Sound player can't read WAV file produced by my program, says missing header

I am coding a program that renders a wave type (sine, square, wavetooth, etc.) and writes it to a file. The problem is that sound player says the header is missing even though I wrote it. The data types are supposed to be written in little endian.

I tried re-writing the I/O functions multiple times in different ways to no avail. I tried filling the array with the data to be written with test data and even that doesn't work. There's something wrong when the file is written.

Main function:

int main (int argc, char** argv){
  if (argc != 6){
    printf("Error: expecting more arguments\n");
    return -1;
  }

  //First argument: voice
  int voice = atof(argv[1]);


  //Second argument: frequency
  float frequency = atof(argv[2]);


  //Third argument: amplitude
  float amplitude = atof(argv[3]);


  //Fourth argument: numsample
  int numsamples = atof(argv[4]); 

  //Fifth argument: output file name
  char* filename = argv[5];

  int16_t buffer[numsamples*2];
  memset(buffer, 0.1, numsamples*2);

  if (check_args(voice, frequency,  amplitude) != 0) {  


    //Sine wave
    if (voice == 0) {
      //render_sine_wave_stero(buffer, numsamples, frequency, amplitude);
    }

    //If square wave
    else if (voice == 1){
      //render_square_wave_stereo(buffer, numsamples, frequency, amplitude);
    }

    //Sawtooth
    else if (voice == 2) {
      //render_sawtooth_wave_stero(buffer, numsamples, frequency, amplitude);
    }





    FILE *outputfile = fopen(filename, "wb+");
    if (outputfile == NULL){
      printf("Error: file failed to open");
      return 1;
    }

    write_wave_header(outputfile, numsamples);

    write_s16_buf(outputfile, buffer, numsamples*2);

    fclose(outputfile);

    printf("Finished\n");

Header writing function:

#define PI                 3.14159265358979323846
#define SAMPLES_PER_SECOND 44100u
#define NUM_CHANNELS       2u
#define BITS_PER_SAMPLE    16u

void write_wave_header(FILE *out, unsigned num_samples) {
  /*
   * See: http://soundfile.sapp.org/doc/WaveFormat/
   */
  uint32_t ChunkSize, Subchunk1Size, Subchunk2Size;
  uint16_t NumChannels = NUM_CHANNELS;
  uint32_t ByteRate = SAMPLES_PER_SECOND * NumChannels * (BITS_PER_SAMPLE/8u);
  uint16_t BlockAlign = NumChannels * (BITS_PER_SAMPLE/8u);

  /* Subchunk2Size is the total amount of sample data */
  Subchunk2Size = num_samples * NumChannels * (BITS_PER_SAMPLE/8u);
  Subchunk1Size = 16u;
  ChunkSize = 4u + (8u + Subchunk1Size) + (8u + Subchunk2Size);

  /* Write the RIFF chunk descriptor */
  write_bytes(out, "RIFF", 4u);
  write_u32(out, ChunkSize);
  write_bytes(out, "WAVE", 4u);

  /* Write the "fmt " sub-chunk */
  write_bytes(out, "fmt ", 4u);       /* Subchunk1ID */
  write_u32(out, Subchunk1Size);
  write_u16(out, 1u);                 /* PCM format */
  write_u16(out, NumChannels);
  write_u32(out, SAMPLES_PER_SECOND); /* SampleRate */
  write_u32(out, ByteRate);
  write_u16(out, BlockAlign);
  write_u16(out, BITS_PER_SAMPLE);

  /* Write the beginning of the "data" sub-chunk, but not the actual data */
  write_bytes(out, "data", 4);        /* Subchunk2ID */
  write_u32(out, Subchunk2Size);
}

IO functions:

void write_byte(FILE* out, char val){

  fwrite(&val, sizeof(char), 1, out);
}

void write_bytes(FILE* out, const char data[], unsigned n){

  for (unsigned i=0; i<n; i++){
    write_byte(out, data[n]);
  }
}

void write_u16(FILE* out, uint16_t value){

  char bytes[] = {0, 0};
  //Lowest byte
  bytes[0] = value & 0xFF;
  //Biggest byte
  bytes[1] = value>>8;
  write_bytes(out, bytes, 2u);


  //fwrite(&value, sizeof(uint16_t), 1, out);
}

void write_u32(FILE* out, uint32_t value){

  uint16_t first = value & 0x0000FFFF;
  uint16_t second = value >> 16;;

  write_u16(out, first);
  write_u16(out, second);

    //fwrite(&value, sizeof(uint32_t), 1, out);
}

void write_s16(FILE* out, int16_t  value){

  uint16_t newV = (uint16_t)value;

  write_u16(out, newV);

  //fwrite(&value, sizeof(int16_t), 1, out);
}

void write_s16_buf(FILE* out, const int16_t  buf[], unsigned n){
  for (unsigned i=0; i<n; i++){
    write_s16(out, buf[n]);
  }
}

I expected the file produced by calling ./render_tone 1 100 0.1 44100 out.wav to be readable by a sound player but the out.wav results in an error saying the header is missing on read.

You have a typo (I assume) in your write_bytes() function:

void write_bytes(FILE* out, const char data[], unsigned n){

  for (unsigned i=0; i<n; i++){
    write_byte(out, data[n]);
  }
}

you should use data[i] instead of data[n]. Right now if you call:

write_bytes(out, "RIFF", 4u);

it will write \0 character (NUL terminating the c-string) 4 times.