The Linux-PAM Application Developers' Guide: A library of miscellaneous helper functions

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## 5. A library of miscellaneous helper functions

To aid the work of the application developer a library of miscellaneous functions is provided. It is called `libpam_misc`, and contains functions for allocating memory (securely), a text based conversation function, and routines for enhancing the standard PAM-environment variable support.

## 5.1 Requirements

The functions, structures and macros, made available by this library can be defined by including `<security/pam_misc.h>`. It should be noted that this library is specific to Linux-PAM and is not referred to in the defining DCE-RFC (see the bibliography) below.

## 5.2 Functions supplied

### Safe string duplication

> `

>     extern char *xstrdup(const char *s)

> `

Return a duplicate copy of the `NUL` terminated string, `s`. `NULL` is returned if there is insufficient memory available for the duplicate or if `s=NULL`.

### A text based conversation function

> `

>     extern int misc_conv(int num_msg, const struct pam_message **msgm,
>                          struct pam_response **response, void *appdata_ptr);

> `

This is a function that will prompt the user with the appropriate comments and obtain the appropriate inputs as directed by authentication modules.

In addition to simply slotting into the appropriate `struct pam_conv`, this function provides some time-out facilities. The function exports five variables that can be used by an application programmer to limit the amount of time this conversation function will spend waiting for the user to type something.

The five variables are as follows:

`extern time_t pam_misc_conv_warn_time;`

  This variable contains the time (as returned by `time()`) that the user should be first warned that the clock is ticking. By default it has the value `0`, which indicates that no such warning will be given. The application may set its value to sometime in the future, but this should be done prior to passing control to the Linux-PAM library.

`extern const char *pam_misc_conv_warn_line;`

  Used in conjuction with `pam_misc_conv_warn_time`, this variable is a pointer to the string that will be displayed when it becomes time to warn the user that the timeout is approaching. Its default value is ``..\a.Time is running out...\n'', but this can be changed by the application prior to passing control to Linux-PAM.

`extern time_t pam_misc_conv_die_time;`

  This variable contains the time (as returned by `time()`) that the conversation will time out. By default it has the value `0`, which indicates that the conversation function will not timeout. The application may set its value to sometime in the future, this should be done prior to passing control to the Linux-PAM library.

`extern const char *pam_misc_conv_die_line;`

  Used in conjuction with `pam_misc_conv_die_time`, this variable is a pointer to the string that will be displayed when the conversation times out. Its default value is ``..\a.Sorry, your time is up!\n'', but this can be changed by the application prior to passing control to Linux-PAM.

`extern int pam_misc_conv_died;`

  Following a return from the Linux-PAM libraray, the value of this variable indicates whether the conversation has timed out. A value of `1` indicates the time-out occurred.

`extern int (*pam_binary_handler_fn)(const union pam_u_packet_p send, union pam_u_packet_p *receive);`

  This function pointer is initialized to `NULL` but can be filled with a function that provides machine-machine (hidden) message exchange. It is intended for use with hidden authentication protocols such as RSA or Diffie-Hellman key exchanges. (This is still under development.)

### Transcribing an environment to that of Linux-PAM

> `

>     extern int pam_misc_paste_env(pam_handle_t *pamh,
>                                   const char * const * user_env);

> `

This function takes the supplied list of environment pointers and uploads its contents to the Linux-PAM environment. Success is indicated by `PAM_SUCCESS`.

### Saving the Linux-PAM environment for later use

> `

>     extern char **pam_misc_copy_env(pam_handle_t *pamh);

> `

This function returns a pointer to a list of environment variables that are a direct copy of the Linux-PAM environment. The memory associated with these variables are the responsibility of the application and should be liberated with a call to `pam_misc_drop_env()`.

### Liberating a locally saved environment

> `

>     extern char **pam_misc_drop_env(char **env);

> `

This function is defined to complement the `pam_misc_copy_env()` function. It liberates the memory associated with `env`, overwriting with `0` all memory before `free()`ing it.

### BSD like Linux-PAM environment variable setting

> `

>     extern int pam_misc_setenv(pam_handle_t *pamh, const char *name,
>                                const char *value, int readonly);

> `

This function performs a task equivalent to `pam_putenv()`, its syntax is, however, more like the BSD style function; `setenv()`. The `name` and `value` are concatenated with an ```=`'' to form a `name_value` and passed to `pam_putenv()`. If, however, the Linux-PAM variable is already set, the replacement will only be applied if the last argument, `readonly`, is zero.

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