{"id":3701,"date":"2019-08-10T00:01:50","date_gmt":"2019-08-10T07:01:50","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=3701"},"modified":"2019-08-03T09:51:36","modified_gmt":"2019-08-03T16:51:36","slug":"extremely-buffered-output","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=3701","title":{"rendered":"Extremely Buffered Output"},"content":{"rendered":"

A few weeks back, I wrote a post on the fflush()<\/em> function<\/a>, which immediately relieves an output buffer, releasing its contents. The mirror opposite of this function might be another output control function, setbuf()<\/em>.
\n
\nTo understand what setbuf()<\/em> does, as well as fflush()<\/em>, I must blather on about how output buffering works in C.<\/p>\n

<blather>
\nFor text output, C uses three types of buffering: unbuffered, block buffered, and line buffered. You’ve probably seen this type of buffering in action, but didn’t recognize what it was or that clever terms existed to describe it.<\/p>\n

Unbuffered<\/strong> output is generated as soon as it’s written. The characters aren’t stored.<\/p>\n

Block buffered<\/strong> output implies that characters are stored in a buffer. When the buffer gets full, its contents are dumped en masse<\/em> to the output device. The buffer is also dumped by an fflush()<\/em> statement or when closing the output device, such as a file.<\/p>\n

Line buffered<\/strong> output works like block buffered, though when a newline is encountered, the buffer is output.<\/p>\n

In C, file output is block buffered. Output to stdout<\/em> is line buffered. The stderr<\/em> device is unbuffered.
\n<\/blather><\/p>\n

The setbuf()<\/em> function alters buffering behavior for an output device, stdout<\/em> or an open file handle. Here’s the format:<\/p>\n

void setbuf(FILE *restrict stream, char *restrict buf);<\/code><\/p>\n

The stream<\/code> argument is a FILE<\/em> handle or stdout<\/em> for the standard output stream. The second argument, buf<\/code>, is a character buffer set to the size BUFSIZ<\/em>. The BUFSIZ<\/em> defined constant is set in the stdio.h<\/code> header. If you want to set a different size, use the setbuffer()<\/em> function instead, where the third argument sets the buffer’s size.<\/p>\n

The following code demonstrates how each line sent to standard output is line-buffered. The getchar()<\/em> function pauses output, then the final line is sent.<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    puts(\"Hold it!\");\r\n    puts(\"Hold it!\");\r\n    puts(\"Go!\");\r\n    getchar();\r\n    puts(\"Thank you.\");\r\n\r\n    return(0);\r\n}<\/pre>\n
Here’s the output:<\/p>\n
Hold it!
\nHold it!
\nGo!<\/code>
\n(Enter is pressed.)<\/em>
\nThank you.<\/code><\/p>\n
In this modification to the code, the setbuf()<\/em> function activates blocked buffering for standard output, holding all text sent to stdout<\/em> until the program stops (or the buffer fills):<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char buffer[BUFSIZ];\r\n\r\n    \/* setup the buffer *\/<\/span>\r\n    setbuf(stdout,buffer);\r\n\r\n    puts(\"Hold it!\");\r\n    puts(\"Hold it!\");\r\n    puts(\"Go!\");\r\n    getchar();\r\n    puts(\"Thank you.\");\r\n\r\n    return(0);\r\n}<\/pre>\n
The setbuf()<\/em> function at Line 8 redirects output to stdout<\/em> to the buffer<\/code> declared at Line 5. Output waits until the buffer is full, the buffer is flushed, or in this case when the program stops:<\/p>\n
(Enter is pressed.)<\/em>
\nHold it!
\nHold it!
\nGo!
\nThank you.<\/code><\/p>\n
If you’re a Unix nerd, I recommend you read the documentation in the man page for setbuf()<\/em>. It’s interesting to see how file I\/O can be manipulated. Further, you can set an environment variable to control buffering in the terminal window. It’s pretty cool stuff, nerdy, but good.<\/p>\n","protected":false},"excerpt":{"rendered":"
You can direct an output buffer to “hold it” until your code finally finds a rest stop. 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-3701","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\/3701","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=3701"}],"version-history":[{"count":3,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3701\/revisions"}],"predecessor-version":[{"id":3706,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3701\/revisions\/3706"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3701"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3701"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3701"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}