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POC: Returning a void versus returning a double * from a subfunction
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copied!<p>I'm working on trying to speed up some general data processing in C. I've written several subroutines of the form:</p> <pre><code>double *do_something(double *arr_in, ...) { double *arr_out; arr_out = malloc(...) for (...) { do the something on arr_in and put into arr_out } return arr_out; } </code></pre> <p>I like this style because it's easy to read and use, but often I call it as:</p> <pre><code> array = do_something(array,...); </code></pre> <p>Would it make for faster code (and maybe prevent memory leaks) if I were to instead use void subfunctions as:</p> <pre><code>void do_something(double *arr_in, ...) { for (...) { arr_in = do that something; } return; } </code></pre> <p>update 1: I ran valgrind --leak-check=full on the executable and it appears there were no memory leaks using the first method. However, the executable links to a library which contains all the subroutines I made with this form, so it might not catch leaks from the library. </p> <p>I'm curious as to how I would write the wrappers to free the memory and what the ** really does, or what a pointer to a pointer is, so I'm avoiding using the ** route (that and maybe I did it wrong because it didn't compile on my mac).</p> <p>Here's one current subroutine:</p> <pre><code>double *cos_taper(double *arr_in, int npts) { int i; double *arr_out; double cos_taper[npts]; int M; M = floor( ((npts - 2) / 10) + .5); arr_out = malloc(npts*sizeof(arr_out)); for (i=0; i<npts; i++) { if (i<M) { cos_taper[i] = .5 * (1-cos(i * PI / (M + 1))); } else if (i<npts - M - 2) { cos_taper[i] = 1; } else if (i<npts) { cos_taper[i] = .5 * (1-cos((npts - i - 1) * PI / (M + 1))); } arr_out[i] = arr_in[i] * cos_taper[i]; } return arr_out; } </code></pre> <p>From the advice I've gotten here, it sounds like a better method would be:</p> <pre><code>void *cos_taper(double *arr_in, double *arr_out, int npts) { int i; double cos_taper[npts]; int M; M = floor( ((npts - 2) / 10) + .5); for (i=0; i<npts; i++) { if (i<M) { cos_taper[i] = .5 * (1-cos(i * PI / (M + 1))); } else if (i<npts - M - 2) { cos_taper[i] = 1; } else if (i<npts) { cos_taper[i] = .5 * (1-cos((npts - i - 1) * PI / (M + 1))); } arr_out[i] = arr_in[i] * cos_taper[i]; } return } </code></pre> <p>call:</p> <pre><code>int main() { int npts; double *data, *cos_tapered; data = malloc(sizeof(data) * npts); cos_tapered = malloc(sizeof(cos_tapered) * npts); //fill data cos_taper(data, cos_tapered, npts); free(data); ... free(cos_tapered); ... return 0; } </code></pre>

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