4 The SANE Application Programmer Interface (API)¶

This Section defines version 1 of the SANE application programmer interface (API). Any SANE frontend must depend on the interface defined in this section only. Converseley, any SANE backend must implement its functionality in accordance with this specification. The interface as documented here is declared as a C callable interface in a file called sane/sane.h. This file should normally be included via a C pre-processor directive of the form:

#include <sane/sane.h>

4.1 Version Control¶

The SANE standard is expected to evolve over time. Whenever a change to the SANE standard is made that may render an existing frontend or backend incompatible with the new standard, the major version number must be increased. Thus, any frontend/backend pair is compatible provided the major version number of the SANE standard they implement is the same. A frontend may implement backwards compatiblity by allowing major numbers that are smaller than the expected major number (provided the frontend really can cope with the older version). In contrast, a backend always provides support for one and only one version of the standard. If a specific application does require that two different versions of the same backend are accessible at the same time, it is possible to do so by installing the two versions under different names.

SANE version control also includes a minor version number and a build revision. While control of these numbers remains with the implementor of a backend, the recommended use is as follows. The minor version is incremented with each official release of a backend. The build revision is increased with each build of a backend.

The SANE API provides the following five macros to manage version numbers.

SANE_CURRENT_MAJOR¶

The value of this macro is the number of the SANE standard that the interface implements.

SANE_VERSION_CODE(maj, min, bld)¶

This macro can be used to build a monotonically increasing version code. A SANE version code consists of the SANE standard major version number (maj), the minor version number min, and the build revision of a backend (bld). The major and minor version numbers must be in the range 0…255 and the build revision must be in the range 0…65535.

Version codes are monotonic in the sense that it is possible to apply relational operators (e.g., equality or less-than test) directly on the version code rather than individually on the three components of the version code.

Note that the major version number alone determines whether a frontend/backend pair is compatible. The minor version and the build revision are used for informational and bug-fixing purposes only.

SANE_VERSION_MAJOR(vc)¶

This macro returns the major version number component of the version code passed in argument vc.

SANE_VERSION_MINOR(vc)¶

This macro returns the minor version number component of the version code passed in argument vc.

SANE_VERSION_BUILD(vc)¶

This macro returns the build revision component of the version code passed in argument vc.

4.2 Data Types¶

4.2.1 Base Types¶

The SANE standard is based on just two SANE-specific base types: the SANE byte and word.

typedef some-scalar-type SANE_Byte;
typedef some-scalar-type SANE_Word;

SANE_Byte must correspond to some scalar C type that is capable of holding values in the range 0 to 255. SANE_Word must be capable of holding any of the following:

the truth values SANE_FALSE and SANE_TRUE
signed integers in the range $-2^{31}\ldots2^{31}-1$
fixed point values in the range $-32768\ldots32767.9999$ with a resolution of $1/65536$
32 bits (for bit sets)

Note that the SANE standard does not define what C type SANE_Byte and SANE_Word map to. For example, on some platforms, the latter may map to long int whereas on others it may map to int. A portable SANE frontend or backend must therefore not depend on a particular mapping.

4.2.2 Boolean Type¶

SANE_Bool is used for variables that can take one of the two truth values SANE_FALSE and SANE_TRUE. The former value is defined to be 0, whereas the latter is 1. 1 The C declarations for this type are given below.

#define SANE_FALSE      0
#define SANE_TRUE       1
typedef SANE_Word SANE_Bool;

Note that SANE_Bool is simply an alias of SANE_Word. It is therefore always legal to use the latter type in place of the former. However, for clarity, it is recommended to use SANE_Bool whenever a given variable or formal argument has a fixed interpretation as a boolean object.

4.2.3 Integer Type¶

SANE_Int is used for variables that can take integer values in the range $-2^{32}$ to $2^{31}-1$. Its C declaration is given below.

typedef SANE_Word SANE_Int;

Note that SANE_Int is simply an alias of SANE_Word. It is therefore always legal to use the latter type in place of the former. However, for clarity, it is recommended to use SANE_Int whenever a given variable or formal argument has a fixed interpretation as an integer object.

4.2.4 Fixed-point Type¶

SANE_Fixed is used for variables that can take fixed point values in the range $-32768$ to $32767.9999$ with a resolution of $1/65535$. The C declarations relating to this type are given below.

#define SANE_FIXED_SCALE_SHIFT  16
typedef SANE_Word SANE_Fixed;

The macro SANE_FIXED_SCALE_SHIFT gives the location of the fixed binary point. This standard defines that value to be 16, which yields a resolution of $1/65536$.

Note that SANE_Fixed is simply an alias of SANE_Word. It is therefore always legal to use the latter type in place of the former. However, for clarity, it is recommended to use SANE_Fixed whenever a given variable or formal argument has a fixed interpretation as a fixed-point object.

For convenience, SANE also defines two macros that convert fixed-point values to and from C double floating point values.

SANE_FIX(d)¶

Returns the largest SANE fixed-point value that is smaller than the double value d. No range checking is performed. If the value of d is out of range, the result is undefined.

SANE_UNFIX(w)¶

Returns the nearest double machine number that corresponds to fixed-point value w.

SANE does not require that the following two expressions hold true (even if the values of w and d are in range):

SANE_UNFIX(SANE_FIX(d)) == d
SANE_FIX(SANE_UNFIX(w)) == w

In other words, conversion between fixed and double values may be lossy. It is therefore recommended to avoid repeated conversions between the two representations.

4.2.5 Text¶

Character Type¶

Type SANE_Char represents a single text character or symbol. At present, this type maps directly to the underlying C char type (typically one byte). The encoding for such characters is currently fixed as ISO LATIN-1. Future versions of this standard may map this type to a wider type and allow multi-byte encodings to support internationalization. As a result of this, care should be taken to avoid the assumption that $\verb|sizeof|(\verb|SANE_Char|) == \verb|sizeof|(\verb|char|)$.

typedef char SANE_Char;

String Type¶

Type SANE_String represents a text string as a sequence of C char values. The end of the sequence is indicated by a '\0' (NUL) character.

typedef SANE_Char *SANE_String;
typedef const SANE_Char *SANE_String_Const;

The type SANE_String_Const is provided by SANE to enable declaring strings whose contents is unchangable. Note that in ANSI C, the declaration

const SANE_String str;

declares a string pointer that is constant (not a string pointer that points to a constant value).

4.2.6 Scanner Handle Type¶

Access to a scanner is provided through an opaque type called SANE_Handle. The C declaration of this type is given below.

typedef void *SANE_Handle;

While this type is declared to be a void pointer, an application must not attempt to interpret the value of a SANE_Handle. In particular, SANE does not require that a value of this type is a legal pointer value.

4.2.7 Status Type¶

Most SANE operations return a value of type SANE_Status to indicate whether the completion status of the operation. If an operation completes successfully, SANE_STATUS_GOOD is returned. In case of an error, a value is returned that indicates the nature of the problem. The complete list of available status codes is listed in Table 4.1. It is recommended to use function sane_strstatus() to convert status codes into a legible string.

Table 4.1 Status Codes¶
Symbol	Code	Description
`SANE_STATUS_GOOD`¶	0	Operation completed succesfully.
`SANE_STATUS_UNSUPPORTED`¶	1	Operation is not supported.
`SANE_STATUS_CANCELLED`¶	2	Operation was cancelled.
`SANE_STATUS_DEVICE_BUSY`¶	3	Device is busy—retry later.
`SANE_STATUS_INVAL`¶	4	Data or argument is invalid.
`SANE_STATUS_EOF`¶	5	No more data available (end-of-file).
`SANE_STATUS_JAMMED`¶	6	Document feeder jammed.
`SANE_STATUS_NO_DOCS`¶	7	Document feeder out of documents.
`SANE_STATUS_COVER_OPEN`¶	8	Scanner cover is open.
`SANE_STATUS_IO_ERROR`¶	9	Error during device I/O.
`SANE_STATUS_NO_MEM`¶	10	Out of memory.
`SANE_STATUS_ACCESS_DENIED`¶	11	Access to resource has been denied.

4.2.8 Device Descriptor Type¶

Each SANE device is represented by a structure of type SANE_Device. The C declaration of this type is given below.

typedef struct
  {
    SANE_String_Const name;
    SANE_String_Const vendor;
    SANE_String_Const model;
    SANE_String_Const type;
  }
SANE_Device;

The structure provides the unique name of the scanner in member name. It is this unique name that should be passed in a call to sane_open(). The format of this name is completely up to the backend. The only constraints are that the name is unique among all devices supported by the backend and that the name is a legal SANE text string. To simplify presentation of unique names, their length should not be excessive. It is recommended that backends keep unique names below 32 characters in length. However, applications must be able to cope with arbitrary length unique names.

The remaining members in the device structure provide additional information on the device corresponding to the unique name. Specifically, members vendor, model, and type are single-line strings that give information on the vendor (manufacturer), model, and the type of the device. For consistency’s sake, the following strings should be used when appropriate (the lists will be expanded as need arises):

Table 4.2 Predefined Device Information Strings¶
Vendor Strings
`AGFA`	`Logitech`
`Artec`	`Microtek`
`Connectix`	`Minolta`
`Epson`	`Mustek`
`Hewlett-Packard`	`UMAX`
`Kodak`	`Noname`

Type Strings
`flatbed scanner`
`frame grabber`
`handheld scanner`
`still camera`
`video camera`
`virtual device`

Note that vendor string Noname can be used for virtual devices that have no physical vendor associated. Also, there are no predefined model name strings since those are vendor specific and therefore completely under control of the respective backends.

4.2.9 Option Descriptor Type¶

Option descriptors are at the same time the most intricate and powerful type in the SANE standard. Options are used to control virtually all aspects of device operation. Much of the power of the SANE API stems from the fact that most device controls are completely described by their respective option descriptor. Thus, a frontend can control a scanner abstractly, without requiring knowledge as to what the purpose of any given option is. Conversely, a scanner can describe its controls without requiring knowledge of how the frontend operates. The C declaration of the SANE_Option_Descriptor type is given below.

typedef struct
  {
    SANE_String_Const name;
    SANE_String_Const title;
    SANE_String_Const desc;
    SANE_Value_Type type;
    SANE_Unit unit;
    SANE_Int size;
    SANE_Int cap;
    SANE_Constraint_Type constraint_type;
    union
      {
        const SANE_String_Const *string_list;
        const SANE_Word *word_list;
        const SANE_Range *range;
      }
    constraint;
  }
SANE_Option_Descriptor;

Option Name¶

Member name is a string that uniquely identifies the option. The name must be unique for a given device (i.e., the option names across different backends or devices need not be unique). The option name must consist of lower-case ASCII letters (a–z), digits (0–9), or the dash character (-) only. The first character must be a lower-case ASCII character (i.e., not a digit or a dash).

Option Title¶

Member title is a single-line string that can be used by the frontend as a title string. This should typically be a short (one or two-word) string that is chosen based on the function of the option.

Option Description¶

Member desc is a (potentially very) long string that can be used as a help text to describe the option. It is the responsibility of the frontend to break the string into managable-length lines. Newline characters in this string should be interpreted as paragraph breaks.

Option Value Type¶

Member type specifies the type of the option value. The possible values for type SANE_Value_Type are described in Table 4.3.

Table 4.3 Option Value Types (`SANE_Value_Type`)¶
Symbol	Code	Description
`SANE_TYPE_BOOL`¶	0	Option value is of type `SANE_Bool`.
`SANE_TYPE_INT`¶	1	Option value is of type `SANE_Int`.
`SANE_TYPE_FIXED`¶	2	Option value is of type `SANE_Fixed`.
`SANE_TYPE_STRING`¶	3	Option value is of type `SANE_String`.
`SANE_TYPE_BUTTON`¶	4	An option of this type has no value. Instead, setting an option of this type has an option-specific side-effect. For example, a button-typed option could be used by a backend to provide a means to select default values or to the tell an automatic document feeder to advance to the next sheet of paper.
`SANE_TYPE_GROUP`¶	5	An option of this type has no value. This type is used to group logically related options. A group option is in effect up to the point where another group option is encountered (or up to the end of the option list, if there are no other group options). For group options, only members `title` and `type` are valid in the option descriptor.

Option Value Unit¶

Member unit specifies what the physical unit of the option value is. The possible values for type SANE_Unit are described in Table 4.4. Note that the specified unit is what the SANE backend expects. It is entirely up to a frontend as to how these units a presented to the user. For example, SANE expresses all lengths in millimeters. A frontend is generally expected to provide appropriate conversion routines so that a user can express quantities in a customary unit (e.g., inches or centimeters).

Table 4.4 Physical Units (`SANE_Unit`)¶
Symbol	Code	Description
`SANE_UNIT_NONE`¶	0	Value is unit-less (e.g., page count).
`SANE_UNIT_PIXEL`¶	1	Value is in number of pixels.
`SANE_UNIT_BIT`¶	2	Value is in number of bits.
`SANE_UNIT_MM`¶	3	Value is in millimeters.
`SANE_UNIT_DPI`¶	4	Value is a resolution in dots/inch.
`SANE_UNIT_PERCENT`¶	5	Value is a percentage.
`SANE_UNIT_MICROSECOND`¶	6	Value is time in $\mu$-seconds.

Option Value Size¶

Member size specifies the size of the option value (in bytes). This member has a slightly different interpretation depending on the type of the option value:

SANE_TYPE_STRING: The size is the maximum size of the string. For the purpose of string size calcuations, the terminating NUL character is considered to be part of the string. Note that the terminating NUL character must always be present in string option values.

SANE_TYPE_INT, SANE_TYPE_FIXED: The size must be a positive integer multiple of the size of a SANE_Word. The option value is a vector of length $\verb|size| / \verb|sizeof|(\verb|SANE_Word|)$.

SANE_TYPE_BOOL: The size must be set to $\verb|sizeof|(\verb|SANE_Word|)$.

SANE_TYPE_BUTTON, SANE_TYPE_GROUP: The option size is ignored.

Option Capabilities¶

Member cap describes what capabilities the option posseses. This is a bitset that is formed as the inclusive logical OR of the capabilities described in Table 4.5. The SANE API provides the following to macros to test certain features of a given capability bitset:

SANE_OPTION_IS_ACTIVE(cap)¶

This macro returns SANE_TRUE if and only if the option with the capability set cap is currently active.

SANE_OPTION_IS_SETTABLE(cap)¶

This macro returns SANE_TRUE if and only if the option with the capability set cap is software settable.

Table 4.5 Option Capabilities¶
Symbol	Code	Description
`SANE_CAP_SOFT_SELECT`¶	1	The option value can be set by a call to `sane_control_option()`.
`SANE_CAP_HARD_SELECT`¶	2	The option value can be set by user-intervention (e.g., by flipping a switch). The user-interface should prompt the user to execute the appropriate action to set such an option. This capability is mutually exclusive with SANE_CAP_SOFT_SELECT (either one of them can be set, but not both simultaneously).
`SANE_CAP_SOFT_DETECT`¶	4	The option value can be detected by software. If `SANE_CAP_SOFT_SELECT` is set, this capability must be set. If `SANE_CAP_HARD_SELECT` is set, this capability may or may not be set. If this capability is set but neither `SANE_CAP_SOFT_SELECT` nor `SANE_CAP_HARD_SELECT` are, then there is no way to control the option. That is, the option provides read-out of the current value only.
`SANE_CAP_EMULATED`¶	8	If set, this capability indicates that an option is not directly supported by the device and is instead emulated in the backend. A sophisticated frontend may elect to use its own (presumably better) emulation in lieu of an emulated option.
`SANE_CAP_AUTOMATIC`¶	16	If set, this capability indicates that the backend (or the device) is capable to picking a reasonable option value automatically. For such options, it is possible to select automatic operation by calling `sane_control_option()` with an action value of `SANE_ACTION_SET_AUTO`.
`SANE_CAP_INACTIVE`¶	32	If set, this capability indicates that the option is not currently active (e.g., because it’s meaningful only if another option is set to some other value).
`SANE_CAP_ADVANCED`¶	64	If set, this capability indicates that the option should be considered an “advanced user option.” A frontend typically displays such options in a less conspicuous way than regular options (e.g., a command line interface may list such options last or a graphical interface may make them available in a seperate “advanced settings” dialog).

Option Value Constraints¶

It is often useful to constrain the values that an option can take. For example, constraints can be used by a frontend to determine how to represent a given option. Member constraint_type indicates what constraint is in effect for the option. The constrained values that are allowed for the option are described by one of the union members of member constraint. The possible values of type SANE_Constraint_Type and the interpretation of the constraint union is described in Table 4.6.

Table 4.6 Option Value Constraints¶
Symbol	Code	Description
`SANE_CONSTRAINT_NONE`¶	0	The value is unconstrained. The option can take any of the values possible for the option’s type.
`SANE_CONSTRAINT_RANGE`¶	1	This constraint is applicable to integer and fixed-point valued options only. It constrains the option value to a possibly quantized range of numbers. Option descriptor member `constraint.range` points to a range of the type `SANE_Range`. This type is illustrated below: typedef struct { SANE_Word min; SANE_Word max; SANE_Word quant; } SANE_Range; All three members in this structure are interpreted according to the option value type (`SANE_TYPE_INT` or `SANE_TYPE_FIXED`). Members `min` and `max` specify the minimum and maximum values, respectively. If member `quant` is non-zero, it specifies the quantization value. If $l$ is the minimum value, $u$ the maximum value and $q$ the (non-zero) quantization of a range, then the legal values are $v=k\cdot q+l$ for all non-negative integer values of $k$ such that $v<=u$.
`SANE_CONSTRAINT_WORD_LIST`¶	2	This constraint is applicable to integer and fixed-point valued options only. It constrains the option value to a list of numeric values. Option descriptor member `constraint.word_list` points to a list of words that enumerates the legal values. The first element in that list is an integer (`SANE_Int`) that specifies the length of the list (not counting the length itself). The remaining elements in the list are interpreted according to the type of the option value (`SANE_TYPE_INT` or `SANE_TYPE_FIXED`).
`SANE_CONSTRAINT_STRING_LIST`¶	3	This constraint is applicable to string-valued options only. It constrains the option value to a list of strings. The option descriptor member `constraint.string_list` points to a `NULL` terminated list of strings that enumerate the legal values for the option value.

4.3 Operations¶

4.3.1 `sane_init()`¶

This function must be called before any other SANE function can be called. The behavior of a SANE backend is undefined if this function is not called first. The version code of the backend is returned in the value pointed to by version_code. If that pointer is NULL, no version code is returned. Argument authorize is either a pointer to a function that is invoked when the backend requires authentication for a specific resource or NULL if the frontend does not support authentication.

SANE_Status sane_init (SANE_Int * version_code,
                       SANE_Authorization_Callback authorize);

The authorization function may be called by a backend in response to any of the following calls:

sane_open(),

sane_control_option(),

sane_start()

If a backend was initialized without authorization function, then authorization requests that cannot be handled by the backend itself will fail automatically and the user may be prevented from accessing protected resources. Backends are encouraged to implement means of authentication that do not require user assistance. E.g., on a multi-user system that authenticates users through a login process a backend could automatically lookup the apporpriate password based on resource- and user-name.

The authentication function type has the following declaration:

#define SANE_MAX_USERNAME_LEN   128
#define SANE_MAX_PASSWORD_LEN   128

typedef void (*SANE_Authorization_Callback)
    (SANE_String_Const resource,
     SANE_Char username[SANE_MAX_USERNAME_LEN],
     SANE_Char password[SANE_MAX_PASSWORD_LEN]);

Three arguments are passed to the authorization function: resource is a string specifying the name of the resource that requires authorization. A frontend should use this string to build a user-prompt requesting a username and a password. The username and password arguments are (pointers to) an array of SANE_MAX_USERNAME_LEN and SANE_MAX_PASSWORD_LEN characters, respectively. The authorization call should place the entered username and password in these arrays. The returned strings must be ASCII-NUL terminated.

4.3.2 `sane_exit()`¶

This function must be called to terminate use of a backend. The function will first close all device handles that still might be open (it is recommended to close device handles explicitly through a call to sane_close(), but backends are required to release all resources upon a call to this function). After this function returns, no function other than sane_init() may be called (regardless of the status value returned by sane_exit(). Neglecting to call this function may result in some resources not being released properly.

void sane_exit (void);

4.3.3 `sane_get_devices()`¶

This function can be used to query the list of devices that are available. If the function executes successfully, it stores a pointer to a NULL terminated array of pointers to SANE_Device structures in *device_list. The returned list is guaranteed to remain unchanged and valid until (a) another call to this function is performed or (b) a call to sane_exit() is performed. This function can be called repeatedly to detect when new devices become available. If argument local_only is true, only local devices are returned (devices directly attached to the machine that SANE is running on). If it is false, the device list includes all remote devices that are accessible to the SANE library.

SANE_Status sane_get_devices (const SANE_Device *** device_list,
                              SANE_Bool local_only);

This function may fail with SANE_STATUS_NO_MEM if an insufficient amount of memory is available.

Backend Implementation Note

SANE does not require that this function is called before a sane_open() call is performed. A device name may be specified explicitly by a user which would make it unnecessary and undesirable to call this function first.

4.3.4 `sane_open()`¶

This function is used to establish a connection to a particular device. The name of the device to be opened is passed in argument name. If the call completes successfully, a handle for the device is returned in *h. As a special case, specifying a zero-length string as the device requests opening the first available device (if there is such a device).

SANE_Status sane_open (SANE_String_Const name, SANE_Handle * h);

This function may fail with one of the following status codes.

SANE_STATUS_DEVICE_BUSY: The device is currently busy (in use by somebody else).

SANE_STATUS_INVAL: The device name is not valid.

SANE_STATUS_IO_ERROR: An error occured while communicating with the device.

SANE_STATUS_NO_MEM: An insufficent amount of memory is available.

SANE_STATUS_ACCESS_DENIED: Access to the device has been denied due to insufficient or invalid authentication.

4.3.5 `sane_close()`¶

This function terminates the association between the device handle passed in argument h and the device it represents. If the device is presently active, a call to sane_cancel() is performed first. After this function returns, handle h must not be used anymore.

void sane_close (SANE_Handle h);

4.3.6 `sane_get_option_descriptor()`¶

This function is used to access option descriptors. The function returns the option descriptor for option number n of the device represented by handle h. Option number 0 is guaranteed to be a valid option. Its value is an integer that specifies the number of options that are available for device handle h (the count includes option 0). If $n$ is not a valid option index, the function returns NULL. The returned option descriptor is guaranteed to remain valid (and at the returned address) until the device is closed.

const SANE_Option_Descriptor *
    sane_get_option_descriptor (SANE_Handle h, SANE_Int n);

4.3.7 `sane_control_option()`¶

This function is used to set or inquire the current value of option number n of the device represented by handle h. The manner in which the option is controlled is specified by parameter a. The possible values of this parameter are described in more detail below. The value of the option is passed through argument v. It is a pointer to the memory that holds the option value. The memory area pointed to by v must be big enough to hold the entire option value (determined by member size in the corresponding option descriptor). The only exception to this rule is that when setting the value of a string option, the string pointed to by argument v may be shorter since the backend will stop reading the option value upon encountering the first NUL terminator in the string. If argument i is not NULL, the value of *i will be set to provide details on how well the request has been met. The meaning of this argument is described in more detail below.

SANE_Status sane_control_option (SANE_Handle h, SANE_Int n,
                                 SANE_Action a, void *v,
                                 SANE_Int * i);

The way the option is affected by a call to this function is controlled by parameter a which is a value of type SANE_Action. The possible values and their meaning is described in Table 4.7.

Table 4.7 Action Values (`SANE_Action`)¶
Symbol	Code	Description
`SANE_ACTION_GET_VALUE`¶	0	Get current option value.
`SANE_ACTION_SET_VALUE`¶	1	Set option value. The option value passed through argument `v` may be modified by the backend if the value cannot be set exactly.
`SANE_ACTION_SET_AUTO`¶	2	Turn on automatic mode. Backend or device will automatically select an appropriate value. This mode remains effective until overridden by an explicit set value request. The value of parameter `v` is completely ignored in this case and may be `NULL`.

After setting a value via an action value of SANE_ACTION_SET_VALUE, additional information on how well the request has been met is returned in *i (if i is non-NULL). The returned value is a bitset that may contain any combination of the values described in Table 4.8.

Table 4.8 Additional Information Returned When Setting an Option¶
Symbol	Code	Description
`SANE_INFO_INEXACT`¶	1	This value is returned when setting an option value resulted in a value being selected that does not exactly match the requested value. For example, if a scanner can adjust the resolution in increments of 30dpi only, setting the resolution to 307dpi may result in an actual setting of 300dpi. When this happens, the bitset returned in `*i` has this member set. In addition, the option value is modified to reflect the actual (rounded) value that was used by the backend. Note that inexact values are admissible for strings as well. A backend may choose to “round” a string to the closest matching legal string for a constrained string value.
`SANE_INFO_RELOAD_OPTIONS`¶	2	The setting of an option may affect the value or availability of one or more other options. When this happens, the SANE backend sets this member in `*i` to indicate that the application should reload all options. This member may be set if and only if at least one option changed.
`SANE_INFO_RELOAD_PARAMS`¶	4	The setting of an option may affect the parameter values (see `sane_get_parameters()`). If setting an option affects the parameter values, this member will be set in `*i`. Note that this member may be set even if the parameters did not actually change. However, it is guaranteed that the parameters never change without this member being set.

This function may fail with one of the following status codes.

SANE_STATUS_UNSUPPORTED: The operation is not supported for the specified handle and option number.

SANE_STATUS_INVAL: The option value is not valid.

SANE_STATUS_IO_ERROR: An error occured while communicating with the device.

SANE_STATUS_NO_MEM: An insufficent amount of memory is available.

SANE_STATUS_ACCESS_DENIED: Access to the option has been denied due to insufficient or invalid authentication.

4.3.8 `sane_get_parameters()`¶

This function is used to obtain the current scan parameters. The returned parameters are guaranteed to be accurate between the time a scan has been started (sane_start() has been called) and the completion of that request. Outside of that window, the returned values are best-effort estimates of what the parameters will be when sane_start() gets invoked. Calling this function before a scan has actually started allows, for example, to get an estimate of how big the scanned image will be. The parameters passed to this function are the handle h of the device for which the parameters should be obtained and a pointer p to a parameter structure. The parameter structure is described in more detail below.

SANE_Status sane_get_parameters (SANE_Handle h,
                                 SANE_Parameters * p);

The scan parameters are returned in a structure of type SANE_Parameters. The C declaration of this structure is given below.

typedef struct
  {
    SANE_Frame format;
    SANE_Bool last_frame;
    SANE_Int lines;
    SANE_Int depth;
    SANE_Int pixels_per_line;
    SANE_Int bytes_per_line;
  }
SANE_Parameters;

Member format specifies the format of the next frame to be returned. The possible values for type SANE_Frame are described in Table 4.9. The meaning of these values is described in more detail in Section 3.2.

Table 4.9 Frame Format (`SANE_Frame`)¶
Symbol	Code	Description
`SANE_FRAME_GRAY`¶	0	Band covering human visual range.
`SANE_FRAME_RGB`¶	1	Pixel-interleaved red/green/blue bands.
`SANE_FRAME_RED`¶	2	Red band of a red/green/blue image.
`SANE_FRAME_GREEN`¶	3	Green band of a red/green/blue image.
`SANE_FRAME_BLUE`¶	4	Blue band of a red/green/blue image.

Member last_frame is set to SANE_TRUE if and only if the frame that is currently being acquired (or the frame that will be acquired next if there is no current frame) is the last frame of a multi frame image (e.g., the current frame is the blue component of a red, green, blue image).

Member lines specifies how many scan lines the frame is comprised of. If this value is -1, the number of lines is not known a priori and the frontend should call sane_read() until it returns a status of SANE_STATUS_EOF.

Member bytes_per_line specifies the number of bytes that comprise one scan line.

Member depth specifies the number of bits per sample.

Member pixels_per_line specifies the number of pixels that comprise one scan line.

Assume $B$ is the number of channels in the frame, then the bit depth $d$ (as given by member depth) and the number of pixels per line $n$ (as given by this member pixels_per_line) are related to $c$, the number of bytes per line (as given by member bytes_per_line) as follows:

\[\begin{split}c >= \left\{ \begin{array}{ll} \lceil B\cdot n / 8\rceil & \mbox{if $d=1$}\\ B\cdot n \cdot \lceil (d + 7)/8 \rceil & \mbox{if $d>1$} \end{array} \right.\end{split}\]

Note that the number of bytes per line can be larger than the minimum value imposed by the right side of this equation. A frontend must be able to properly cope with such “padded” image formats.

4.3.9 `sane_start()`¶

This function initiates aquisition of an image from the device represented by handle h.

SANE_Status sane_start (SANE_Handle h);

This function may fail with one of the following status codes.

SANE_STATUS_CANCELLED: The operation was cancelled through a call to sane_cancel().

SANE_STATUS_DEVICE_BUSY: The device is busy. The operation should be retried later.

SANE_STATUS_JAMMED: The document feeder is jammed.

SANE_STATUS_NO_DOCS: The document feeder is out of documents.

SANE_STATUS_COVER_OPEN: The scanner cover is open.

SANE_STATUS_IO_ERROR: An error occurred while communicating with the device.

SANE_STATUS_NO_MEM: An insufficent amount of memory is available.

4.3.10 `sane_read()`¶

This function is used to read image data from the device represented by handle h. Argument buf is a pointer to a memory area that is at least maxlen bytes long. The number of bytes returned is stored in *len. A backend must set this to zero when a status other than SANE_STATUS_GOOD is returned). When the call succeeds, the number of bytes returned can be anywhere in the range from 0 to maxlen bytes.

SANE_Status sane_read (SANE_Handle h, SANE_Byte * buf,
                       SANE_Int maxlen, SANE_Int * len);

If this function is called when no data is available, one of two things may happen, depending on the I/O mode that is in effect for handle h.

If the device is in blocking I/O mode (the default mode), the call blocks until at least one data byte is available (or until some error occurs).
If the device is in non-blocking I/O mode, the call returns immediately with status SANE_STATUS_GOOD and with *len set to zero.

The I/O mode of handle h can be set via a call to sane_set_io_mode().

This function may fail with one of the following status codes.

SANE_STATUS_CANCELLED: The operation was cancelled through a call to sane_cancel().

SANE_STATUS_EOF: No more data is available for the current frame.

SANE_STATUS_JAMMED: The document feeder is jammed.

SANE_STATUS_NO_DOCS: The document feeder is out of documents.

SANE_STATUS_COVER_OPEN: The scanner cover is open.

SANE_STATUS_IO_ERROR: An error occurred while communicating with the device.

SANE_STATUS_NO_MEM: An insufficent amount of memory is available.

SANE_STATUS_ACCESS_DENIED: Access to the device has been denied due to insufficient or invalid authentication.

4.3.11 `sane_cancel()`¶

This function is used to immediately or as quickly as possible cancel the currently pending operation of the device represented by handle h.

void sane_cancel (SANE_Handle h);

This function can be called at any time (as long as handle h is a valid handle) but usually affects long-running operations only (such as image is acquisition). It is safe to call this function asynchronously (e.g., from within a signal handler). It is important to note that completion of this operaton does not imply that the currently pending operation has been cancelled. It only guarantees that cancellation has been initiated. Cancellation completes only when the cancelled call returns (typically with a status value of SANE_STATUS_CANCELLED). Since the SANE API does not require any other operations to be re-entrant, this implies that a frontend must not call any other operation until the cancelled operation has returned.

4.3.12 `sane_set_io_mode()`¶

This function is used to set the I/O mode of handle h. The I/O mode can be either blocking or non-blocking. If argument m is SANE_TRUE, the mode is set to non-blocking mode, otherwise it’s set to blocking mode.

SANE_Status sane_set_io_mode (SANE_Handle h, SANE_Bool m);

By default, newly opened handles operate in blocking mode. A backend may elect not to support non-blocking I/O mode. In such a case the status value SANE_STATUS_UNSUPPORTED is returned. Blocking I/O must be supported by all backends, so calling this function with argument m set to SANE_FALSE is guaranteed to complete successfully.

This function may fail with one of the following status codes:

SANE_STATUS_INVAL: No image acquisition is pending.

SANE_STATUS_UNSUPPORTED: The backend does not support this operation.

4.3.13 `sane_get_select_fd()`¶

This function is used to obtain a (platform-specific) file-descriptor for handle h that is readable if and only if image data is available (i.e., when a call to sane_read() will return at least one byte of data). If the call completes successfully, the select file-descriptor is returned in *fd.

SANE_Status sane_get_select_fd (SANE_Handle h, SANE_Int *fd);

This function can be called only after a call to sane_start() has been performed and the returned file-descriptor is guaranteed to remain valid for the duration of the current image acquisition (i.e., until sane_cancel() or sane_start() get called again or until sane_read() returns with status SANE_STATUS_EOF). Indeed, a backend must guarantee to close the returned select file descriptor at the point when the next sane_read() call would return SANE_STATUS_EOF. This is necessary to ensure the application can detect when this condition occurs without actually having to call sane_read().

A backend may elect not to support this operation. In such a case, the function returns with status code SANE_STATUS_UNSUPPORTED.

Note that the only operation supported by the returned file-descriptor is a host operating-system dependent test whether the file-descriptor is readable (e.g., this test can be implemented using select() or poll() under UNIX). If any other operation is performed on the file descriptor, the behavior of the backend becomes unpredictable. Once the file-descriptor signals “readable” status, it will remain in that state until a call to sane_read() is performed. Since many input devices are very slow, support for this operation is strongly encouraged as it permits an application to do other work while image acquisition is in progress.

This function may fail with one of the following status codes:

SANE_STATUS_INVAL: No image acquisition is pending.

SANE_STATUS_UNSUPPORTED: The backend does not support this operation.

4.3.14 `sane_strstatus()`¶

This function can be used to translate a SANE status code into a printable string. The returned string is a single line of text that forms a complete sentence, but without the trailing period (full-stop). The function is guaranteed to never return NULL. The returned pointer is valid at least until the next call to this function is performed.

const SANE_String_Const sane_strstatus (SANE_Status status);

4.4 Code Flow¶

The code flow for the SANE API is illustrated in Figure 4.1. Functions sane_init() and sane_exit() initialize and exit the backend, respectively. All other calls must be performed after initialization and before exiting the backend.

Figure 4.1 Code flow¶

Function sane_get_devices() can be called any time after sane_init() has been called. It returns the list of the devices that are known at the time of the call. This list may change over time since some devices may be turned on or off or a remote host may boot or shutdown between different calls. It should be noted that this operation may be relatively slow since it requires contacting all configured devices (some of which may be on remote hosts). A frontend may therefore want to provide the ability for a user to directly select a desired device without requiring a call to this function.

Once a device has been chosen, it is opened using a call to sane_open(). Multiple devices can be open at any given time. A SANE backend must not impose artificial constraints on how many devices can be open at any given time.

An opened device can be setup through the corresponding device handle using functions sane_get_option_descriptor() and sane_control_option(). While setting up a device, obtaining option descriptors and setting and reading of option values can be mixed freely. It is typical for a frontend to read out all available options at the beginning and then build a dialog (either graphical or a command-line oriented option list) that allows to control the available options. It should be noted that the number of options is fixed for a given handle. However, as options are set, other options may become active or inactive. Thus, after setting an option, it maybe necessary to re-read some or all option descriptors. While setting up the device, it is also admissible to call sane_get_parameters() to get an estimate of what the image parameters will look like once image acquisition begins.

The device handle can be put in blocking or non-blocking mode by a call to sane_set_io_mode(). Devices are required to support blocking mode (which is the default mode), but support for non-blocking I/O is strongly encouraged for operating systems such as UNIX.

After the device is setup properly, image acquisition can be started by a call to sane_start(). The backend calculates the exact image parameters at this point. So future calls to sane_get_parameters() will return the exact values, rather than estimates. Whether the physical image acquisition starts at this point or during the first call to sane_read() is unspecified by the SANE API. If non-blocking I/O and/or a select-style interface is desired, the frontend may attempt to call sane_set_io_mode() and/or sane_get_select_fd() at this point. Either of these functions may fail if the backend does not support the requested operation.

Image data is collected by repeatedly calling sane_read(). Eventually, this function will return an end-of-file status (SANE_STATUS_EOF). This indicates the end of the current frame. If the frontend expects additional frames (e.g., the individual channels in of a red/green/blue image or multiple images), it can call sane_start() again. Once all desired frames have been acquired, function sane_cancel() must be called. This operation can also be called at any other time to cancel a pending operation. Note that sane_cancel() must be called even if the last read operation returned SANE_STATUS_EOF.

When done using the device, the handle should be closed by a call to sane_close(). Finally, before exiting the application, function sane_exit() must be called. It is important not to forget to call this function since otherwise some resources (e.g., temporary files or locks) may remain unclaimed.

4.5 Well-Known Options¶

While most backend options are completely self-describing, there are a cases where a user interface might want to special-case the handling of certain options. For example, the scan area is typically defined by four options that specify the top-left and bottom-right corners of the area. With a graphical user interface, it would be tedious to force the user to type in these four numbers. Instead, most such interfaces will want to present to the user a preview (low-resolution scan) of the scanner surface and let the user pick the scan area by dragging a rectangle into the desired position. For this reason, the SANE API specifies a small number of option names that have well-defined meanings.

4.5.1 Option Number Count¶

Option number 0 has an empty string as its name. The value of this option is of type SANE_TYPE_INT and it specifies the total number of options available for a given device (the count includes option number 0). This means that there are two ways of counting the number of options available: a frontend can either cycle through all option numbers starting at one until sane_get_option_descriptor() returns NULL, or a frontend can directly read out the value of option number 0.

4.5.2 Scan Resolution Option¶

Option resolution is used to select the resolution at which an image should be acquired. The type of this option is either SANE_TYPE_INT or SANE_TYPE_FIXED. The unit is SANE_UNIT_DPI (dots/inch).

This option is not mandatory, but if a backend does support it, it must implement it in a manner consistent with the above definition.

4.5.3 Preview Mode Option¶

The boolean option preview is used by a frontend to inform the backend when image acquisition should be optimized for speed, rather than quality (“preview mode”). When set to SANE_TRUE, preview mode is in effect, when set to SANE_FALSE image acquisition should proceed in normal quality mode. The setting of this option must not affect any other option. That is, as far as the other options are concerned, the preview mode is completely side effect free. A backend can assume that the frontend will take care of appropriately setting the scan resolution for preview mode (through option resolution). A backend is free to override the resolution value with its own choice for preview mode, but it is advised to leave this choice to the frontend wherever possible.

This option is not mandatory, but if a backend does support it, it must implement it in a manner consistent with the above definition.

4.5.4 Scan Area Options¶

The four most important well-known options are the ones that define the scan area. The scan area is defined by two points (x/y coordinate pairs) that specify the top-left and the bottom-right corners. This is illustrated in Figure 4.2. Note that the origin of the coordinate system is at the top-left corner of the scan surface as seen by the sensor (which typically is a mirror image of the scan surface seen by the user). For this reason, the top-left corner is the corner for which the abscissa and ordinate values are simultaneously the smallest and the bottom-right corner is the corner for which the abscissa and ordinate values are simulatenously the largest. If this coordinate system is not natural for a given device, it is the job of the backend to perform the necessary conversions.

Figure 4.2 Scan area options¶

The names of the four options that define the scan area are given in the table below:

Name	Description
`tl-x`	Top-left $x$ coordinate value
`tl-y`	Top-left $y$ coordinate value
`br-x`	Bottom-right $x$ coordinate value
`br-y`	Bottom-right $y$ coordinate value

There are several rules that should be followed by front and backends regarding these options:

Backends must attach a unit of either pixels (SANE_UNIT_PIXEL) or millimeters (SANE_UNIT_MM) to these options. The unit of all four options must be identical.
Whenever meaningful, a backend should attach a range or a word-list constraint to these options.
A frontend can determine the size of the scan surface by first checking that the options have range constraints associated. If a range or word-list constraints exist, the frontend can take the minimum and maximum values of one of the x and y option range-constraints to determine the scan surface size.
A frontend must work properly with any or all of these options missing.

1: This is different from ANSI C where any non-zero integer value represents logical TRUE.

4 The SANE Application Programmer Interface (API)¶

4.1 Version Control¶

4.2 Data Types¶

4.2.1 Base Types¶

4.2.2 Boolean Type¶

4.2.3 Integer Type¶

4.2.4 Fixed-point Type¶

4.2.5 Text¶

Character Type¶

String Type¶

4.2.6 Scanner Handle Type¶

4.2.7 Status Type¶

4.2.8 Device Descriptor Type¶

4.2.9 Option Descriptor Type¶

Option Name¶

Option Title¶

Option Description¶

Option Value Type¶

Option Value Unit¶

Option Value Size¶

Option Capabilities¶

Option Value Constraints¶

4.3 Operations¶

4.3.1 sane_init()¶

4.3.2 sane_exit()¶

4.3.3 sane_get_devices()¶

4.3.4 sane_open()¶

4.3.5 sane_close()¶

4.3.6 sane_get_option_descriptor()¶

4.3.7 sane_control_option()¶

4.3.8 sane_get_parameters()¶

4.3.9 sane_start()¶

4.3.10 sane_read()¶

4.3.11 sane_cancel()¶

4.3.12 sane_set_io_mode()¶

4.3.13 sane_get_select_fd()¶

4.3.14 sane_strstatus()¶

4.4 Code Flow¶

4.5 Well-Known Options¶

4.5.1 Option Number Count¶

4.5.2 Scan Resolution Option¶

4.5.3 Preview Mode Option¶

4.5.4 Scan Area Options¶

4.3.1 `sane_init()`¶

4.3.2 `sane_exit()`¶

4.3.3 `sane_get_devices()`¶

4.3.4 `sane_open()`¶

4.3.5 `sane_close()`¶

4.3.6 `sane_get_option_descriptor()`¶

4.3.7 `sane_control_option()`¶

4.3.8 `sane_get_parameters()`¶

4.3.9 `sane_start()`¶

4.3.10 `sane_read()`¶

4.3.11 `sane_cancel()`¶

4.3.12 `sane_set_io_mode()`¶

4.3.13 `sane_get_select_fd()`¶

4.3.14 `sane_strstatus()`¶