Environment Variables

NuttX supports environment variables that can be used to control the behavior of programs. In the spirit of NuttX the environment variable behavior attempts to emulate the behavior of environment variables in the multi-processing OS:

Task environments. When a new task is created using task_create, the environment of the child task is an inherited, exact copy of the environment of the parent. However, after child task has been created, subsequent operations by the child task on its environment does not alter the environment of the parent. No do operations by the parent effect the child’s environment. The environments start identical but are independent and may diverge.
Thread environments. When a pthread is created using pthread_create, the child thread also inherits that environment of the parent. However, the child does not receive a copy of the environment but, rather, shares the same environment. Changes to the environment are visible to all threads with the same parentage.

Programming Interfaces

The following environment variable programming interfaces are provided by NuttX and are described in detail in the following paragraphs.

getenv()

putenv()

clearenv()

setenv()

unsetenv()

Disabling Environment Variable Support

All support for environment variables can be disabled by setting CONFIG_DISABLE_ENVIRON in the board configuration file.

Functions

FAR char *getenv(const char *name)

Searches the environment list for a string that matches the string pointed to by name.

Parameters:

name – The name of the variable to find.

Returns:

The value of the variable (read-only) or NULL on failure.

int putenv(char *string)

Adds or changes the value of environment variables. The argument string is of the form name=value. If name does not already exist in the environment, then string is added to the environment. If name does exist, then the value of name in the environment is changed to value.

Parameters:

string – name=value string describing the environment setting to add/modify.

Returns:

Zero on success.

int clearenv(void)

Clears the environment of all name-value pairs and sets the value of the external variable environ to NULL.

Returns:: Zero on success.

int setenv(const char *name, const char *value, int overwrite)

Adds the variable name to the environment with the specified value if the variable name does not exist. If the name does exist in the environment, then its value is changed to value if overwrite is non-zero; if overwrite is zero, then the value of name is unaltered.

Parameters:

name – The name of the variable to change.
value – The new value of the variable.
value – Replace any existing value if non-zero.

Returns:

Zero on success.

int unsetenv(const char *name)

Deletes the variable name from the environment.

Parameters:

name – The name of the variable to delete.

Returns:

Zero on success.