library. The library can decompress files generated by the same CODEC only. To ensure that the zfp.h header matches the binary library linked to, zfp_codec_version should match ZFP_CODEC.

const uint zfp_library_version

The library version. The binary library and headers are compatible if zfp_library_version matches ZFP_VERSION.

const char *const zfp_version_string

A constant string representing the zfp library version and release date. One can search for this string in executables and libraries that link to libzfp when built as a static library.


size_t zfp_type_size(zfp_type type)

Return byte size of the given scalar type, e.g., zfp_type_size(zfp_type_float) = 4.

Compressed Stream

zfp_stream *zfp_stream_open(bitstream *stream)

Allocate compressed stream and associate it with bit stream for reading and writing bits to/from memory. stream may be NULL and attached later via zfp_stream_set_bit_stream().

void zfp_stream_close(zfp_stream *stream)

Close and deallocate compressed stream. This does not affect the attached bit stream.

void zfp_stream_rewind(zfp_stream *stream)

Rewind bit stream to beginning for compression or decompression.

bitstream *zfp_stream_bit_stream(const zfp_stream *stream)

Return bit stream associated with compressed stream (see zfp_stream_set_bit_stream()).

void zfp_stream_set_bit_stream(zfp_stream *stream, bitstream *bs)

Associate bit stream with compressed stream.

size_t zfp_stream_compressed_size(const zfp_stream *stream)

Number of bytes of compressed storage. This function returns the current byte offset within the bit stream from the beginning of the bit stream memory buffer. To ensure all buffered compressed data has been output call zfp_stream_flush() first.

size_t zfp_stream_maximum_size(const zfp_stream *stream, const zfp_field *field)

Conservative estimate of the compressed byte size for the compression parameters stored in stream and the array whose scalar type and dimensions are given by field. This function may be used to determine how large a memory buffer to allocate to safely hold the entire compressed array. The buffer may then be resized (using realloc()) after the actual number of bytes is known, as returned by zfp_compress().

Compression Parameters

zfp_mode zfp_stream_compression_mode(const zfp_stream *stream)

Return compression mode associated with compression parameters. Return zfp_mode_null when compression parameters are invalid.

void zfp_stream_set_reversible(zfp_stream *stream)

Enable reversible (lossless) compression.

double zfp_stream_rate(const zfp_stream *stream, uint dims)

Return rate in compressed bits per value if stream is in fixed-rate mode (see zfp_stream_set_rate()), else zero. dims is the dimensionality of the compressed data.

double zfp_stream_set_rate(zfp_stream *stream, double rate, zfp_type type, uint dims, zfp_bool align)

Set rate for fixed-rate mode in compressed bits per value. The target scalar type and array dimensionality are needed to correctly translate the rate to the number of bits per block. The Boolean align should be zfp_true if word alignment is needed, e.g., to support random access writes of blocks for zfp’s compressed arrays. Such alignment may further constrain the rate. The closest supported rate is returned, which may differ from the requested rate.

uint zfp_stream_precision(const zfp_stream *stream)

Return precision in uncompressed bits per value if stream is in fixed-precision mode (see zfp_stream_set_precision()), else zero.

uint zfp_stream_set_precision(zfp_stream *stream, uint precision)

Set precision for fixed-precision mode. The precision specifies how many uncompressed bits per value to store, and indirectly governs the relative error. The actual precision is returned, e.g., in case the desired precision is out of range. To preserve a certain floating-point mantissa or integer precision in the decompressed data, see FAQ #21.

double zfp_stream_accuracy(const zfp_stream *stream)

Return accuracy as an absolute error tolerance if stream is in fixed-accuracy mode (see zfp_stream_set_accuracy()), else zero.

double zfp_stream_set_accuracy(zfp_stream *stream, double tolerance)

Set absolute error tolerance for fixed-accuracy mode. The tolerance ensures that values in the decompressed array differ from the input array by no more than this tolerance (in all but exceptional circumstances; see FAQ #17). This compression mode should be used only with floating-point (not integer) data.

uint64 zfp_stream_mode(const zfp_stream *stream)

Return compact encoding of compression parameters. If the return value is no larger than ZFP_MODE_SHORT_MAX, then the least significant ZFP_MODE_SHORT_BITS (12 in the current version) suffice to encode the parameters. Otherwise all 64 bits are needed, and the low ZFP_MODE_SHORT_BITS bits will be all ones. Thus, this variable-length encoding can be used to economically encode and decode the compression parameters, which is especially important if the parameters are to vary spatially over small regions. Such spatially adaptive coding would have to be implemented via the low-level API.

zfp_mode zfp_stream_set_mode(zfp_stream *stream, uint64 mode)

Set all compression parameters from compact integer representation. See zfp_stream_mode() for how to encode the parameters. Return the mode associated with the newly-set compression parameters. If the decoded compression parameters are invalid, they are not set and the function returns zfp_mode_null.

void zfp_stream_params(const zfp_stream *stream, uint *minbits, uint *maxbits, uint *maxprec, int *minexp)

Query compression parameters. For any parameter not needed, pass NULL for the corresponding pointer.

zfp_bool zfp_stream_set_params(zfp_stream *stream, uint minbits, uint maxbits, uint maxprec, int minexp)

Set all compression parameters directly. See the section on expert mode for a discussion of the parameters. The return value is zfp_true upon success.

Execution Policy

zfp_exec_policy zfp_stream_execution(const zfp_stream *stream)

Return current execution policy.

uint zfp_stream_omp_threads(const zfp_stream *stream)

Return number of OpenMP threads to request for compression. See zfp_stream_set_omp_threads().

uint zfp_stream_omp_chunk_size(const zfp_stream *stream)

Return number of blocks to compress together per OpenMP thread. See zfp_stream_set_omp_chunk_size().

zfp_bool zfp_stream_set_execution(zfp_stream *stream, zfp_exec_policy policy)

Set execution policy. If different from the previous policy, initialize the execution parameters to their default values. zfp_true is returned if the execution policy is supported.

zfp_bool zfp_stream_set_omp_threads(zfp_stream *stream, uint threads)

Set the number of OpenMP threads to use during compression. If threads is zero, then the number of threads is given by the value of the OpenMP nthreads-var internal control variable when zfp_compress() is called (usually the maximum number available). This function also sets the execution policy to OpenMP. Upon success, zfp_true is returned.

zfp_bool zfp_stream_set_omp_chunk_size(zfp_stream *stream, uint chunk_size)

Set the number of consecutive blocks to compress together per OpenMP thread. If zero, use one chunk per thread. This function also sets the execution policy to OpenMP. Upon success, zfp_true is returned.

Compression Configuration

These functions encode a desired compression mode and associated parameters (if any) in a single struct, e.g., for configuring zfp’s read-only array classes.

zfp_config zfp_config_none()

Unspecified configuration.

zfp_config zfp_config_rate(double rate, zfp_bool align)

Fixed-rate mode using rate compressed bits per value. When align is true, word alignment is enforced to further constrain the rate (see zfp_stream_set_rate()).

zfp_config zfp_config_precision(uint precision)

Fixed-precision mode using precision uncompressed bits per value (see also zfp_stream_set_precision()).

zfp_config zfp_config_accuracy(double tolerance)

Fixed-accuracy mode with absolute error no larger than tolerance (see also zfp_stream_set_accuracy()).

zfp_config zfp_config_reversible()

Reversible (lossless) mode (see also zfp_stream_set_reversible()).

zfp_config zfp_config_expert(uint minbits, uint maxbits, uint maxprec, int minexp)

Expert mode with given parameters (see also zfp_stream_set_params()).

Array Metadata

zfp_field *zfp_field_alloc()

Allocates and returns a default initialized zfp_field struct. The caller must free this struct using zfp_field_free().

zfp_field *zfp_field_1d(void *pointer, zfp_type type, size_t nx)

Allocate and return a field struct that describes an existing 1D array, a[nx], of nx uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

zfp_field *zfp_field_2d(void *pointer, zfp_type type, size_t nx, size_t ny)

Allocate and return a field struct that describes an existing 2D array, a[ny][nx], of nx × ny uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

zfp_field *zfp_field_3d(void *pointer, zfp_type type, size_t nx, size_t ny, size_t nz)

Allocate and return a field struct that describes an existing 3D array, a[nz][ny][nx], of nx × ny × nz uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

zfp_field *zfp_field_4d(void *pointer, zfp_type type, size_t nx, size_t ny, size_t nz, size_t nw)

Allocate and return a field struct that describes an existing 4D array, a[nw][nz][ny][nx], of nx × ny × nz × nw uncompressed scalars of given type stored at pointer, which may be NULL and specified later.

void zfp_field_free(zfp_field *field)

Free zfp_field struct previously allocated by one of the functions above.

void *zfp_field_pointer(const zfp_field *field)

Return pointer to the first scalar in the field with index x = y = z = w = 0.

void *zfp_field_begin(const zfp_field *field)

Return pointer to the lowest memory address occupied by the field. Equals zfp_field_pointer() if all strides are positive. Available since zfp 1.0.0.

zfp_type zfp_field_type(const zfp_field *field)

Return array scalar type.

uint zfp_field_precision(const zfp_field *field)

Return scalar precision in number of bits, e.g., 32 for zfp_type_float.

uint zfp_field_dimensionality(const zfp_field *field)

Return array dimensionality (1, 2, 3, or 4).

size_t zfp_field_size(const zfp_field *field, size_t *size)

Return total number of scalars stored in the array, e.g., nx × ny × nz for a 3D array. If size is not NULL, then store the number of scalars for each dimension, e.g., size[0] = nx; size[1] = ny; size[2] = nz for a 3D array.

size_t zfp_field_size_bytes(const zfp_field *field)

Return number of bytes spanned by the field payload data. This includes gaps in memory in case the field layout, as given by the strides, is not contiguous (see zfp_field_is_contiguous()). Available since zfp 1.0.0.

size_t zfp_field_blocks(const zfp_field *field)

Return total number of d-dimensional blocks (whether partial or whole) spanning the array. Each whole block consists of 4d scalars. Available since zfp 1.0.0.

zfp_bool zfp_field_stride(const zfp_field *field, ptrdiff_t *stride)

Return zfp_false if the array is stored contiguously as a[nx], a[ny][nx], a[nz][ny][nx], or a[nw][nz][ny][nx] depending on dimensionality. Return zfp_true if the array is strided and laid out differently in memory. If stride is not NULL, then store the stride for each dimension, e.g., stride[0] = sx; stride[1] = sy; stride[2] = sz; for a 3D array. See zfp_field for more information on strides. Return false if the array is stored contiguously (the default) as a[nx], a[ny][nx], a[nz][ny][nx], or a[nw][nz][ny][nx] depending on dimensionality. Return true if nonzero strides have been specified.

zfp_bool zfp_field_is_contiguous(const zfp_field *field)

Return true if the field occupies a contiguous portion of memory. Note that the field layout may be contiguous even if a raster order traversal does not visit memory in a monotonically increasing or decreasing order, e.g., if the layout is simply a permutation of the default layout. Available since zfp 1.0.0.

uint64 zfp_field_metadata(const zfp_field *field)

Return 52-bit compact encoding of the scalar type and array dimensions. This function returns ZFP_META_NULL on failure, e.g., if the array dimensions are too large to be encoded in 52 bits.

void zfp_field_set_pointer(zfp_field *field, void *pointer)

Set pointer to first scalar in the array.

zfp_type zfp_field_set_type(zfp_field *field, zfp_type type)

Set array scalar type.

void zfp_field_set_size_1d(zfp_field *field, size_t nx)

Specify dimensions of 1D array a[nx].

void zfp_field_set_size_2d(zfp_field *field, size_t nx, size_t ny)

Specify dimensions of 2D array a[ny][nx].

void zfp_field_set_size_3d(zfp_field *field, size_t nx, size_t ny, size_t nz)

Specify dimensions of 3D array a[nz][ny][nx].

void zfp_field_set_size_4d(zfp_field *field, size_t nx, size_t ny, size_t nz, size_t nw)

Specify dimensions of 4D array a[nw][nz][ny][nx].

void zfp_field_set_stride_1d(zfp_field *field, ptrdiff_t sx)

Specify stride for 1D array: sx = &a[1] - &a[0].

void zfp_field_set_stride_2d(zfp_field *field, ptrdiff_t sx, ptrdiff_t sy)

Specify strides for 2D array: sx = &a[0][1] - &a[0][0]; sy = &a[1][0] - &a[0][0].

void zfp_field_set_stride_3d(zfp_field *field, ptrdiff_t sx, ptrdiff_t sy, ptrdiff_t sz)

Specify strides for 3D array: sx = &a[0][0][1] - &a[0][0][0]; sy = &a[0][1][0] - &a[0][0][0]; sz = &a[1][0][0] - &a[0][0][0].

void zfp_field_set_stride_4d(zfp_field *field, ptrdiff_t sx, ptrdiff_t sy, ptrdiff_t sz, ptrdiff_t sw)

Specify strides for 4D array: sx = &a[0][0][0][1] - &a[0][0][0][0]; sy = &a[0][0][1][0] - &a[0][0][0][0]; sz = &a[0][1][0][0] - &a[0][0][0][0]; sw = &a[1][0][0][0] - &a[0][0][0][0].

zfp_bool zfp_field_set_metadata(zfp_field *field, uint64 meta)

Specify array scalar type and dimensions from compact 52-bit representation. Return zfp_true upon success. See zfp_field_metadata() for how to encode meta.

Compression and Decompression

size_t zfp_compress(zfp_stream *stream, const zfp_field *field)

Compress the whole array described by field using parameters given by stream. Then flush the stream to emit any buffered bits and align the stream on a word boundary. The resulting byte offset within the bit stream is returned, which equals the total number of bytes of compressed storage if the stream was rewound before the zfp_compress() call. Zero is returned if compression failed.

size_t zfp_decompress(zfp_stream *stream, zfp_field *field)

Decompress from stream to array described by field and align the stream on the next word boundary. Upon success, the nonzero return value is the same as would be returned by a corresponding zfp_compress() call, i.e., the current byte offset or the number of compressed bytes consumed. Zero is returned if decompression failed.

size_t zfp_write_header(zfp_stream *stream, const zfp_field *field, uint mask)

Write an optional variable-length header to the stream that encodes compression parameters, array metadata, etc. The header information written is determined by the bit mask (see macros). Unlike in zfp_compress(), no word alignment is enforced. See the limitations section for limits on the maximum array size supported by the header. The return value is the number of bits written, or zero upon failure.

size_t zfp_read_header(zfp_stream *stream, zfp_field *field, uint mask)

Read header if one was previously written using zfp_write_header(). The stream and field data structures are populated with the information stored in the header, as specified by the bit mask (see macros). The caller must ensure that mask agrees between header read and write calls. The return value is the number of bits read, or zero upon failure.