C: Split a buffer to a list of segments of a specific size in bits

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The following code will split a buffer in C to a list of segments.
The size of the segments does not have to be a multiple of a byte.
User defines the size of the segments in bits when calling node_t *segment(const unsigned char buffer[], const unsigned int buffer_bytes_size, const unsigned int segment_bit_size, const unsigned int first_segment_bit_size);.

Each segment is an instance of element_t structure as follows:

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struct element_t {
  unsigned char *segment;
  unsigned int unused_bits;
  unsigned int size;
};

Variable unused_bits defines the bits in the last byte that should not be used in future operations.

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Following is the code that performs the segmentation:

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#include "segmentation.h"
 
#include <math.h>
#include <limits.h>
#include <malloc.h>
#include <string.h>
 
//This method will create a string made of 0s and 1s representing the bits in an object.
//It will skip printing the last n bits as per the input
char *create_bit_representation_string(const void *object, const unsigned int size,
                                       const unsigned int skip_last_bits)
{
    unsigned int i = 0;
    const unsigned char *byte;
    unsigned int temp_size = size;
    const double mask_filter = pow(2, skip_last_bits);
    const unsigned int skip_last_bytes = skip_last_bits / CHAR_BIT;
    char *result = malloc(sizeof(char) * size * CHAR_BIT - skip_last_bits + 1);
 
    for (byte = object; temp_size--; ++byte)
    {
        unsigned char mask;
        for (mask = 1 << (CHAR_BIT - 1); mask; mask >>= 1)
        {
            //We do not want to print the last n bits of the last byte as they should always be 0
            if ((temp_size < skip_last_bytes) || (temp_size == 0 && mask < mask_filter))
            {
                break;
            }
            result[i++] = (char) (mask & *byte ? '1' : '0');
        }
    }
 
    result[i] = '\0';
    return result;
}
 
//Creating a mask where the first n bits are 1s and the rest are 0s to zero the unused bits of the segment
unsigned char create_left_mask(const unsigned int bits)
{
 
    unsigned char left_mask = 0;
    unsigned int i;
    for (i = 0; i < bits; i++)
    {
        left_mask |= (1 << (CHAR_BIT - 1 - i));
    }
    return left_mask;
}
 
//This function will shift to the left a char array for up to 7 bits.
//It will update the object and return the number of bits shifted
unsigned int
shift_left_char_array_n_bits(void *object, const unsigned int size, const unsigned int bits)
{
    if (bits == 0)
    {
        return 0;
    }
 
    if (bits < 1 || bits > CHAR_BIT - 1)
    {
        fprintf(stderr, "%s: Bad value %u for 'bits', it should be [1,7]"
                "\n\tIgnoring operation\n", __FUNCTION__, bits);
        return 0;
    }
 
    //Creating a mask where the first n bits are 1s and the rest are 0s.
    const unsigned char left_mask = create_left_mask(bits);
 
    unsigned char *byte;
    unsigned int temp_size = size;
    //We use temp_size as a counter (until it reaches 0) and we move the byte pointer at each loop
    for (byte = object; temp_size--; ++byte)
    {
        unsigned char carry = 0;
        if (temp_size)
        {
            //We get the bits we want to carry using the mask
            carry = byte[1] & left_mask;
            //Then shift them to the right, as this is where they will be in the new byte.
            carry >>= (CHAR_BIT - bits);
        }
        //Shifting the new byte to make space for the carry
        *byte <<= bits;
        //Applying carry
        *byte |= carry;
    }
    return bits;
}
 
const unsigned int calculate_unused_bits(const unsigned int segment_bit_size)
{
    return (CHAR_BIT - (segment_bit_size % CHAR_BIT)) % CHAR_BIT;
}
 
element_t *create_element(const unsigned char buffer[], const unsigned int byte_size,
                          const unsigned int unused_bits, const unsigned int bytes_skipped,
                          const unsigned char left_mask)
{
    element_t *element = (element_t *) malloc(sizeof(element_t));
    element->segment = malloc(byte_size);
    element->size = byte_size;
    element->unused_bits = unused_bits;
    memcpy(element->segment, &(buffer[bytes_skipped]), byte_size);
    //Zeroing the unused bits at the end of the segment
    element->segment[byte_size - 1] &= left_mask;
    return element;
}
 
//This method will split a buffer to segments of specific size in bits and it will return them as a list
//(each element contains the segment data, its size in bytes and the number of bits that are not used from the last byte)
//If the input buffer is less than the segment size, it will return one segment with all the data.
//The user can set the bit size of the first segment to be different than the rest using first_segment_bit_size > 0
node_t *segment(const unsigned char buffer[], const unsigned int buffer_bytes_size,
                const unsigned int segment_bit_size, const unsigned int first_segment_bit_size)
{
    if (buffer_bytes_size == 0)
    {
        fprintf(stderr, "%s: Bad value %u for 'buffer_bytes_size', it should be greater than 0"
                "\n\tIgnoring operation\n", __FUNCTION__, buffer_bytes_size);
        return NULL;
    }
    if (segment_bit_size == 0)
    {
        fprintf(stderr, "%s: Bad value %u for 'segment_bit_size', it should be greater than 0"
                "\n\tIgnoring operation\n", __FUNCTION__, segment_bit_size);
        return NULL;
    }
 
    node_t *head = NULL;
 
    const double char_bit = CHAR_BIT;
    const unsigned int first_segment_byte_size = (unsigned int) ceil(
            first_segment_bit_size / char_bit);
    if (first_segment_byte_size > buffer_bytes_size)
    {
        append(&head, create_element(buffer, buffer_bytes_size, 0, 0, UCHAR_MAX));
        return head;
    }
 
    unsigned char *temp_buffer = malloc(buffer_bytes_size);
    memcpy(temp_buffer, buffer, buffer_bytes_size);
 
    unsigned int bits_shifted = 0;
    unsigned int bytes_skipped = 0;
 
    if (first_segment_bit_size > 0)
    {
        const unsigned int first_segment_unused_bits = calculate_unused_bits(
                first_segment_bit_size);
        const unsigned int first_segment_byte_size_without_incomplete_byte =
                first_segment_bit_size / CHAR_BIT;
 
        const unsigned int first_segment_bits = CHAR_BIT - first_segment_unused_bits;
        const unsigned char left_mask = create_left_mask(first_segment_bits);
 
        append(&head, create_element(temp_buffer, first_segment_byte_size,
                                     first_segment_unused_bits, bytes_skipped, left_mask));
 
        bytes_skipped += first_segment_byte_size_without_incomplete_byte;
 
        if (bytes_skipped == buffer_bytes_size)
        {
            free(temp_buffer);
            return head;
        }
        if (first_segment_bits > 0 && first_segment_bits < CHAR_BIT)
        {
            bits_shifted += shift_left_char_array_n_bits(&(temp_buffer[bytes_skipped]),
                                                         buffer_bytes_size - bytes_skipped -
                                                         (bits_shifted / CHAR_BIT),
                                                         first_segment_bits);
        }
    }
 
    const unsigned int segment_byte_size = (unsigned int) ceil(segment_bit_size / char_bit);
    const unsigned int buffer_bits_size =
            (buffer_bytes_size - bytes_skipped) * CHAR_BIT - bits_shifted;
    const unsigned int segments_count = buffer_bits_size / segment_bit_size;
 
    if (segments_count == 0)
    {
        append(&head, create_element(temp_buffer, buffer_bytes_size - bytes_skipped, bits_shifted, bytes_skipped, UCHAR_MAX));
        free(temp_buffer);
        return head;
    }
 
    //Creating a mask where first n bits are 1s and the rest are 0s to zero the unused bits of the segment
    const unsigned int segment_unused_bits = calculate_unused_bits(segment_bit_size);
    const unsigned int last_segment_bits = CHAR_BIT - segment_unused_bits;
    const unsigned char left_mask = create_left_mask(last_segment_bits);
    const unsigned int segment_byte_size_without_incomplete_byte = segment_bit_size / CHAR_BIT;
    const unsigned int extra_bits = buffer_bits_size % segment_bit_size;
 
    unsigned int i;
    for (i = 0; i < segments_count; i++)
    {
        append(&head,
               create_element(temp_buffer, segment_byte_size, segment_unused_bits, bytes_skipped,
                              left_mask));
        bytes_skipped += segment_byte_size_without_incomplete_byte;
 
        if ((segments_count > 1 || extra_bits > 0) &&
            (last_segment_bits > 0 && last_segment_bits < CHAR_BIT))
        {
            bits_shifted += shift_left_char_array_n_bits(&(temp_buffer[bytes_skipped]),
                                                         buffer_bytes_size - bytes_skipped -
                                                         (bits_shifted / CHAR_BIT),
                                                         last_segment_bits);
        }
    }
 
    if (extra_bits)
    {
        const unsigned int last_segment_bytes_size =
                buffer_bytes_size - bytes_skipped - (bits_shifted / CHAR_BIT);
        const unsigned int unused_bytes_for_last_segment =
                segment_byte_size - last_segment_bytes_size;
        const unsigned int last_segment_unused_bits =
                segment_bit_size - (buffer_bits_size % segment_bit_size) + segment_unused_bits -
                (unused_bytes_for_last_segment * CHAR_BIT);
        append(&head, create_element(temp_buffer, last_segment_bytes_size,
                                     last_segment_unused_bits, bytes_skipped, UCHAR_MAX));
    }
 
    free(temp_buffer);
    return head;
}

Sample code that uses the function:

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#include <stdio.h>
#include <malloc.h>
#include <string.h>
#include <limits.h>
#include <stdlib.h>
#include <time.h>
 
#include "libs/segmentation/segmentation.h"
 
 
// This application will create a char array of size BUFFER_BYTE_SIZE that contains random values
// and later it will split it in segments of size SEGMENT_BIT_SIZE.
// The first segment will be of size FIRST_SEGMENT_BIT_SIZE.
 
#define BUFFER_BYTE_SIZE 420
#define SEGMENT_BIT_SIZE 222
#define FIRST_SEGMENT_BIT_SIZE 11
#define POSSIBLE_VALUES 256
 
int main()
{
    srand(time(NULL));
    const unsigned int buffer_byte_size = BUFFER_BYTE_SIZE;
    fprintf(stdout, "Buffer Size: %uB\n", buffer_byte_size);
    const unsigned int segment_bit_size = SEGMENT_BIT_SIZE;
    fprintf(stdout, "Segment Size: %ub\n", segment_bit_size);
    const unsigned int first_segment_bit_size = FIRST_SEGMENT_BIT_SIZE;
    fprintf(stdout, "First Segment Size: %ub\n", first_segment_bit_size);
    unsigned char buffer[buffer_byte_size];
    unsigned int i;
    for (i = 0; i < buffer_byte_size; i++)
    {
        buffer[i] = (unsigned char) (rand() % POSSIBLE_VALUES);
    }
    char *buffer_bits = create_bit_representation_string(buffer, buffer_byte_size, 0);
    const size_t buffer_length = strlen(buffer_bits);
    fprintf(stdout, "\tBuffer: '%s'\n", buffer_bits);
    node_t *head = segment(buffer, buffer_byte_size, segment_bit_size, first_segment_bit_size);
 
    element_t *element = pop(&head);
    unsigned int bytes_skipped = 0;
    unsigned int segment_count = 0;
    unsigned int total_segment_bit_size = 0;
    while (element != NULL)
    {
 
        char *segment_bits = create_bit_representation_string(element->segment,
                                                              element->size,
                                                              element->unused_bits);
        const size_t segment_length = strlen(segment_bits);
        fprintf(stdout,
               "\t\tSegment %04u: Size in bytes %02u - Unused bits %04u - '%.*s'\n",
               ++segment_count,
               element->size, element->unused_bits,
               element->size * CHAR_BIT - element->unused_bits, segment_bits);
        if (segment_length == 0)
        {
            fprintf(stderr,
                    "Data validation failed."
                            "\n\tBuffer size in bytes %d"
                            "\n\tSegment size in bits %d"
                            "\n\tFirst Segment size in bits %d"
                            "\n\tFound empty segment\n",
                    buffer_byte_size, segment_bit_size, first_segment_bit_size);
            clear(&head);
            free(segment_bits);
            free(element->segment);
            free(element);
            free(buffer_bits);
            return EXIT_FAILURE;
        }
        for (i = 0; i < segment_length && bytes_skipped + i < buffer_length; i++)
        {
            if (segment_bits[i] != buffer_bits[bytes_skipped + i])
            {
                fprintf(stderr,
                        "Data validation failed."
                                "\n\tBuffer size in bytes %d"
                                "\n\tSegment size in bits %d"
                                "\n\tFirst Segment size in bits %d"
                                "\n\tPosition %u of the buffer"
                                "\n\tPosition %u of the segment\n",
                        buffer_byte_size, segment_bit_size, first_segment_bit_size, bytes_skipped + i, i);
                clear(&head);
                free(segment_bits);
                free(element->segment);
                free(element);
                free(buffer_bits);
                return EXIT_FAILURE;
            }
        }
        free(segment_bits);
        bytes_skipped += segment_length;
 
        const unsigned int current_segment_bit_size = ((element->size - 1) * CHAR_BIT) + CHAR_BIT - element->unused_bits;
        if (segment_length != current_segment_bit_size)
        {
            fprintf(stderr,
                    "Data validation failed."
                            "\n\tBuffer size in bytes %d"
                            "\n\tSegment size in bits %d"
                            "\n\tFirst Segment size in bits %d"
                            "\n\tCurrent Segment bit size (%u) not equal to its string representation (%lu)\n",
                    buffer_byte_size, segment_bit_size, first_segment_bit_size, current_segment_bit_size, segment_length);
            clear(&head);
            free(segment_bits);
            free(element->segment);
            free(element);
            free(buffer_bits);
            return EXIT_FAILURE;
        }
        total_segment_bit_size += current_segment_bit_size;
 
        free(element->segment);
        free(element);
        element = pop(&head);
    }
 
    free(buffer_bits);
 
    if (buffer_length != total_segment_bit_size) {
        fprintf(stderr,
                "Data validation failed."
                        "\n\tBuffer size in bytes %d"
                        "\n\tSegment size in bits %d"
                        "\n\tFirst Segment size in bits %d"
                        "\n\tTotal Segment bit size (%u) not equal to full string representation (%lu)\n",
                buffer_byte_size, segment_bit_size, first_segment_bit_size, total_segment_bit_size, buffer_length);
        return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
}

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