{"id":7153,"date":"2025-09-13T00:01:43","date_gmt":"2025-09-13T07:01:43","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=7153"},"modified":"2025-09-06T14:13:34","modified_gmt":"2025-09-06T21:13:34","slug":"more-array-more-decay","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=7153","title":{"rendered":"More Array, More Decay"},"content":{"rendered":"

I’ll be blunt: If you want to pass an array to a function, or have a function return an array, just give up now and use pointers. It’s a far easier operation, and it would save me the bother of having to write this post to further discuss the topic of array decay.
\n
\nPicking up from last week’s Lesson<\/a>, array decay can be most notable when working with a function. In the following code, an array is passed to a function, then output as an array within the function:<\/p>\n

2025_09_13-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nvoid output( int a[] )\r\n{\r\n    for( int x=0; x<4; x++ )\r\n        printf(\"%d\\n\",a[x]);\r\n}\r\n\r\nint main()\r\n{\r\n    int array[] = { 10, 20, 30, 40 };\r\n\r\n    output(array);\r\n\r\n    return 0;\r\n}<\/pre>\n
From the main()<\/em> function, array[]<\/code> is passed to function output(<\/em>). This function declares the array as its sole argument, a[]<\/code>. Array notation is used within the function to output each element’s value:<\/p>\n
10
\n20
\n30
\n40<\/code><\/p>\n
Here is the same code, but using pointer notation in function output()<\/em>:<\/p>\n
2025_09_13-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nvoid output( int a[] )\r\n{\r\n    for( int x=0; x<4; x++ )\r\n        printf(\"%d\\n\",*(a+x) );\r\n}\r\n\r\nint main()\r\n{\r\n    int array[] = { 10, 20, 30, 40 };\r\n\r\n    output(array);\r\n\r\n    return 0;\r\n}<\/pre>\n
The printf(<\/em>) statement uses pointer notation to output array a[]<\/code>‘s elements. This switch works because the array is passed by name as an address. In fact, you could declare the output()<\/em> function’s argument as int *a<\/code> and the code still works — though you can’t use array notation in the function when this switch is made.<\/p>\n
One of the more technical aspects of array decay is that is removes the code’s capability to determine the array’s size. Consider this code:<\/p>\n
2025_09_13-Lesson-c.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nchar *title(void)\r\n{\r\n    static char t[] = \"Fun Array Stuff\";\r\n\r\n    printf(\"'%s' contains %zu bytes\\n\",\r\n            t,\r\n            sizeof(t)\r\n          );\r\n\r\n    return(t);\r\n}\r\n\r\nint main()\r\n{\r\n    printf(\"'%s' contains %zu bytes\\n\",\r\n            title(),\r\n            sizeof( title() )\r\n          );\r\n\r\n    return 0;\r\n}<\/pre>\n
Here is the output:<\/p>\n
'Fun Array Stuff' contains 16 bytes
\n'Fun Array Stuff' contains 8 bytes<\/code><\/p>\n
In the title()<\/em> function, array t[]<\/code> must be declared static<\/em> or its data is lost and cannot be returned. Within the function, the sizeof<\/em> operator reports the number of bytes that the string occupies in storage.<\/p>\n
Back in the main(<\/em>) function, title()<\/em> returns the string — its address, as the function is a char<\/em> pointer. But now, the sizeof<\/em> operator reports the number of bytes that the address<\/em> (the pointer) occupies in memory. The string’s size is lost due to array decay. (Yes, the strlen()<\/em> function  still works, but array decay has transformed the data.)<\/p>\n
Some might say that such an issue isn’t a problem. Even so, it may come as a surprise when you expect to return an array from a function but it’s form is lost. I don’t find array decay an issue in my code because I allocate storage and use pointers when I can. But one area where it might be a problem is with an array of structures. I can’t think of a good example to mess with at this point, though it seems like an interesting problem.<\/p>\n","protected":false},"excerpt":{"rendered":"
Array decay is most evident when it comes to using arrays with functions. 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-7153","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\/7153","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=7153"}],"version-history":[{"count":3,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7153\/revisions"}],"predecessor-version":[{"id":7160,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7153\/revisions\/7160"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7153"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7153"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7153"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}