{"id":3863,"date":"2019-11-30T00:01:22","date_gmt":"2019-11-30T08:01:22","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=3863"},"modified":"2019-12-21T08:47:18","modified_gmt":"2019-12-21T16:47:18","slug":"non-standard-function-strcasecmp","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=3863","title":{"rendered":"Non-Standard Function: strcasecmp()<\/em>"},"content":{"rendered":"

As part of my research, I run my C code on different platforms using different compilers. Occasionally I’m crushed to discover that my code won’t compile because my development computer uses a customized version of the C library, one that features a non-standard function, such as strcasecmp()<\/em>.
\n
\nThe C17\/C18 library standard defines the strcmp()<\/em> function and its siblings, which compare two strings. It does not define the strcasecmp()<\/em> function, which works the same as strcmp()<\/em>, but ignores the difference between uppercase and lowercase letters.<\/p>\n

Surprising, many libraries are littered with non-standard C functions. They’re documented, but the man<\/em> page doesn’t proclaim that the functions are non-standard:<\/p>\n

The strcasecmp()<\/em> and strncasecmp()<\/em> functions first appeared in 4.4BSD. Their prototypes existed previously in <\/code> before they were moved to <strings.h><\/code> for IEEE Std 1003.1-2001 (“POSIX.1”) compliance.<\/p><\/blockquote>\n

A quick look in the string.h<\/code> header file on my system confirms that it also includes the (non-standard) strings.h<\/code> header file, in which the strcasecmp()<\/em> function is defined. Sneaky.<\/p>\n

Without knowing otherwise, I’ve used strcasecmp()<\/em> in several programs. Only when I attempted to build the code on another system do I discover that the function isn’t defined. Yeah, that ticks me off a tad.<\/p>\n

Digging into the topic of non-standard functions further, I discovered two more: strlcpy()<\/em> and strlcat()<\/em>. I’m certain other non-standard functions exist as well, available in one C library but not another. Yet, these functions serve valid purposes.<\/p>\n

Like strcpy()<\/em> and strcat()<\/em> functions, strlcpy()<\/em> and strlcat()<\/em> copy and concatenate strings, respectively. The difference is in a new, third argument, which sets a size limit for the destination string. This addition makes the functions safer than their standard C library counterparts — an excellent improvement.<\/p>\n

The good news is that if your compiler’s C library fails to implement these functions, you can code them yourself. Below you see my attempt at replicating the strcasecmp()<\/em> function. It performs similar to strcasecmp()<\/em> as documented, returning a value of zero when the strings match (regardless of case) and values less than or greater than zero depending on how the strings compare.<\/p>\n

\r\n#include <stdio.h>\r\n#include <ctype.h>\r\n\r\nint strcasecmp(const char *s1, const char *s2)\r\n{\r\n    int offset,ch;\r\n    unsigned char a,b;\r\n\r\n    offset = 0;\r\n    ch = 0;\r\n    while( *(s1+offset) != '\\0' )\r\n    {\r\n        \/* check for end of s2 *\/<\/span>\r\n        if( *(s2+offset)=='\\0')\r\n            return( *(s1+offset) );\r\n\r\n        a = (unsigned)*(s1+offset);\r\n        b = (unsigned)*(s2+offset);\r\n        ch = toupper(a) - toupper(b);\r\n        if( ch<0 || ch>0 )\r\n            return(ch);\r\n        offset++;\r\n    }\r\n\r\n    return(ch);\r\n}\r\n\r\nvoid test(const char *s1, const char *s2)\r\n{\r\n    printf(\"%s v. %s = \",s1,s2);\r\n    if( strcasecmp(s1,s2)==0 )\r\n        puts(\"match\");\r\n    else\r\n        puts(\"no match\");\r\n}\r\n\r\nint main()\r\n{\r\n    char string1[] = \"I drink coffee\";\r\n    char string2[] = \"I DRINK COFFEE\";\r\n    char string3[] = \"I drink tea\";\r\n\r\n    test(string1,string1);\r\n    test(string1,string2);\r\n    test(string1,string3);\r\n\r\n    return(0);\r\n}<\/pre>\n
The main()<\/em> function defines three test strings, which are run through the test()<\/em> function for comparison and output. The test()<\/em> function uses my version of the strcasecmp()<\/em> function to compare the strings, then outputs match<\/code> or nomatch<\/code> depending on the results.<\/p>\n
The strcasecmp()<\/em> function (which doesn’t generate a compiler warning in this code because I didn’t include the string.h<\/code> or strings.h<\/code> header) uses a while<\/em> loop to compare both strings, s1<\/code> and s2<\/code>. Both strings are compared letter-by-letter, thanks to the offset<\/code> variable, at Line 19. At Line 20, if variable ch<\/code> isn’t equal to zero, its value is returned, otherwise, the while<\/em> loop continues:<\/p>\n
ch = toupper(a) - toupper(b);
\nif( ch<0 || ch>0 )
\n    return(ch);<\/code><\/p>\n
Lines 14 and 15 perform a vital end-of-string test for s2<\/code>, just in case it’s shorter than s1<\/code>. If so, the current character in s1<\/code> is returned:<\/p>\n
if( *(s2+offset)=='\\0')
\n    return( *(s1+offset) );<\/code><\/p>\n
Here’s sample output:<\/p>\n
I drink coffee v. I drink coffee = match
\nI drink coffee v. I DRINK COFFEE = match
\nI drink coffee v. I drink tea = no match<\/code><\/p>\n
For next week’s Lesson<\/a> I cover a substitute function for strlcpy()<\/em>. A later Lesson<\/a> covers strlcat()<\/em>.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the first of a short series, non-standard C library functions are discussed and substitutes presented. This week, the strcasecmp<\/em> function is covered, which compares strings regardless of case. 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-3863","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\/3863","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=3863"}],"version-history":[{"count":8,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3863\/revisions"}],"predecessor-version":[{"id":3913,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3863\/revisions\/3913"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3863"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3863"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3863"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}