{"id":3921,"date":"2020-01-04T01:01:23","date_gmt":"2020-01-04T09:01:23","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=3921"},"modified":"2019-12-28T10:05:11","modified_gmt":"2019-12-28T18:05:11","slug":"the-snprintf-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=3921","title":{"rendered":"The snprintf()<\/em> Function"},"content":{"rendered":"

Another non-standard library function is snprintf()<\/em>. Like the strlcpy()<\/em><\/a> and strlcat()<\/em><\/a> functions shown in previous Lessons, snprintf()<\/em> is safer than its standard Library counterpart, sprintf()<\/em>.
\n
\nThe sprintf()<\/em> function is part of the family of printf()<\/em> functions, each of which formats output. Here is its man<\/em> page format:<\/p>\n

int sprintf(char * restrict str, const char * restrict format, ...);<\/code><\/p>\n

The function’s first argument is a buffer into which the formatted output is placed. No bounds-checking is performed, so the buffer can overflow, as the following code demonstrates:<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char buffer[12];\r\n    int r;\r\n\r\n    r = sprintf(buffer,\"The total is %d\\n\",5+8);\r\n\r\n    puts(buffer);\r\n    printf(\"%d characters stored\\n\",r);\r\n\r\n    return(0);\r\n}<\/pre>\n
This code saves the sprintf()<\/em> function’s return value. As with printf()<\/em>, this value is equal to the characters output, or negative for an error. Here’s a sample run under Ubuntu Linux:<\/p>\n
The total is 13<\/p>\n
16 characters stored<\/code><\/p>\n
The output on my Macintosh is far more telling:<\/p>\n
Illegal instruction: 4<\/code><\/p>\n
Even when the program doesn’t crash, you can see that 16 characters were stored in a 12 character buffer. The snprintf()<\/em> function prevents this overflow from occurring. Here is its man<\/em> page format:<\/p>\n
int snprintf(char * restrict str, size_t size, const char * restrict format, ...);<\/code><\/p>\n
As with sprintf()<\/em>, the first argument is a buffer into which the formatted output is stored. A new second argument sets the buffer’s size. This size<\/code> value caps the string at n<\/em>-1 characters (one for the null character), preventing overflow. Here is the snprintf()<\/em> version of the sample code:<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char buffer[12];\r\n    int r;\r\n\r\n    r = snprintf(buffer,12,\"The total is %d\\n\",5+8);\r\n\r\n    puts(buffer);\r\n    printf(\"%d characters stored\\n\",r);\r\n\r\n    return(0);\r\n}<\/pre>\n
And sample output:<\/p>\n
The total i
\n16 characters stored<\/code><\/p>\n
The output is capped at 12 bytes; 11 for the string and one for the null character.<\/p>\n
The return value, 16, is the same as for the sprintf()<\/em> function. The logic here is that the return value can be compared with the function’s second argument to determine whether overflow would have occurred.<\/p>\n
Coding my own snprintf()<\/em> function would be truly complex. Not only must I emulate its behavior, but I must also reverse engineer the printf()<\/em> function; as a standard I\/O function, I couldn’t include its definition in stdio.h<\/code>, which truly complicates matters.<\/p>\n
Next week, I explore another interesting variation on the printf()<\/em> function, asprintf()<\/em>.<\/p>\n","protected":false},"excerpt":{"rendered":"
The sprintf()<\/em> can lead to buffer overflow, which is why the non-standard snprintf()<\/em> function was developed. 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-3921","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\/3921","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=3921"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3921\/revisions"}],"predecessor-version":[{"id":3935,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3921\/revisions\/3935"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3921"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3921"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3921"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}