Different compilers, different output?

Question

I have a bit of c code that I've been working on in Xcode on my mac. I then wanted to work with it on a Windows machine and compile it with TinyC. When I run it, the output is different.

Is it possible that this is due to using different compilers?

Thanks!

EDIT 1

The code is a simple script that opens up a wav file to throw all the samples into an array.

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



void read_wav_header(unsigned int *samp_rate, unsigned int *bits_per_samp,
                 unsigned int *num_samp);
void read_wav_data(int *data, unsigned int samp_rate,
               unsigned int bits_per_samp, unsigned int num_samp);
int conv_bit_size(unsigned int in, int bps);



int main(void)
// int read_wav(void)
{


unsigned int samp_rate, bits_per_samp, num_samp;
read_wav_header(&samp_rate, &bits_per_samp, &num_samp);

printf("samp_rate=[%d] bits_per_samp=[%d] num_samp=[%d]\n",
       samp_rate, bits_per_samp, num_samp);

int *data = (int *) malloc(num_samp * sizeof(int));
read_wav_data(data, samp_rate, bits_per_samp, num_samp);

unsigned int i;
// for (i = 0; i < num_samp; ++i) {

for (i = 0; i < 100; ++i) {
    printf("%d\n", data[i]);
}

return EXIT_SUCCESS;
    }

   void read_wav_header(unsigned int *samp_rate, unsigned int *bits_per_samp,
                 unsigned int *num_samp)
{
unsigned char buf[5];

// freopen ("/Users/ericbrotto/Desktop/iPhoneData/tmp/Hack.wav","r",stdin);

 freopen ("C:/Documents and Settings/Eric.Brotto/Desktop/Eric_Other/Files/Hack.wav","r",stdin);



/* ChunkID (RIFF for little-endian, RIFX for big-endian) */
fread(buf, 1, 4, stdin);
buf[4] = '\0';
if (strcmp((char*)buf, "RIFF")) exit(EXIT_FAILURE);


/* ChunkSize */
fread(buf, 1, 4, stdin);

/* Format */
fread(buf, 1, 4, stdin);
buf[4] = '\0';

printf("IS THIS WAVE?  -->%s<--\n",(char*)buf);


if (strcmp((char*)buf, "WAVE")) exit(EXIT_FAILURE);

/* Subchunk1ID */
fread(buf, 1, 4, stdin);
buf[4] = '\0';
printf("IS THIS fmt?  -->%s<--\n",(char*)buf);

if (strcmp((char*)buf, "fmt ")) exit(EXIT_FAILURE);

/* Subchunk1Size (16 for PCM) */
fread(buf, 1, 4, stdin);
if (buf[0] != 16 || buf[1] || buf[2] || buf[3]) exit(EXIT_FAILURE);

/* AudioFormat (PCM = 1, other values indicate compression) */
fread(buf, 1, 2, stdin);
if (buf[0] != 1 || buf[1]) exit(EXIT_FAILURE);

/* NumChannels (Mono = 1, Stereo = 2, etc) */
fread(buf, 1, 2, stdin);
unsigned int num_ch = buf[0] + (buf[1] << 8);
if (num_ch != 1) exit(EXIT_FAILURE);

/* SampleRate (8000, 44100, etc) */
fread(buf, 1, 4, stdin);
*samp_rate = buf[0] + (buf[1] << 8) +
(buf[2] << 16) + (buf[3] << 24);

/* ByteRate (SampleRate * NumChannels * BitsPerSample / 8) */
fread(buf, 1, 4, stdin);
const unsigned int byte_rate = buf[0] + (buf[1] << 8) +
(buf[2] << 16) + (buf[3] << 24);

/* BlockAlign (NumChannels * BitsPerSample / 8) */
fread(buf, 1, 2, stdin);
const unsigned int block_align = buf[0] + (buf[1] << 8);

/* BitsPerSample */
fread(buf, 1, 2, stdin);
*bits_per_samp = buf[0] + (buf[1] << 8);

if (byte_rate != ((*samp_rate * num_ch * *bits_per_samp) >> 3))
    exit(EXIT_FAILURE);

if (block_align != ((num_ch * *bits_per_samp) >> 3))
    exit(EXIT_FAILURE);



/* Subchunk2ID */
// fread reads line by line until the end. 

fread(buf, 1, 4, stdin);
  buf[4] = '\0';

if (strcmp((char*)buf, "data")) exit(EXIT_FAILURE);



/* Subchunk2Size (NumSamples * NumChannels * BitsPerSample / 8) */
fread(buf, 1, 4, stdin);

const unsigned int subchunk2_size = buf[0] + (buf[1] << 8) +
(buf[2] << 16) + (buf[3] << 24);
*num_samp = (subchunk2_size << 3) / (
                                     num_ch * *bits_per_samp);
 }


 void read_wav_data(int *data, unsigned int samp_rate,
               unsigned int bits_per_samp, unsigned int num_samp)
 {

// freopen ("/Users/ericbrotto/Desktop/iPhoneData/tmp/Hack.wav","r",stdin);
 freopen ("C:/Documents and Settings/Eric.Brotto/Desktop/Eric_Other/Files/Hack.wav","r",stdin);

  unsigned char buf;
unsigned int i, j;
for (i=0; i < num_samp; ++i) {
    unsigned int tmp = 0;
    for (j=0; j != bits_per_samp; j+=8) {
        fread(&buf, 1, 1, stdin);           
        tmp += buf << j;
    }
    data[i] = conv_bit_size(tmp, bits_per_samp);
}
 }


 int conv_bit_size(unsigned int in, int bps)
 {
const unsigned int max = (1 << (bps-1)) - 1;
 return in > max ? in - (max<<1) : in; // Supposedly this is not correct: http://ubuntuforums.org/showthread.php?p=10442711
//  return in > max ? in - ((max<<1)+2) : in;

 }

EDIT 2

On my mac it outputs all the samples in the array (ints roughly between -32000 and 32000). Here I get the output you see in the image followed by a few million zeros.

enter image description here

It's entirely possible. It depends what code you are writing. Some code is deemed "implementation dependent", which means that results will vary from one compiler to another — vmpstr, Feb 18 '11 at 13:45

paxdiablo · Accepted Answer · 2011-02-18T13:59:19.297

Yes. Even assuming that both compilers comply with the ISO standard (which is not necessarily so), there's still a lot of leeway in that standard.

For example, if your program uses implementation defined or locale-specific behaviour, the output can be different.

If whoever wrote the program used undefined behaviour, then that's also a possibility for different output.

Your best bet would be to show us the code for proper analysis.

If you're interested in the sorts of things that can differ, Annex J of C99 (you can downlaod the draft with TC1, 2 and 3 from the bottom of that page - the differences between that and the final product are minimal) lists the portability issues (unspecified, undefined, implementation-defined and locale-specific behaviour).

One thing you may want to be careful of. This may not apply to Tiny C but I know that the Microsoft compilers I've used are one of that class where "r" and "rb" are treated differently in fopen/freopen. If you just specify "r", translation takes place which may give you the wrong data from a binary file such as a wav file.

@Eric: no, I'm pointing out that it's one _possible_ problem from a cursory glance over your code. There's little chance I'm going to do an in-depth analysis since I normally get paid pretty well for that sort of stuff :-) My advice is to create an incredibly short wav file for debugging so that you're not getting several million zeroes. That will ease your task somewhat. Then it's just a matter of comparing the output. — paxdiablo, Feb 18 '11 at 14:12

score 0 · Answer 2 · answered Feb 18 '11 at 13:57

Any reasonably simple code (i.e. printf("Hello World!") should behave the same way in both environments. More exotic expressions are not guaranteed.

Xcode uses gcc, so your code may be using a gnu-specific extension (which is not guaranteed to be portable)

Different compilers, different output?

2 Answers2

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