LengthFramer.c

//
// Created by tw on 2018/3/2.
//
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <netinet/in.h>
#include "Practical.h"

/**
 * Read 2-byte length and place in big-endian order.
 * Then read the indicated number of bytes.
 * If the input buffer is too small for the date, truncate to fit and
 * return the negation of the *indicated* length. Thus a negative return
 * other than -1 indicate that the message was truncated.
 * (Ambiguity is possiable only if the caller passses an empty buffer.)
 * Input stream is always left empty.
 */
 int GetNextMsg(FILE *in, uint8_t *buf, size_t bufSize) {
    uint16_t mSize = 0;
    uint16_t extra = 0;

    if (fread(&mSize, sizeof(uint16_t), 1, in) != 1)
        return -1;
    mSize = ntohs(mSize);
    if (mSize > bufSize) {
        extra = mSize - bufSize;
        mSize = bufSize;
    }
    if (fread(buf, sizeof(uint8_t), mSize, in) != mSize) {
        fprintf(stderr, "Framing error: expected %d, read less\n", mSize);
        return -1;
    }
    if (extra > 0) {
        uint8_t waste[BUFSIZ];
        fread(waste, sizeof(uint8_t), extra, in);
        return -(mSize+extra);
    } else
        return mSize;
}

/**
 * Write the given message to the output stream, followed by
 * the delimiter. Precondidion: buf[] is at least msgSize.
 * Returns -1 on any error.
 */
int PutMsg(uint8_t buf[], size_t msgSize, FILE *out) {
    if (msgSize > UINT16_MAX)
        return -1;
    uint16_t payloadSize = htons(msgSize);
    if ((fwrite(&payloadSize, sizeof(uint16_t), 1, out) != 1) || (fwrite(buf, sizeof(uint8_t), msgSize, out) != msgSize))
        return -1;
    fflush(out);
    return msgSize;
}