{"id":6366,"date":"2024-04-27T00:01:29","date_gmt":"2024-04-27T07:01:29","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6366"},"modified":"2024-04-20T10:57:45","modified_gmt":"2024-04-20T17:57:45","slug":"from-void-to-integer","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6366","title":{"rendered":"From Void to Integer"},"content":{"rendered":"

You can’t just slap a data type upon a variable declared as void<\/em>. No, a void<\/em> type can be assigned a pointer. But even then, you can’t just typecast that pointer as an int<\/em> and get away with it.
\n
\nFrom last week’s Lesson<\/a>, I devised the code below to properly allocate void<\/em> variable a<\/code> as storage for an integer. But the code generates a few mismatch warnings and crashes when the program runs:<\/p>\n

2024_04_20-Lesson-c.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    void *a;\r\n\r\n    a = malloc(sizeof(int));\r\n\r\n    printf(\"Enter an integer: \");\r\n    scanf(\"%d\",(int)a);\r\n    printf(\"You entered %d\\n\",(int)a);\r\n\r\n    return 0;\r\n}<\/pre>\n
Starting with the scanf()<\/em> function, the (int)<\/code> typecast attempts to convert a pointer into an integer, which doesn’t work. Pointers are memory locations — an address. You must fetch the value<\/em> at the address. The nerds say “to dereference” the address, which requires the asterisk pointer operator. But where to put it? Here are some options, which I’m sure every C programmer new to pointers would attempt:<\/p>\n
A: (int)*a<\/code>
\nB: *(int)a<\/code>
\nC: (int *)a<\/code><\/p>\n
You can rule out using the &<\/code> address-of operator. The scanf()<\/em> function requires an address as its second argument in this instance. Variable a<\/code> is already an address. So which of the above typecasts work?<\/p>\n
A<\/strong> doesn’t work because variable a<\/code> is an address of a void<\/em> type. You don’t want the value<\/em> at the address, you want the address itself. B<\/strong> doesn’t work because a<\/code> is a pointer, which you can’t cast as an integer. The answer is C<\/strong> because it typecasts the address as the location<\/em> of an integer value, which is what the %d<\/code> placeholder desires.<\/p>\n
To fix the printf()<\/em> statement takes a bit more brainwork. You need an integer value at an address, so the format (int)a<\/code> (in the above code) doesn’t do anything. Rather than throw every combination of the characters (<\/code>, )<\/code>, *<\/code>, and the word int<\/code> at the wall to see what sticks, here is the proper format:<\/p>\n
*((int *)a)<\/code><\/p>\n
The (int *)<\/code> cast is required as it was for the scanf()<\/em> function to reference pointer a<\/code> as containing an integer value. This expression, (int *)a<\/code>, is then enclosed in parentheses and prefixed with the asterisk operator to fetch the integer value at the address stored at a<\/code>. It’s ugly, but it works.<\/p>\n
2024_04_27-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    void *a;\r\n\r\n    a = malloc(sizeof(int));\r\n\r\n    printf(\"Enter an integer: \");\r\n    scanf(\"%d\",(int *)a);\r\n    printf(\"You entered %d\\n\",*((int *)a));\r\n\r\n    return 0;\r\n}<\/pre>\n
The final code runs like this:<\/p>\n
Enter an integer: 100
\nYou entered 100<\/code><\/p>\n
These steps are convoluted and completely unnecessary. In practice, if I wanted to mess with void<\/em> variable a<\/code> as an integer, I would just declare a proper pointer type, int *b<\/code>, and assign the void<\/em> pointer to it:<\/p>\n
2024_04_27-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    void *a;\r\n    int *b;\r\n\r\n    a = malloc(sizeof(int));\r\n    b = a;\r\n\r\n    printf(\"Enter an integer: \");\r\n    scanf(\"%d\",b);\r\n    printf(\"You entered %d\\n\",*b);\r\n\r\n    return 0;\r\n}<\/pre>\n
This solution avoids the overhead (shown earlier) of referencing void<\/em> variable a<\/code> as an integer. Pointer b<\/code> is a straight-up int<\/em> pointer. It’s assigned the address stored in void<\/em> pointer a<\/code>, which handles the typecasting ordeal and renders unnecessary the extra decorations.<\/p>\n
Finally, remember that just declaring a<\/code> as an int<\/em> in the first place, would have avoided all this nonsense altogether. But then I’d have less to write about.<\/p>\n","protected":false},"excerpt":{"rendered":"
Yes, it’s possible to pervert a void<\/em> type to an integer, but not easy or even necessary. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-6366","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6366","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6366"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6366\/revisions"}],"predecessor-version":[{"id":6393,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6366\/revisions\/6393"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6366"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6366"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6366"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}