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POMemory allocation/reallocation question
 

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    <p>I just finished working out a memory allocation problem with the current program I'm writing, but I am not happy with what I had to do to fix it.</p> <p>In my program, I was building up an array of structs, reallocating space for the array every time I wanted to add a struct to it. Here is a generic version of my struct and the function that would add a struct to the array:</p> <pre><code>typedef struct Example { const char* name; int (*func)(int, int); int bool_switch; } int add_struct_to_array( Example **example_array, int *ex_array_size, int name, int (*func)(int, int), int bool_switch) { // first, make a new struct Example *new_example = (Example *) calloc( 1, sizeof( Example ) ); if( new_example != NULL ) { new_example-&gt;name = name; new_example-&gt;func = func; new_example-&gt;bool_switch = bool_switch; ( *ex_array_size )++; } else { printf( "Errror allocating %s\n", name ); exit( -1 ); } // now, realloc the array of structs and add the new member to it Example **temp_example_array = ( Example** )realloc( example_array, ( *ex_array_size ) * sizeof( Example* ) ); if( temp_example_array != NULL ) { example_array = temp_example_array; example_array[ ( *ex_array_size ) - 1 ] = new_example; } else { printf( "Reallocation failed\n" ) exit( -1 ); } return 0; } </code></pre> <p>And here is where I would call the functions (notice how I'm initially allocating the array of structs, cuz that's where the problem was)</p> <pre><code>#include "example_struct.h" int main( int argc, char **argv ) { int ex_array_size = 0; Example **example_array = ( Example** )calloc( 0, sizeof( Example* ) ); add_struct_to_array( example_array, &amp;ex_array_size, "name", &amp;function, 1 ); ... ... add_struct_to_array( example_array, &amp;ex_array_size, "other_name", &amp;other_func, 0 ); /* Do stuff here */ example_array_free( example_array ); return 0; } </code></pre> <p>In my ignorance, I apparently thought that allocating the array with size 0 would be ok, since it was initially empty, and I could add structs to it after that. Obviously, this did not work, I would get runtime errors about <code>error for object 0x100100080: pointer being reallocated was not allocated</code>. The <code>example_array</code> was at address <code>0x100100080</code> and the first struct I would allocate for would be at address <code>0x100100090</code>, and after a few reallocations the <code>example_array</code> would run out of room.</p> <p>So, finally, to my question. I solved this problem by allocating more space for my <code>example_array</code> than I would need, but that seems very inelegant. Is there a better way to do this?</p> <p><strong>**EDIT**</strong></p> <p>Ok, so from the looks of most of the responses, I shouldn't be using pointers to pointers. So, I am trying it a slightly different way, mixing <code>pmg</code> and <code>crypto</code>'s responses. Here is my code now:</p> <pre><code>/* example_struct.h */ int add_struct_to_array( Example *example_array, int *ex_array_size, int name, int (*func)(int, int), int bool_switch) { Example temp_example_array = realloc( example_array, ( ( *ex_array_size ) + 1 ) * sizeof( Example ) ); if( temp_example_array != NULL ) { example_array = temp_example_array; Example new_example; new_example.name = name; new_example.func = func; new_example.bool_switch = bool_switch; example_array[ ( *ex_array_size ) ] = new_example; ++( *ex_array_size ); } else { fprintf( stderr, "Error reallocating for %s", name ); exit( -1 ); } return 0; } /* main.c */ ... ... #include "example_struct.h" int main( int argc, char **argv ) { int ex_array_size = 0; Example *example_array = NULL; add_struct_to_array( example_array, &amp;ex_array_size, "name", &amp;func, 1 ); add_struct_to_array( ... ); ... add_struct_to_array( example_array, &amp;ex_array_size, "other name", &amp;other_func, 0 ); example_free( example_array ); } </code></pre> <p>Everything compiles and <code>realloc</code>'s alright, but I have trouble accessing the structs within the array. </p> <pre><code>/* main.c */ ... ... #include "example_struct.h" int main( int argc, char **argv ) { int ex_array_size = 0; Example *example_array = NULL; add_struct_to_array( example_array, &amp;ex_array_size, "name", &amp;func, 1 ); add_struct_to_array( ... ); ... add_struct_to_array( example_array, &amp;ex_array_size, "other name", &amp;other_func, 0 ); printf( "%s\n", example_array[0].name ) /* Segfault */ example_free( example_array ); } </code></pre> <p>Thanks again for all your help.</p>
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    1. COWhy do you need a pointer to pointer to Example? A single pointer is enough to make as large an "array" as you need.
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    2. COShouldn't the line 2 in the add_struct_to_array() function use (Example *) as typecast, rather than (example *)?
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    3. COTip: You don't need to explicitly typecast the void * returned by calloc, malloc or realloc. Read this: http://stackoverflow.com/questions/1835193/is-it-a-better-practice-to-typecast-the-pointer-returned-by-malloc
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