{"id":6237,"date":"2024-02-10T00:01:25","date_gmt":"2024-02-10T08:01:25","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6237"},"modified":"2024-02-17T09:03:22","modified_gmt":"2024-02-17T17:03:22","slug":"peering-into-the-environment","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6237","title":{"rendered":"Peering into the Environment"},"content":{"rendered":"

The operating system’s environment provides temporary storage for variables, settings, and options. These values are easily accessible from any C program. One of the first posts<\/a> I made on this blog was about accessing the environment. Time to revisit.
\n
\nYou can view the environment from a terminal window. In Linux, type the env<\/em> command to see a huge dump of environment variables. For Windows\/DOS, type the set<\/em> command.<\/p>\n

The output shows a list of environment variables. It’s often long and detailed. The format for each variable is the same. For example:<\/p>\n

HOME=\/home\/dang<\/code><\/p>\n

The HOME<\/code> variable in Linux indicates the user’s home directory. It’s followed by an equal sign, then the home directory path, which is \/home\/dang<\/code> for my account on this system.<\/p>\n

Internally, the environment exists as a pointer array. It’s a list of addresses, with each address referencing a string. This array exists as a global variable available to any C program. Include the stdlib.h<\/code> header file to ensure that your system has access to this variable and the various environment function prototypes.<\/p>\n

extern char **environ;<\/code><\/p>\n

To view the environment, you cycle through the pointers stored at **environ<\/code> until the NULL pointer is encountered:<\/p>\n

2024_02_10-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    extern char **environ;\r\n\r\n    while( *environ )\r\n    {\r\n        printf(\"%s\\n\",*environ);\r\n        environ++;\r\n    }\r\n    return 0;\r\n}<\/pre>\n
The **environ<\/code> variable is declared externally. A while<\/em> loop processes each address stored at **environ<\/code> until the NULL is ecountered at the end of the list. A printf()<\/em> statement outputs each string.<\/p>\n
Because environ<\/code> is a pointer-pointer, its value is the size of an address. Incrementing the environ<\/code> variable references the next address (string) in the list: environ++<\/code><\/p>\n
The program’s output is the same as the env<\/em> or set<\/em> command’s dump at the command prompt.<\/p>\n
You might think that to fetch an item from the list, you use something like this code:<\/p>\n
2024_02_10-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\nint main()\r\n{\r\n    extern char **environ;\r\n\r\n    puts(\"Search path:\");\r\n    while( *environ )\r\n    {\r\n        if( strncmp(*environ,\"PATH\",4)==0 )\r\n        {\r\n            printf(\"%s\\n\",*environ);\r\n            break;\r\n        }\r\n        environ++;\r\n    }\r\n\r\n    return 0;\r\n}<\/pre>\n
This code adds the strncmp()<\/em> function to the original example. The function scans the first four characters of the *environ<\/code> string to match PATH<\/code>. Once found, the search path is output. This output can be quite lengthy, so I’m not duplicating it here.<\/p>\n
The environ man<\/em> page, however, recommends that you not search the environment manually. Instead, use the getenv()<\/em> function to fetch a specific entry:<\/p>\n
2024_02_10-Lesson-c.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main()\r\n{\r\n    puts(\"Search path:\");\r\n    printf(\"%s\\n\",getenv(\"PATH\"));\r\n\r\n    return 0;\r\n}<\/pre>\n
The getenv()<\/em> function is prototyped in the stdlib.h<\/code> header file. Its argument is a string matching a variable to find in the environment table. The return value is the variable’s value or NULL when no match is found.<\/p>\n
Just as a user can set an environment variable, so can a program. I cover the required tools in next week’s Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
The environment lists system variables and values your program can access. 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-6237","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\/6237","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=6237"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6237\/revisions"}],"predecessor-version":[{"id":6260,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6237\/revisions\/6260"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6237"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6237"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6237"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}