High-Level C API

The C API is broken down into a high-level API, which handles compression of entire arrays, and a low-level-api for processing individual blocks and managing the underlying bit stream.

The high-level API should be the API of choice for applications that compress and decompress entire arrays. A low-level API exists for processing individual, possibly partial blocks as well as reduced-precision integer data less than 32 bits wide.

The following sections are available:



Macros identifying the zfp library version. ZFP_VERSION is a single integer constructed from the previous three macros. ZFP_VERSION_STRING is a string literal. See also zfp_library_version and zfp_version_string.


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 3D 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.


Bit masks for specifying which portions of a header to output (if any). These constants 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.

ZFP_HEADER_META in essence encodes the information stored in the zfp_field struct, while ZFP_HEADER_MODE encodes the compression parameters stored in the zfp_stream struct. The magic, which includes the CODEC version, can be used to uniquely identify the stream as a zfp stream.

See zfp_read_header() and zfp_write_header() for how to read and write header information.


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_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;

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, ...)
  zfp_exec_params params; // execution parameters
} zfp_execution;

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;

Execution parameters are shared among policies in a union. Currently the only parameters available are for OpenMP.

typedef union {
  zfp_exec_params_omp omp; // OpenMP parameters
} zfp_exec_params;

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 1D blocks to assign to each thread when compressing 1D arrays.

typedef struct {
  uint threads;    // number of requested threads
  uint chunk_size; // number of blocks per chunk (1D only)
} zfp_exec_params_omp;

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;

Enumerates the scalar types supported by the compressor, and is used to describe 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;

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)
  uint nx, ny, nz, nw; // sizes (zero for unused dimensions)
  int 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. 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.


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 discussion for further details.


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 use zfp to determine which version of the library the application was compiled against.


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.

bitstream* zfp_stream_bit_stream(const zfp_stream* stream)

Return bit stream associated with compressed stream.

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.

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.

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.

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.

void zfp_stream_set_bit_stream(zfp_stream* stream, bitstream* bs)

Associate bit stream with compressed stream.

void zfp_stream_rewind(zfp_stream* stream)

Rewind bit stream to beginning for compression or decompression.

Compression Parameters

void zfp_stream_set_reversible(zfp_stream* stream)

Enable reversible (lossless) compression.

double zfp_stream_set_rate(zfp_stream* stream, double rate, zfp_type type, uint dims, int wra)

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 parameter wra should be nonzero if random access writes of blocks into the compressed bit stream is needed, for example for implementing zfp’s compressed arrays. This requires blocks to be aligned on bit stream word boundaries, and therefore constrains the rate. The closest supported rate is returned, which may differ from the desired rate.

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_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.

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.

int 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 nonzero 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().

int 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. Nonzero is returned if the execution policy is supported.

int 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, nonzero is returned.

int 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, nonzero is returned.

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, uint 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, uint nx, uint 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, uint nx, uint ny, uint 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, uint nx, uint ny, uint nz, uint 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 array.

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, uint* 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.

int zfp_field_stride(const zfp_field* field, int* stride)

Return zero 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 nonzero 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.

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, uint nx)

Specify dimensions of 1D array a[nx].

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

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

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

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

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

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

void zfp_field_set_stride_1d(zfp_field* field, int sx)

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

void zfp_field_set_stride_2d(zfp_field* field, int sx, int 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, int sx, int sy, int 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, int sx, int sy, int sz, int 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].

int zfp_field_set_metadata(zfp_field* field, uint64 meta)

Specify array scalar type and dimensions from compact 52-bit representation. Return nonzero 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 and then flush the stream to emit any buffered bits and align the stream on a word boundary. The number of bytes of compressed storage is returned, if the stream were rewound before compression, and otherwise the current byte offset within the bit stream. 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). The return value is the number of bits written, or zero upon failure. Unlike in zfp_compress(), no word alignment is enforced. See the limitations section for limits on the maximum array size supported by the header.

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 return value is the number of bits read, or zero upon failure. The caller must ensure that the bit mask agrees between header read and write calls.