Pointer in struct question. Have I done it correct?

Question

Hi,

I am rewriting some code from having a regular pointer to where the pointer is in a struct.

This is the original code which works:

int wrote = sf_writef_double(outfile, *mono_channel, frames);

In the new code, where I have put the mono_channel pointer in a struct have I written

int wrote = sf_writef_double(outfile, data->mono_channel, frames);

The compiler doesn't complain, but the program crashes.

So the question is. Is data->mono_channel the same as *mono_channel?

Hugs, Louise

Edit: To be more precise here is what I do:

ltfat_complex* fm;
fm = malloc(data->L * sizeof(ltfat_complex));
if (fm == NULL) { puts("fm malloc failed"); exit(1); }

/* Writes output to fm */      
idgt_fac(data->coef, gdf, data->L, 1, 1, data->a, data->M, fm);

free(data->mono_channel);

data->mono_channel = (double*) fm;
free(fm);

...

int wrote = sf_writef_double(outfile_handler, data->mono_channel, frames);

Could the problem be, that I try to re-use the data->mono_channel pointer?

Edit2: Here are the entire source codes (~ 700 lines each):

Old which works: http://www.student.dtu.dk/~s011392/gabor-io.zip

New: http://www.student.dtu.dk/~s011392/gui.zip

Sadly they require a lot of libraries to compile: ltfat from SVN, fftw3, lapack, blas, sndfile.

But there tey are =)

Answer 1

You're dereferencing mono_channel in the first version but not in the second. Try

int wrote = sf_writef_double(outfile, *(data->mono_channel), frames);

Answer 2

data->mono_channel is the same as (*data).mono_channel

If you want the contents of the pointed to variable I suggest you want *(data->mono_channel)

You may want to re-read K&R.

Answer 3

No, data->mono_channel and *mono_channel are not the same thing. *(data->mono_channel) is the same as *mono_channel.

I really wonder how you got by with *mono_channel in your program if you wanted to pass a pointer to a double instead of a double. Clearly mono_channel was a pointer to a pointer to a vector of doubles. Most likely mono_channel was a pointer to an array (which is actually a vector) of doubles.

double *mono_channel[1000];   // let's say

Anyway, if you're crashing, it's probably because you haven't allocated any storage and pointed mono_channel to it, instead you've just defined it as a member of your struct that is a double * and are using it. You either need to point mono_channel to a predefined global array or allocate it with malloc/new.