{"id":4938,"date":"2021-09-04T00:01:00","date_gmt":"2021-09-04T07:01:00","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4938"},"modified":"2021-08-28T12:04:51","modified_gmt":"2021-08-28T19:04:51","slug":"when-passing-a-string-to-a-function-be-careful-that-you-dont","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4938","title":{"rendered":"When Passing a String to a Function, Be Careful That You Don’t . . ."},"content":{"rendered":"

Passing a string to a function? Check!
\nPassing an array of strings to a function? Watch out!
\n
\nFrom last week’s Lesson<\/a>, I’m attempting to pass an array of strings to a function, one at a time. Within the function, the string’s characters are modified. The modified string is output from the main()<\/em> function.<\/p>\n

My original code blew up. The strings are passed to the function just fine, but once the string’s characters are modified, BOOM<\/em>.<\/p>\n

I kept working and working the function, looking in a debugger, and getting cross with my cats. Nothing helped.<\/p>\n

Then I looked at the string definition:<\/p>\n

char *strings[3] = {
\n    \"Hello, world!\",
\n    \"This is a string test\",
\n    \"Once upon a time, there was a string\"<\/code><\/p>\n

May the gods forgive me.<\/p>\n

This declaration commits the sin of declaring a string as a pointer. You can do it, but lord help you if you attempt to modify the string. I’ve even written about this topic<\/a> before! How stupid can I be?<\/p>\n

You want to be extremely careful when you declare a string literal as a pointer: individually or in an array. If the strings aren’t to be modified, fine. Declare them as const<\/em> types. No problem.<\/p>\n

If you choose to modify the strings, just don’t initialize the array of pointers. This way, you can pass the string pointer to a function and manipulate it. To avoid the mistake, the strings must be initialized elsewhere, then their addresses assigned to the char<\/em> pointer array.<\/p>\n

For example, you start like this:<\/p>\n

char *strings[3];<\/code><\/p>\n

This array is declared as storage for three char<\/em> pointers, which eventually will reference strings. Observe that this declaration doesn’t assign anything to the array; it’s just storage. The strings to manipulate, and eventually reference in the strings[]<\/code> array, are declared on their own:<\/p>\n

char a[] = \"Hello, world!\";
\nchar b[] = \"This is a string test\";
\nchar c[] = \"Once upon a time, there was a string\";<\/code><\/p>\n

These strings are char<\/em> arrays, not pointers. The array name serves as the base address for each string, like a pointer. In fact, the next step is to reference each array as an element within the strings[]<\/code> pointer array:<\/p>\n

strings[0] = a;
\nstrings[1] = b;
\nstrings[2] = c;<\/code><\/p>\n

These statements copy the address of each string into the string[]<\/code> array’s elements. That’s it. The char<\/em> pointer array is now initialized properly. The strings referenced in the array can be sent to a function for direct manipulation, as this modification to my earlier code demonstrates:<\/p>\n

2021_09_04-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nvoid increase(char *a)\r\n{\r\n    while(*a)\r\n    {\r\n        *a = *a + 1;\r\n        a++;\r\n    }\r\n}\r\n\r\nint main()\r\n{\r\n    char *strings[3];\r\n    char a[] = \"Hello, world!\";\r\n    char b[] = \"This is a string test\";\r\n    char c[] = \"Once upon a time, there was a string\";\r\n    int x;\r\n\r\n    \/* allocate pointers *\/<\/span>\r\n    strings[0] = a;\r\n    strings[1] = b;\r\n    strings[2] = c;\r\n\r\n    \/* copy the strings *\/<\/span>\r\n\r\n    for( x=0; x<3; x++ )\r\n    {\r\n        printf(\"%s = \",strings[x]);\r\n        increase(strings[x]);\r\n        printf(\"%s\\n\",strings[x]);\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
The program now works, with no excess choking taking place in the increase()<\/em> function:<\/p>\n
Hello, world! = Ifmmp-!xpsme\"
\nThis is a string test = Uijt!jt!b!tusjoh!uftu
\nOnce upon a time, there was a string = Podf!vqpo!b!ujnf-!uifsf!xbt!b!tusjoh<\/code><\/p>\n
What I’ve learned from this episode is, first, always remember those things I tell others never to forget: The direct pointer-string assignment is fine for referencing strings, but such strings cannot be modified!<\/p>\n
Second, I learned that if you do want to modify strings in an array, assign the string values separately. May I never forget that.<\/p>\n","protected":false},"excerpt":{"rendered":"
Initializing a string as a pointer works, but . . . 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-4938","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\/4938","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=4938"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4938\/revisions"}],"predecessor-version":[{"id":4970,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4938\/revisions\/4970"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4938"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4938"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4938"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}