{"id":5445,"date":"2022-07-16T00:01:02","date_gmt":"2022-07-16T07:01:02","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5445"},"modified":"2022-07-09T09:21:01","modified_gmt":"2022-07-09T16:21:01","slug":"having-fun-with-the-getline-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5445","title":{"rendered":"Having Fun with the getline()<\/em> Function"},"content":{"rendered":"

In last week’s post<\/a>, I updated information I had originally written about the useful getline()<\/em> function. The topic is worthy of further exploration, specifically with the mechanics behind the function’s internal allocation — and reallocation — of memory.
\n
\nThe getline()<\/em> function allocates memory when passed a char<\/em> pointer initialized to NULL<\/code>. If more storage is required, the function automatically re-allocates the pointer. To demonstrate how this internal operation behaves, I concocted the following code.<\/p>\n

2022_07_16-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    char *buffer = NULL;\r\n    size_t size;\r\n    int r;\r\n\r\n    \/* repeat until a blank line is input *\/<\/span>\r\n    do\r\n    {\r\n        \/* prompt and gather input *\/<\/span>\r\n        printf(\"Type: \");\r\n        r = getline( &buffer, &size, stdin );\r\n\r\n        \/* show input *\/<\/span>\r\n        printf(\"You typed (%d): %s\",r,buffer);\r\n\r\n    } while( r>1 );\r\n\r\n    return(0);\r\n}<\/pre>\n
The above code doesn’t even bother to initialize variable size<\/code>; it doesn’t matter! The getline()<\/em> function allocates as much storage as needed.<\/p>\n
At Line 15, the function returns the number characters input, including the newline. The value -1 is returned upon error and zero for an empty string (which can’t be input directly from the keyboard). The do-while<\/em> loop repeats until a blank line is input (r<\/code> is one or less). Here’s a sample run:<\/p>\n
Type: This is some text
\nYou typed (18): This is some text
\nType: Even more text
\nYou typed (15): Even more text
\nType:
\nYou typed (1):<\/code><\/p>\n
This code demonstrates that the function can be called again and again, each time processing the text input regardless of the extant values of buffer<\/code> and size<\/code>. I wondered whether the buffer is allocated afresh for every call, in which case it could be saved. This way getline()<\/em> can process several lines of text while reusing the same variables. So I wrote this code:<\/p>\n
2022_07_16-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    const int range = 5;\r\n    char *buffer = NULL;\r\n    char *text[range];\r\n    size_t size;\r\n    int x;\r\n\r\n    \/* gather `range` lines of input *\/<\/span>\r\n    for( x=0; x<range; x++ )\r\n    {\r\n        printf(\"Line #%d\/%d: \",x+1,range);        \/* add one for humans *\/<\/span>\r\n        getline( &buffer, &size, stdin );\r\n        text[x] = buffer;\r\n    }\r\n\r\n    \/* output the text *\/<\/span>\r\n    for( x=0; x<range; x++ )\r\n    {\r\n        printf(\"%s\",text[x]);\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
In this update to the code, the first for<\/em> loop uses getline()<\/em> to fetch five lines of text. The buffer for each allocated line is saved in the text[]<\/code> char<\/em> pointer array. The second loop outputs each of the stored lines. But it doesn’t really work:<\/p>\n
Line #1\/5: Line one
\nLine #2\/5: This is line two
\nLine #3\/5: Line three
\nLine #4\/5: Almost done
\nLine #5\/5: Last line!
\nLine one
\nLast line!
\nLast line!
\nLast line!
\nLast line!<\/code><\/p>\n
Because the second line is longer than the first, buffer<\/code> is reallocated by getline()<\/em>. As the remaining lines are shorter, buffer<\/code> isn’t reallocated. Even then, copying the address of buffer<\/code> into text[x]<\/code> (at Line 17) is a poor way to retain the text input. It relies upon the assumption that each call to getline()<\/em> allocates another buffer, which demonstrably isn’t the case.<\/p>\n
The moral of the story is that you can use getline()<\/em> to capture a single line of text, use the text in some manner, then use getline()<\/em> again with the same arguments. Never assume that a new buffer is created for each call. Further, to save the text you must allocate a new buffer and copy the text into it.<\/p>\n
Oh, and you can free the buffer after you’ve used it. I don’t in my examples because getline()<\/em> appears only in the main()<\/em> function, where memory is freed when the program terminates.<\/p>\n","protected":false},"excerpt":{"rendered":"
To enjoy the getline()<\/em> function you must understand its internal workings. 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-5445","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\/5445","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=5445"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5445\/revisions"}],"predecessor-version":[{"id":5453,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5445\/revisions\/5453"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5445"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5445"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5445"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}