#### 25.4.1 Environment Access

The value of an environment variable can be accessed with the `getenv` function. This is declared in the header file `stdlib.h`. Libraries should use `secure_getenv` instead of `getenv`, so that they do not accidentally use untrusted environment variables. Modifications of environment variables are not allowed in multi-threaded programs. The `getenv` and `secure_getenv` functions can be safely used in multi-threaded programs.

— Function: char * getenv (const char *name)

> This function returns a string that is the value of the environment variable name. You must not modify this string. In some non-Unix systems not using the GNU C Library, it might be overwritten by subsequent calls to `getenv` (but not by any other library function). If the environment variable name is not defined, the value is a null pointer.

— Function: char * secure_getenv (const char *name)

> This function is similar to `getenv`, but it returns a null pointer if the environment is untrusted. This happens when the program file has SUID or SGID bits set. General-purpose libraries should always prefer this function over `getenv` to avoid vulnerabilities if the library is referenced from a SUID/SGID program.

> This function is a GNU extension.

— Function: int putenv (char *string)

> The `putenv` function adds or removes definitions from the environment. If the string is of the form ‘`name=value`’, the definition is added to the environment. Otherwise, the string is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

> If the function is successful it returns `0`. Otherwise the return value is nonzero and `errno` is set to indicate the error.

> The difference to the `setenv` function is that the exact string given as the parameter string is put into the environment. If the user should change the string after the `putenv` call this will reflect in automatically in the environment. This also requires that string is no automatic variable which scope is left before the variable is removed from the environment. The same applies of course to dynamically allocated variables which are freed later.

> This function is part of the extended Unix interface. Since it was also available in old SVID libraries you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header.

— Function: int setenv (const char *name, const char *value, int replace)

> The `setenv` function can be used to add a new definition to the environment. The entry with the name name is replaced by the value ‘`name=value`’. Please note that this is also true if value is the empty string. To do this a new string is created and the strings name and value are copied. A null pointer for the value parameter is illegal. If the environment already contains an entry with key name the replace parameter controls the action. If replace is zero, nothing happens. Otherwise the old entry is replaced by the new one.

> Please note that you cannot remove an entry completely using this function.

> If the function is successful it returns `0`. Otherwise the environment is unchanged and the return value is `-1` and `errno` is set.

> This function was originally part of the BSD library but is now part of the Unix standard.

— Function: int unsetenv (const char *name)

> Using this function one can remove an entry completely from the environment. If the environment contains an entry with the key name this whole entry is removed. A call to this function is equivalent to a call to `putenv` when the value part of the string is empty.

> The function return `-1` if name is a null pointer, points to an empty string, or points to a string containing a `=` character. It returns `0` if the call succeeded.

> This function was originally part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.

There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.

— Function: int clearenv (void)

> The `clearenv` function removes all entries from the environment. Using `putenv` and `setenv` new entries can be added again later.

> If the function is successful it returns `0`. Otherwise the return value is nonzero.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see Executing a File).

— Variable: char ** environ

> The environment is represented as an array of strings. Each string is of the format ‘`name=value`’. The order in which strings appear in the environment is not significant, but the same name must not appear more than once. The last element of the array is a null pointer.

> This variable is declared in the header file `unistd.h`.

> If you just want to get the value of an environment variable, use `getenv`.

Unix systems, and GNU systems, pass the initial value of `environ` as the third argument to `main`. See Program Arguments.