pam_conv − PAM conversation function
const char *msg;
int (*conv)(int num_msg, const struct pam_message **msg,
struct pam_response **resp, void *appdata_ptr);
The PAM library uses an application−defined callback to allow a direct communication between a loaded module and the application. This callback is specified by the struct pam_conv passed to pam_start(3) at the start of the transaction.
When a module calls the referenced conv() function, the argument appdata_ptr is set to the second element of this structure.
The other arguments of a call to conv() concern the information exchanged by module and application. That is to say, num_msg holds the length of the array of pointers, msg. After a successful return, the pointer resp points to an array of pam_response structures, holding the application supplied text. The resp_retcode member of this struct is unused and should be set to zero. It is the caller´s responsibility to release both, this array and the responses themselves, using free(3). Note, *resp is a struct pam_response array and not an array of pointers.
The number of responses is always equal to the num_msg conversation function argument. This does require that the response array is free(3)´d after every call to the conversation function. The index of the responses corresponds directly to the prompt index in the pam_message array.
On failure, the conversation function should release any resources it has allocated, and return one of the predefined PAM error codes.
Each message can have one of four types, specified by the msg_style member of struct pam_message:
Obtain a string without echoing any text.
Obtain a string whilst echoing text.
Display an error message.
Display some text.
The point of having an array of messages is that it becomes possible to pass a number of things to the application in a single call from the module. It can also be convenient for the application that related things come at once: a windows based application can then present a single form with many messages/prompts on at once.
In passing, it is worth noting that there is a descrepency between the way Linux−PAM handles the const struct pam_message **msg conversation function argument from the way that Solaris´ PAM (and derivitives, known to include HP/UX, are there others?) does. Linux−PAM interprets the msg argument as entirely equivalent to the following prototype const struct pam_message *msg (which, in spirit, is consistent with the commonly used prototypes for argv argument to the familiar main() function: char **argv; and char *argv). Said another way Linux−PAM interprets the msg argument as a pointer to an array of num_msg read only ´struct pam_message´ pointers. Solaris´ PAM implementation interprets this argument as a pointer to a pointer to an array of num_msg pam_message structures. Fortunately, perhaps, for most module/application developers when num_msg has a value of one these two definitions are entirely equivalent. Unfortunately, casually raising this number to two has led to unanticipated compatibility problems.
For what its worth the two known module writer work−arounds for trying to maintain source level compatibility with both PAM implementations are:
• never call the conversation function with num_msg greater than one.
• set up msg as doubly referenced so both types of conversation function can find the messages. That is, make
msg[n] = & (( *msg )[n])
Memory buffer error.
Conversation failure. The application should not set *resp.
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.