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POCasting the void pointer and printing with format specifiers
 

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    <p>Okay this is for a school project but my question has <strong>nothing to do with anything regarding a specific implementation</strong>, nor am I asking for any help regarding the project itself. I am only giving this warning because I want to give context for what I am doing, but this is rather a general question regarding the casting of void pointers, framed within the context of a project... This function I'm writing isn't even FOR the project, it's just something I wrote as a testing mechanism to see if my generation of a data structure is working... and it lead me to a problem involving printf(), format specifiers, and casting.... I basically have to implement a linked-list (called a 'queue' but is not really a queue at all), and I wrote a function to test it.</p> <p>There is basically a struct called a "Chunk" with some variables (<strong>first</strong> is the index within the larger array that is the first element of a given "Chunk", and <strong>arr</strong> is basically larger_array[c->first], but it's actually a void pointer as such: <strong>void *arr</strong> where c is a pointer to a Chunk) that indicate a position in a larger array. So basically if you have an array like a = {5,7,3,4,6,9,1,2}, and you were given a chunk size of two, you have 4 chunks each of whom have an "arr" void pointer variable which point respectively to 5, then 3, then 6, then 1. </p> <p>Anyways, I wrote a "print_queue" function (actually a linked-list of Chunks) and yay! It prints all the information as desired, and here is the core part that I will share:</p> <pre><code> while (index &lt; num_chunks) { first = c-&gt;first; printf("Chunk %d: first is %d and a[%d] is ", index, first, first); if (elem_size == LONG_SIZE) /* LONG_SIZE defined above as 8 */ printf("%ld\n", c-&gt;arr); else if (elem_size == INT_SIZE) /* INT_SIZE defined above as 4 */ printf("%d\n", c-&gt;arr); else if (elem_size == CHAR_SIZE) /* CHAR_SIZE defined above as 1 */ printf("%c\n", c-&gt;arr); index++; if (c-&gt;next != NULL) c = c-&gt;next; } </code></pre> <p>I basically wanted to write a function that would be able to print my linked-list for any of the three types (longs, ints, and chars) for testing purposes while I implement the actual functionality of the project (multithreaded merge-sort). So the code above actually works! This is the output for this array input: </p> <pre><code> char original[] = {'z', 'y', 'x', 'w', 'v', 'u', 't', 's'}; </code></pre> <p>Output:</p> <pre><code> Chunk 0: first is 0 and a[0] is z Chunk 1: first is 2 and a[2] is x Chunk 2: first is 4 and a[4] is v Chunk 3: first is 6 and a[6] is t </code></pre> <p>So it works! Yay! However, I get these compiler warnings:</p> <pre><code>mergesort.c: In function 'print_chunk_queue': mergesort.c:85:7: warning: format '%ld' expects argument of type 'long int', but argument 2 has type 'void *' [-Wformat] mergesort.c:87:7: warning: format '%d' expects argument of type 'int', but argument 2 has type 'void *' [-Wformat] mergesort.c:89:7: warning: format '%c' expects argument of type 'int', but argument 2 has type 'void *' [-Wformat] </code></pre> <p>So what I did was cast all the c->arr 's to (type *) c->arr, but that gave me more warnings about "oh we want an int but you have us an int pointer" so what I did was then:</p> <pre><code> * ((type *) c-&gt;arr) </code></pre> <p>Basically dereferencing my casted void pointer (which is NEVER null! It always is pointing to valid numbers, at least with the input I provided it!), and then that gives me a segmentation fault!. So I'm pretty frustrated because I went from working output with a ton of "warnings" to useless segmentation faults. </p> <p>EDIT:</p> <p>definition of the data structure, as requested:</p> <pre><code>typedef struct chunk { void *arr; struct chunk *next; int first; } Chunk; </code></pre> <p>this is how I am setting the state of a single chunk and creating a linked-list of chunks:</p> <pre><code> while (index &lt; number_of_chunks) { if (index == 0) { if ((current = malloc(sizeof(Chunk))) == NULL) err(EX_OSERR, "memory allocation has failed\n"); head = current; } current-&gt;first = (chunk_size * index); current-&gt;arr = ((char *) array)[current-&gt;first]; current-&gt;size = chunk_size; if (index != (number_of_chunks - 1)) { if ((current-&gt;next = malloc(sizeof(Chunk))) == NULL) err(EX_OSERR, "memory allocation has failed\n"); current = current-&gt;next; } else { tail = current; } index += 1; } </code></pre>
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    1. COCould you post the definitions of the data structures?
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    2. COEdited, as requested
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      1. USArthur Collé
    3. COHow are you setting `arr`? Please show that code. It's worrisome that `printf("%c\n", c->arr)` actually prints the right values since `c->arr` ought to be an address, not a `char`.
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      1. USJohn Kugelman
 

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