{"id":5223,"date":"2022-03-05T00:01:46","date_gmt":"2022-03-05T08:01:46","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5223"},"modified":"2022-02-26T09:37:54","modified_gmt":"2022-02-26T17:37:54","slug":"how-many-characters-output-so-far","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5223","title":{"rendered":"How Many Characters Output – So Far?"},"content":{"rendered":"

I doubt any C programmer has memorized all of the printf()<\/em> function’s placeholders, options, and settings. It’s a character salad of interesting and weird things. One of the strangest has to be the %n<\/code> placeholder.
\n
\nFor a placeholder review, refer to my series of Lessons on Conversion Character Mania, which starts here<\/a>.<\/p>\n

From last week’s Lesson<\/a>, the printf()<\/em> function returns the number of characters output. The %n<\/code> placeholder, used within the format string, returns the number of characters output so far.<\/p>\n

Suppose that the format string is \"123%n456\"<\/code>. If so, the %n<\/code> placeholder represents the value three, the number of characters output before its position in the string. The value reflects output, so another placeholder appearing before %n<\/code> affects the value returned.<\/p>\n

To make the %n<\/code> placeholder work, its corresponding argument must be the address of an int<\/em> value. The following code demonstrates:<\/p>\n

2022_03_05-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int n;\r\n\r\n    printf(\"Oh no!%n Here comes Godzilla!\\n\",&n);\r\n    printf(\"%d\\n\",n);\r\n\r\n    return(0);\r\n}<\/pre>\n
Integer variable n<\/code> is declared at Line 5. Its address is referenced in the printf()<\/em> statement at Line 7. The value stored is the number of characters output before the %n<\/code> placeholder appears (most likely six).<\/p>\n
At Line 8, the value of variable n<\/code> is output.<\/p>\n
Here’s a sample run:<\/p>\n
Oh no! Here comes Godzilla!
\n6<\/code><\/p>\n
Yup. It’s six.<\/p>\n
The disappointing news is that you cannot access this value until after the printf()<\/em> statement returns. So the following code doesn’t do what you think it does:<\/p>\n
2022_03_05-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int n;\r\n\r\n    printf(\"The total is %d!%n\\n%d characters output above\\n\",\r\n            500*80,\r\n            &n,\r\n            n\r\n          );\r\n\r\n    return(0);\r\n}<\/pre>\n
The %n<\/code> placeholder appears after the exclamation point in the printf()<\/em> format string at Line 7. Its argument appears at Line 9, but the value is used at Line 10. This value is unset, as the output proves:<\/p>\n
The total is 40000!
\n24342624 characters output above<\/code><\/p>\n
The number 24342624 represents whatever garbage is in memory before variable n<\/code> is initialized. That’s because the value is initialized after the function returns.<\/p>\n
Add the following statement at Line 12:<\/p>\n
printf(\"n==%d\\n\",n);<\/code><\/p>\n
Save, build, and run to see this output:<\/p>\n
The total is 40000!
\n24342624 characters output above
\nn==19<\/code><\/p>\n
Seeing how the %n<\/code> placeholder’s value isn’t available until after the function executes makes this placeholder seem less-than-useful to me. While you could use this value and the printf()<\/em> function’s return value to do some character output math, the %n<\/code> placeholder does have a warning attached to it.<\/p>\n
It’s possible that the %n<\/code> placeholder can be used to nefariously write data to a sensitive memory location. The method is complex, but the format string can be manipulated in a way to access memory in a malicious manner. Even for secure functions such as snprintf()<\/em>, avoid using a string variable to set the format string when the %n<\/code> placeholder is used.<\/p>\n
I believe the warning about the %n<\/code> placeholder outweighs any useful value it may have, though I remain curious which type of code would require this placeholder in the first place.<\/p>\n","protected":false},"excerpt":{"rendered":"
The obscure and potentially dangerous %n<\/code> placeholder deserves an explanation. 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-5223","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\/5223","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=5223"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5223\/revisions"}],"predecessor-version":[{"id":5240,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5223\/revisions\/5240"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5223"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5223"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5223"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}