Quantization noise during writing wav file using C - c

Question

I'm just trying to write a library for read and write Wav files (just need it for audio processing), just as test, I read samples from a Wave File convert them to double (just standardize them to -1 ~ 1), and do nothing but transform them back to integer, according to the bit per sample (assume the Wav file have N bits per sample, I divided them through 2^(N-1)-1 and multiply with the same factor after to restore it)
But the problem is, I get a wav file with background noise (id say it seems like quantisization noise) and I don't know why, can you help me find it out?
the library is here: https://pastebin.com/mz5TWMPN
the header file is: https://pastebin.com/Lr2tbmnv
and a demo main function is like:
#include <stdio.h>
#include <math.h>
#include "wavreader.h"
#define FRAMESIZE 512
int main()
{
FILE *fh;
FILE *fhWrite;
struct WavHeader * header;
struct WavHeader * newHeader;
double frame[FRAMESIZE];
int iBytesWritten;
int i;
char test;
fh = fopen("D:/ArbeitsOrdner/advanced_pacev/AudioSample/spfg.wav", "rb+");
if (fh == NULL)
{
printf("Failed to open organ.wav\n");
return 1;
}
fhWrite = fopen("D:/ArbeitsOrdner/MyC/test_organ.wav", "wb+");
if (fhWrite == NULL)
{
printf("Failed to create test_organ.wav\n");
return 1;
}
header = readWaveHeader(fh);
printWaveHeader(header);
newHeader = createWaveHeader(header->iChannels, header->iSampleRate, header->iBitsPerSample);
WaveWriteHeader(fhWrite, newHeader);
while (WaveReadFrame(fh, header, FRAMESIZE, frame) != -1)
{
iBytesWritten = WaveWriteFrame(fhWrite, newHeader, FRAMESIZE, frame);
if (iBytesWritten < 0)
{
printf("Error occured while writing to new file\n");
return 1;
}
}
WaveWriteHeader(fhWrite, newHeader);
fclose(fhWrite);
fclose(fh);
return 0;
}

Answer 1

thx for viewing this post. I have found the problem myself, it is that, i used char instead of unsigned char for raw data (raw bytes). By converting them to int16 or int32, i haven't considered the sign bit. that means they are not exact the same value during convertion as it except to be.
the solution for this is:
either stay with signed char and use:
buffer[i] & 0xff
to get the correct raw data for convertion, or change the char types into unsigned char:
unsgiend char * buffer;