Portable C function to read a line from a file

read_line.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUF_SIZE 512

size_t next_buffer(const size_t buffer, const size_t count)
{
    return (buffer + 1) % count;
}

size_t read_line(FILE *file, char **line)
{
    if (file == NULL) {
        return 0;
    }
    size_t buffer_size = BUF_SIZE;
    size_t start = 0;
    char *buffers[2];
    size_t buffer = 0;
    size_t next = next_buffer(buffer, sizeof(buffers));
    buffers[buffer] = calloc(buffer_size, sizeof(char));
    buffers[next_buffer(buffer, sizeof(buffers))] = NULL;
    while (1) {
        char *result = fgets(buffers[buffer] + start, buffer_size - start, file);
        if (result == NULL) {
            break;
        }
        // The line is longer than the buffer
        if (buffers[buffer][buffer_size - 2] != 0 && buffers[buffer][buffer_size - 2] != '\n') {
            if (buffers[next] != NULL) {
                free(buffers[next]);
            }
            // Allocate the next buffer and copy this buffer's contents into it
            buffers[next] = calloc(buffer_size * 2, sizeof(char));
            if (!memcpy(buffers[next], buffers[buffer], buffer_size)) {
                break;
            }
            // Swap the buffers around
            start = buffer_size - 1;
            buffer_size *= 2;
            buffer = next;
            next = next_buffer(next, sizeof(buffers));
        } else {
            // The whole line was read
            size_t length = strlen(buffers[buffer]) + 1;
            // The final \n is replaced by 0, so the length (including 0)
            // is decreased by 1
            if (buffers[buffer][length - 1] == '\n') {
                buffers[buffer][length - 1] = 0;
                --length;
            }
            *line = calloc(length, sizeof(char));
            if (memcpy(*line, buffers[buffer], length) == NULL) {
                break;
            }
            // Free the buffers used for reading in the line
            if (buffers[buffer] != NULL) {
                free(buffers[buffer]);
            }
            if (buffers[next] != NULL) {
                free(buffers[next]);
            }
            return length - 1;
        }
    }
    // Free the buffers used for reading in the line
    if (buffers[buffer] != NULL) {
        free(buffers[buffer]);
    }
    if (buffers[next] != NULL) {
        free(buffers[next]);
    }
    return 0;
}