BufReader.c

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

#define BUFFER_SIZE 1024

typedef struct
{
    int fd;
    char *buffer;
    ssize_t len;
} BufReader;

static void bufreader_init(BufReader *bufreader, int fd)
{
    bufreader->fd = fd;
    bufreader->buffer = NULL;
    bufreader->len = 0;
}

static void bufreader_destroy(BufReader *bufreader)
{
    free(bufreader->buffer);

    bufreader->fd = -1;
    bufreader->buffer = NULL;
    bufreader->len = 0;
}

static int bufreader_read(BufReader *bufreader)
{
    // Read
    char buffer[BUFFER_SIZE];
    ssize_t bytes_read = read(bufreader->fd, buffer, BUFFER_SIZE);
    if (bytes_read <= 0 && bufreader->len == 0)
    {
        return -1;
    }

    // Resize bufreader according to bytes_read
    bufreader->buffer = realloc(bufreader->buffer, bufreader->len + bytes_read);
    if (bufreader->buffer == NULL)
    {
        perror("Failed to realloc");
        exit(EXIT_FAILURE);
    }

    // Concat read buffer to bufreader
    memcpy(bufreader->buffer + bufreader->len, buffer, bytes_read);
    bufreader->len += bytes_read;

    return 0;
}

static char *bufreader_readline(BufReader *bufreader)
{
    // Find newline character
    ssize_t newline = 0;
    while (newline < bufreader->len && bufreader->buffer[newline] != '\n')
    {
        newline += 1;
    }

    // If newline character not found
    if (newline == bufreader->len)
    {
        return NULL;
    }

    // Duplicate a line
    bufreader->buffer[newline] = '\0';
    char *line = strdup(bufreader->buffer);

    // Shift remain data to the front
    ssize_t dst = 0;
    ssize_t src = newline + 1;
    while (src < bufreader->len)
    {
        bufreader->buffer[dst++] = bufreader->buffer[src++];
    }
    bufreader->len = dst;

    return line;
}

int main()
{
    BufReader bufreader;
    bufreader_init(&bufreader, 0);

    while (1)
    {
        if (bufreader_read(&bufreader) < 0)
        {
            printf("End\n");
            break;
        }

        char *line = bufreader_readline(&bufreader);
        if (line == NULL)
        {
            continue;
        }

        printf("read: \"%s\"\n", line);
        free(line);
    }

    bufreader_destroy(&bufreader);
}