StackOverflow2013

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copied!<p>This code compiles with 'gcc -Wall -Werror -c':</p> <h3>data.h</h3> <pre><code>typedef struct { double ***grid; } Solver; </code></pre> <h3>gs.h</h3> <pre><code>extern void GS_init(double ***grid); </code></pre> <h3>gs.c</h3> <pre><code>#include "data.h" #include "gs.h" #include <stdlib.h> static Solver _solver; void anonymous(void) { _solver.grid = malloc(32 * sizeof(double)); GS_init(_solver.grid); } </code></pre> <hr> <p>Derek asked:</p> <blockquote> <p>Why does this work? Is it because of the extern keyword?</p> </blockquote> <p>The 'extern' is not material to making it work, though I always use it.</p> <blockquote> <p>When I have to flesh out GS_init() in, say compute.c, would I write <code>void GS_init(double ***grid){ //loop over grid[i][j][k] setting to zero }</code></p> </blockquote> <p>Sort of...yes, the GS_init() code could do that if the data structure is set up properly, which is going to need more information than there is currently visible in the structure.</p> <p>For the compiler to process:</p> <pre><code>grid[i][j][k] = 0.0; </code></pre> <p>the code has to know the valid ranges for each of i, j, and k; assume the number of rows in each dimension are N<sub>i</sub>, N<sub>j</sub>, N<sub>k</sub>. The data 'structure' pointed to by <code>grid</code> must be an array of N<sub>i</sub> 'double **' values - which must be allocated. Each of those entries must point to N<sub>j</sub> 'double *' values. So, you have to do more allocation than a single <code>malloc()</code>, and you have to do more initialization than just setting everything to zero.</p> <p>If you want to use a single array of doubles only, you will have to write a different expression to access the data:</p> <pre><code>grid[(i * Ni + j) * Nj + k] = 0.0; </code></pre> <p>And under this scenario, <code>grid</code> would be a simple <code>double *</code> and not a triple pointer.</p>

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