Overview

The CUPS API provides the convenience functions needed to support applications, filters, printer drivers, and backends that need to interface with the CUPS scheduler.

Clients and Servers

CUPS is based on the Internet Printing Protocol ("IPP"), which allows clients (applications) to communicate with a server (the scheduler) to get a list of printers, send print jobs, and so forth. You identify which server you want to communicate with using a pointer to the opaque structure http_t. All of the examples in this document use the CUPS_HTTP_DEFAULT constant, referring to the default connection to the scheduler. The HTTP and IPP APIs document provides more information on server connections.

Printers and Classes

Printers and classes (collections of printers) are accessed through the cups_dest_t structure which includes the name (name), instance (instance - a way of selecting certain saved options/settings), and the options and attributes associated with that destination (num_options and options). Destinations are created using the cupsGetDests function and freed using the cupsFreeDests function. The cupsGetDest function finds a specific destination for printing:

#include <cups/cups.h>

cups_dest_t *dests;
int num_dests = cupsGetDests(&dests);
cups_dest_t *dest = cupsGetDest("name", NULL, num_dests, dests);

/* do something with dest */

cupsFreeDests(num_dests, dests);

Passing NULL to cupsGetDest for the destination name will return the default destination. Similarly, passing a NULL instance will return the default instance for that destination.

Table 1: Printer Attributes
Attribute Name Description
"auth-info-required" The type of authentication required for printing to this destination: "none", "username,password", "domain,username,password", or "negotiate" (Kerberos)
"printer-info" The human-readable description of the destination such as "My Laser Printer".
"printer-is-accepting-jobs" "true" if the destination is accepting new jobs, "false" if not.
"printer-is-shared" "true" if the destination is being shared with other computers, "false" if not.
"printer-location" The human-readable location of the destination such as "Lab 4".
"printer-make-and-model" The human-readable make and model of the destination such as "HP LaserJet 4000 Series".
"printer-state" "3" if the destination is idle, "4" if the destination is printing a job, and "5" if the destination is stopped.
"printer-state-change-time" The UNIX time when the destination entered the current state.
"printer-state-reasons" Additional comma-delimited state keywords for the destination such as "media-tray-empty-error" and "toner-low-warning".
"printer-type" The cups_printer_t value associated with the destination.

Options

Options are stored in arrays of cups_option_t structures. Each option has a name (name) and value (value) associated with it. The cups_dest_t num_options and options members contain the default options for a particular destination, along with several informational attributes about the destination as shown in Table 1. The cupsGetOption function gets the value for the named option. For example, the following code lists the available destinations and their human-readable descriptions:

#include <cups/cups.h>

cups_dest_t *dests;
int num_dests = cupsGetDests(&dests);
cups_dest_t *dest;
int i;
const char *value;

for (i = num_dests, dest = dests; i > 0; i --, dest ++)
  if (dest->instance == NULL)
  {
    value = cupsGetOption("printer-info", dest->num_options, dest->options);
    printf("%s (%s)\n", dest->name, value ? value : "no description");
  }

cupsFreeDests(num_dests, dests);

You can create your own option arrays using the cupsAddOption function, which adds a single named option to an array:

#include <cups/cups.h>

int num_options = 0;
cups_option_t *options = NULL;

/* The returned num_options value is updated as needed */
num_options = cupsAddOption("first", "value", num_options, &options);

/* This adds a second option value */
num_options = cupsAddOption("second", "value", num_options, &options);

/* This replaces the first option we added */
num_options = cupsAddOption("first", "new value", num_options, &options);

Use a for loop to copy the options from a destination:

#include <cups/cups.h>

int i;
int num_options = 0;
cups_option_t *options = NULL;
cups_dest_t *dest;

for (i = 0; i < dest->num_options; i ++)
  num_options = cupsAddOption(dest->options[i].name, dest->options[i].value,
                              num_options, &options);

Use the cupsFreeOptions function to free the options array when you are done using it:

cupsFreeOptions(num_options, options);

Print Jobs

Print jobs are identified by a locally-unique job ID number from 1 to 231-1 and have options and one or more files for printing to a single destination. The cupsPrintFile function creates a new job with one file. The following code prints the CUPS test page file:

#include <cups/cups.h>

cups_dest_t *dest;
int num_options;
cups_option_t *options;
int job_id;

/* Print a single file */
job_id = cupsPrintFile(dest->name, "/usr/share/cups/data/testprint.ps",
                        "Test Print", num_options, options);

The cupsPrintFiles function creates a job with multiple files. The files are provided in a char * array:

#include <cups/cups.h>

cups_dest_t *dest;
int num_options;
cups_option_t *options;
int job_id;
char *files[3] = { "file1.pdf", "file2.pdf", "file3.pdf" };

/* Print three files */
job_id = cupsPrintFiles(dest->name, 3, files, "Test Print", num_options, options);

Finally, the cupsCreateJob function creates a new job with no files in it. Files are added using the cupsStartDocument, cupsWriteRequestData, and cupsFinishDocument functions. The following example creates a job with 10 text files for printing:

#include <cups/cups.h>

cups_dest_t *dest;
int num_options;
cups_option_t *options;
int job_id;
int i;
char buffer[1024];

/* Create the job */
job_id = cupsCreateJob(CUPS_HTTP_DEFAULT, dest->name, "10 Text Files",
                       num_options, options);

/* If the job is created, add 10 files */
if (job_id > 0)
{
  for (i = 1; i <= 10; i ++)
  {
    snprintf(buffer, sizeof(buffer), "file%d.txt", i);

    cupsStartDocument(CUPS_HTTP_DEFAULT, dest->name, job_id, buffer,
                      CUPS_FORMAT_TEXT, i == 10);

    snprintf(buffer, sizeof(buffer),
             "File %d\n"
             "\n"
             "One fish,\n"
             "Two fish,\n
             "Red fish,\n
             "Blue fish\n", i);

    /* cupsWriteRequestData can be called as many times as needed */
    cupsWriteRequestData(CUPS_HTTP_DEFAULT, buffer, strlen(buffer));

    cupsFinishDocument(CUPS_HTTP_DEFAULT, dest->name);
  }
}

Once you have created a job, you can monitor its status using the cupsGetJobs function, which returns an array of cups_job_t structures. Each contains the job ID (id), destination name (dest), title (title), and other information associated with the job. The job array is freed using the cupsFreeJobs function. The following example monitors a specific job ID, showing the current job state once every 5 seconds until the job is completed:

#include <cups/cups.h>

cups_dest_t *dest;
int job_id;
int num_jobs;
cups_job_t *jobs;
int i;
ipp_jstate_t job_state = IPP_JOB_PENDING;
 
while (job_state < IPP_JOB_STOPPED)
{
  /* Get my jobs (1) with any state (-1) */
  num_jobs = cupsGetJobs(&jobs, dest->name, 1, -1);

  /* Loop to find my job */
  job_state = IPP_JOB_COMPLETED;

  for (i = 0; i < num_jobs; i ++)
    if (jobs[i].id == job_id)
    {
      job_state = jobs[i].state;
      break;
    }

  /* Free the job array */
  cupsFreeJobs(num_jobs, jobs);

  /* Show the current state */
  switch (job_state)
  {
    case IPP_JOB_PENDING :
        printf("Job %d is pending.\n", job_id);
        break;
    case IPP_JOB_HELD :
        printf("Job %d is held.\n", job_id);
        break;
    case IPP_JOB_PROCESSING :
        printf("Job %d is processing.\n", job_id);
        break;
    case IPP_JOB_STOPPED :
        printf("Job %d is stopped.\n", job_id);
        break;
    case IPP_JOB_CANCELED :
        printf("Job %d is canceled.\n", job_id);
        break;
    case IPP_JOB_ABORTED :
        printf("Job %d is aborted.\n", job_id);
        break;
    case IPP_JOB_COMPLETED :
        printf("Job %d is completed.\n", job_id);
        break;
  }

  /* Sleep if the job is not finished */
  if (job_state < IPP_JOB_STOPPED)
    sleep(5);
}

To cancel a job, use the cupsCancelJob function with the job ID:

#include <cups/cups.h>

cups_dest_t *dest;
int job_id;

cupsCancelJob(dest->name, job_id);

Error Handling

If any of the CUPS API printing functions returns an error, the reason for that error can be found by calling the cupsLastError and cupsLastErrorString functions. cupsLastError returns the last IPP error code (ipp_status_t) that was encountered, while cupsLastErrorString returns a (localized) human-readable string that can be shown to the user. For example, if any of the job creation functions returns a job ID of 0, you can use cupsLastErrorString to show the reason why the job could not be created:

#include <cups/cups.h>

int job_id;

if (job_id == 0)
  puts(cupsLastErrorString());

Passwords and Authentication

CUPS supports authentication of any request, including submission of print jobs. The default mechanism for getting the username and password is to use the login user and a password from the console.

To support other types of applications, in particular Graphical User Interfaces ("GUIs"), the CUPS API provides functions to set the default username and to register a callback function that returns a password string.

The cupsSetPasswordCB function is used to set a password callback in your program. Only one function can be used at any time.

The cupsSetUser function sets the current username for authentication. This function can be called by your password callback function to change the current username as needed.

The following example shows a simple password callback that gets a username and password from the user:

#include <cups/cups.h>

const char *
my_password_cb(const char *prompt)
{
  char	user[65];


  puts(prompt);

  /* Get a username from the user */
  printf("Username: ");
  if (fgets(user, sizeof(user), stdin) == NULL)
    return (NULL);

  /* Strip the newline from the string and set the user */
  user[strlen(user) - 1] = '\0';

  cupsSetUser(user);

  /* Use getpass() to ask for the password... */
  return (getpass("Password: "));
}

cupsSetPasswordCB(my_password_cb);

Similarly, a GUI could display the prompt string in a window with input fields for the username and password. The username should default to the string returned by the cupsUser function.