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copied!<p>You should write an abstract data type called <code>bitstream</code> for this purpose. It could have the following interface:</p> <p>This is file <code>bitstream.h</code>:</p> <pre> #ifndef BITSTREAM_H #define BITSTREAM_H typedef struct bitstream_impl bitstream; struct bitstream_impl; /** * Creates a bit stream and allocates memory for it. Later, that memory * must be freed by calling bitstream_free(). */ extern bitstream *bitstream_new(); /** * Writes the lower 'bits' bits from the 'value' into the stream, starting with * the least significand bit ("little endian"). * * Returns nonzero if the writing was successful, and zero if the writing failed. */ extern int bitstream_writebits_le(bitstream *bs, unsigned int value, unsigned int bits); /** * Writes the lower 'bits' bits from the 'value' into the stream, starting with * the most significand bit ("big endian"). * * Returns nonzero if the writing was successful, and zero if the writing failed. */ extern int bitstream_writebits_be(bitstream *bs, unsigned int value, unsigned int bits); /** * Returns a pointer to the buffer of the bitstream. * * The returned pointer remains valid until the next time that one of the * bitstream_write* functions is called. The returned buffer must not be * modified. All bits in the buffer that have not yet been written are zero. * (This applies only to the last byte of the buffer.) Each byte of the buffer * contains at most 8 bits of data, even if CHAR_BITS is larger. */ extern unsigned char *bitstream_getbuffer(const bitstream *bs); /** * Returns the number of bits that have been written to the stream so far. */ extern unsigned int bitstream_getsize(const bitstream *bs); /** * Frees all the memory that is associated with this bitstream. */ extern void bitstream_free(bitstream *bs); #endif </pre> <p>Then you need to write an implementation for this interface. It should be in a file called <code>bitstream.c</code>. This is left as an excercise.</p> <p>To use the bitstream should then be pretty simple:</p> <pre> #include "bitstream.h" static void die(const char *msg) { perror(msg); exit(EXIT_FAILURE); } int main(void) { bitstream *bs; unsigned char *buf; bs = bitstream_new(); if (bs == NULL) die("bitstream_new"); if (!bitstream_writebits_be(bs, 0x000b, 5)) die("write 5 bits"); if (!bitstream_writebits_be(bs, 0x0005, 3)) die("write 3 bits"); if (bitstream_getsize(bs) != 8) die("FAIL: didn't write exactly 8 bits."); buf = bitstream_getbuffer(bs); if (buf[0] != 0x005dU) die("FAIL: didn't write the expected bits."); bitstream_free(bs); return 0; } </pre>

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