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GNU MP 4.1.2

5.14 Integer Import and Export

mpz_t variables can be converted to and from arbitrary words of binary data with the following functions.

Function: void mpz_import (mpz_t rop, size_t count, int order, int size, int endian, size_t nails, const void *op)

Set rop from an array of word data at op.

The parameters specify the format of the data. count many words are read, each size bytes. order can be 1 for most significant word first or -1 for least significant first. Within each word endian can be 1 for most significant byte first, -1 for least significant first, or 0 for the native endianness of the host CPU. The most significant nails bits of each word are skipped, this can be 0 to use the full words.

There are no data alignment restrictions on op, any address is allowed.

Here's an example converting an array of unsigned long data, most significant element first and host byte order within each value.

unsigned long a[20]; mpz_t z; mpz_import (z, 20, 1, sizeof(a[0]), 0, 0, a);

This example assumes the full sizeof bytes are used for data in the given type, which is usually true, and certainly true for unsigned long everywhere we know of. However on Cray vector systems it may be noted that short and int are always stored in 8 bytes (and with sizeof indicating that) but use only 32 or 46 bits. The nails feature can account for this, by passing for instance 8*sizeof(int)-INT_BIT.

Function: void *mpz_export (void *rop, size_t *count, int order, int size, int endian, size_t nails, mpz_t op)

Fill rop with word data from op.

The parameters specify the format of the data produced. Each word will be size bytes and order can be 1 for most significant word first or -1 for least significant first. Within each word endian can be 1 for most significant byte first, -1 for least significant first, or 0 for the native endianness of the host CPU. The most significant nails bits of each word are unused and set to zero, this can be 0 to produce full words.

The number of words produced is written to *count. rop must have enough space for the data, or if rop is NULL then a result array of the necessary size is allocated using the current GMP allocation function (see section 14. Custom Allocation). In either case the return value is the destination used, rop or the allocated block.

If op is non-zero then the most significant word produced will be non-zero. If op is zero then the count returned will be zero and nothing written to rop. If rop is NULL in this case, no block is allocated, just NULL is returned.

There are no data alignment restrictions on rop, any address is allowed. The sign of op is ignored, just the absolute value is used.

When an application is allocating space itself the required size can be determined with a calculation like the following. Since mpz_sizeinbase always returns at least 1, count here will be at least one, which avoids any portability problems with malloc(0), though if z is zero no space at all is actually needed.

numb = 8*size - nail; count = (mpz_sizeinbase (z, 2) + numb-1) / numb; p = malloc (count * size);