{"id":6245,"date":"2024-02-17T00:01:25","date_gmt":"2024-02-17T08:01:25","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6245"},"modified":"2024-02-10T15:20:36","modified_gmt":"2024-02-10T23:20:36","slug":"creating-your-own-environment-variables","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6245","title":{"rendered":"Creating Your Own Environment Variables"},"content":{"rendered":"

Programs use environment variables, thanks to the getenv()<\/em> function shown in last week’s Lesson<\/a>. They can also create their own environment variables, reset variable values, and remove variables.
\n
\nTo add your own environment variable, use the setenv()<\/em> function, prototyped in the stdlib.h<\/code> header file. Here is the man<\/em> page format:<\/p>\n

int setenv(const char *name, const char *value, int overwrite);<\/code><\/p>\n

Strings name<\/em> and value<\/em> are strings representing the environment variable name and value to add. The overwrite<\/em> option is zero or non-zero. If overwrite<\/em> is zero and name<\/em> already exists, it’s not overwritten. Otherwise, if name<\/em> exists (and overwrite<\/em> is non-zero) the new value replaces the original value.<\/p>\n

Remember that the function returns zero on success even if overwrite<\/em> is zero and name<\/em> already exists. In this case, name<\/em> isn’t overwritten, which the function considered a “success.”<\/p>\n

2024_02_17-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    const char newvar[] = \"language\";\r\n    const char value[] = \"C\";\r\n\r\n    printf(\"Setting variable '%s' to '%s'\\n\",\r\n            newvar,\r\n            value\r\n          );\r\n    setenv(newvar,value,0);\r\n\r\n    printf(\"Result: '%s'\\n\",getenv(newvar) );\r\n\r\n    return 0;\r\n}<\/pre>\n
This code sets the language<\/code> variable to the value C<\/code> in the environment: language=C<\/code> If language<\/code> already exists, it’s overwritten. Here’s a sample run:<\/p>\n
Setting variable 'langauge' to 'C'
\nResult: 'C'<\/code><\/p>\n
Another function similar to setenv()<\/em> is putenv()<\/em>, also prototyped in the stdlib.h<\/code> header file. The difference between the two is that putenv()<\/em> requires the entire string to be set at once. For example:<\/p>\n
putenv(\"language=C\");<\/code><\/p>\n
Otherwise, both functions perform the same task.<\/p>\n
One thing to keep in mind with environment variables is that any variables created or modified in a program are local to that program. When the program ends, the variables are lost. After running the program above, you won’t find language=C<\/code> in the environment.<\/p>\n
To deliberately remove created environment variables from your program. The unsetenv()<\/em> function handles the job, as shown in this code:<\/p>\n
2024_02_17-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    const char newvar[] = \"language\";\r\n    const char value[] = \"C\";\r\n\r\n    printf(\"Setting variable '%s' to '%s'\\n\",\r\n            newvar,\r\n            value\r\n          );\r\n    setenv(newvar,value,0);\r\n\r\n    printf(\"Result: '%s'\\n\",getenv(newvar) );\r\n\r\n    printf(\"Freeing '%s'...\",newvar);\r\n    unsetenv(newvar);\r\n    printf(\"done!\\n\");\r\n\r\n    return 0;\r\n}<\/pre>\n
The unsetenv()<\/em> function’s sole argument is the environment variable to zap. Like setenv()<\/em>, the function returns zero on success, -1 otherwise.<\/p>\n
I’ve never written code that required an environment variable to created on-the-fly, or even to reset a variable’s value. A weather program I wrote relies upon the variable WXSTN<\/code> to obtain the local weather station. But this variable is set when the shell starts, not in the program.<\/p>\n
The environment is great for storing variables accessible when the program runs. For more detailed information, say longer than a string, use temporary or hidden files that contain the settings. Examples include the “dot” files used by many terminal window programs, often found lurking in the user’s home directory.<\/p>\n","protected":false},"excerpt":{"rendered":"
A program can add a variable to the environment — and remove it as well. 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-6245","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\/6245","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=6245"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6245\/revisions"}],"predecessor-version":[{"id":6259,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6245\/revisions\/6259"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6245"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6245"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6245"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}