{"id":6646,"date":"2024-11-02T00:01:00","date_gmt":"2024-11-02T07:01:00","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6646"},"modified":"2024-10-26T10:26:37","modified_gmt":"2024-10-26T17:26:37","slug":"testing-for-the-random-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6646","title":{"rendered":"Testing For the random()<\/em> Function"},"content":{"rendered":"

Silly me. I once assumed that just because my compiler offered the random()<\/em> function — a superior version of the C library standard rand()<\/em> function — that every compiler fatured this function. Boy, was I wrong!
\n
\nThe random()<\/em> function improves upon the randomness of rand()<\/em> using technical mumbo jumbo that I don’t want to understand. It also features a companion, srandom()<\/em> function to seed its randomizer. All I know is that these functions are better than rand()<\/em> and srand()<\/em>. But I also learned is that not every compiler offers these POSIX\/BSD functions.<\/p>\n

To write the best code, you should check for compatibility before implementing a POSIX or BSD function like random()<\/em>. The test parameters are listed in the function’s man<\/em> page documentation:<\/p>\n

CONFORMING TO      POSIX.1-2001, POSIX.1-2008, 4.3BSD.<\/code><\/p>\n

The POSIX defined constant to test is named _POSIX_C_SOURCE<\/code>. This value must be greater than or equal to 200112L (long int<\/em>) to ensure that the relevant function is available as documented.<\/p>\n

The following code tests the POSIX version, confirming whether the random()<\/em> function is available.<\/p>\n

2024_11_02-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\n#ifndef _POSIX_C_SOURCE\r\n#define _POSIX_C_SOURCE 0\r\n#endif\r\n\r\nint main()\r\n{\r\n    \/* test for presence of random() function *\/<\/span>\r\n\r\n    if( _POSIX_C_SOURCE >= 200112L )\r\n        puts(\"The random() function is available\");\r\n    else\r\n        puts(\"The random() function is not available\");\r\n\r\n    return 0;\r\n}<\/pre>\n
Before the main()<\/em> function, the precompiler tests for the presence of the _POSIX_C_SOURCE<\/code> defined constant. If the constant is undefined, the code defines it. This test ensures that the code compiles, otherwise an error occurs should the defined constant be undefined.<\/p>\n
Within the main()<\/em> function, the value of _POSIX_C_SOURCE<\/code> is compared with 200112L. When true (the value is greater than or equal to), the output shows that the random()<\/em> function is available.<\/p>\n
Here’s a sample run:<\/p>\n
The random() function is available<\/code><\/p>\n
If I compile the code with the -ansi<\/code> switch, setting it to conform with the original ANSI C standard, I see this result:<\/p>\n
The random() function is not available<\/code><\/p>\n
This is the output a user sees when the compiler doesn’t use the POSIX C extensions.<\/p>\n
The following code is more practical, showing how to implement the test:<\/p>\n
2024_11_02-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <time.h>\r\n\r\nint main()\r\n{\r\n    int x;\r\n\r\n#ifdef _POSIX_C_SOURCE\r\n    if( _POSIX_C_SOURCE >= 200112L )\r\n    {\r\n        srandom( (unsigned)time(NULL) );\r\n        puts(\"The random() function:\");\r\n        for( x=0; x<10; x++ )\r\n            printf(\"%ld\\n\",random());\r\n    }\r\n    else\r\n    {\r\n        srand( (unsigned)time(NULL) );\r\n        puts(\"The rand() function:\");\r\n        for( x=0; x<10; x++ )\r\n            printf(\"%d\\n\",rand());\r\n    }\r\n#else\r\n    srand( (unsigned)time(NULL) );\r\n    puts(\"The rand() function:\");\r\n    for( x=0; x<10; x++ )\r\n        printf(\"%d\\n\",rand());\r\n#endif\r\n\r\n    return 0;\r\n}<\/pre>\n
With this example, the precompiler directives are checked inside the code. When the _POSIX_C_SOURCE<\/code> constant is defined (#ifdef<\/code>), a if<\/em> test ensures that the version is compatible with the random()<\/em> function:<\/p>\n
if( _POSIX_C_SOURCE >= 200112L )<\/code><\/p>\n
Otherwise, the<\/em> else part of the code uses the rand()<\/em> function.<\/p>\n
The #else<\/code> directive handles the case when the _POSIX_C_SOURCE<\/code> constant is undefined. This part of the code is compiled using only the rand()<\/em> function.<\/p>\n
An #endif<\/code> directive ends the #ifdef<\/code> – #else<\/code> construction.<\/p>\n
Here is the output:<\/p>\n
The random() function:
\n1650599212
\n420354732
\n1514728248
\n139032251
\n1888432268
\n1534300142
\n157468978
\n1753183110
\n1160242387
\n22997577<\/code><\/p>\n
When the -ansi<\/code> switch is used to compile the code, here is the output:<\/p>\n
The rand() function:
\n1512047070
\n1354075150
\n158955988
\n1182828959
\n332400900
\n1817095792
\n1455458813
\n418963527
\n1268248241
\n1994796880<\/code><\/p>\n
This code is built to handle three situations: When the POSIX standard is compatible with random()<\/em>, when it’s not, and when the POSIX standard is undefined. Thanks to the precompiler directives, two different programs are generated depending on the presence of the _POSIX_C_SOURCE<\/code> constant. The program can build on any platform and do its job, even when the random()<\/em> function is unavailable.<\/p>\n","protected":false},"excerpt":{"rendered":"
The random()<\/em> function is better than rand()<\/em>, though it may not be available with all C compilers. 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-6646","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\/6646","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=6646"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6646\/revisions"}],"predecessor-version":[{"id":6662,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6646\/revisions\/6662"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6646"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6646"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6646"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}