curl_getenv(3) -- Linux man page
NAMEcurl_getenv - return value for environment name
DESCRIPTIONcurl_getenv() is a portable wrapper for the getenv() function, meant to emulate its behaviour and provide an identical interface for all operating systems libcurl builds on (including win32).
AVAILABILITYThis function will be removed from the public libcurl API in a near future. It will instead be made "available" by source code access only, and then as curlx_getenv().
RETURN VALUEIf successful, curl_getenv() returns a pointer to the value of the specified environment. The memory it refers to is malloc()ed why the application must free() this when the data has completed to serve its purpose. When curl_getenv(3) fails to find the specified name, it returns a null pointer.
NOTEUnder unix operating systems, there isn't any point in returning an allocated memory, although other systems won't work properly if this isn't done. The unix implementation thus have to suffer slightly from the drawbacks of other systems.