GRASS GIS 7 Programmer's Manual  7.9.dev(2021)-e5379bbd7
GRASS GIS General Library (aka GIS Library)

Table of Contents

by GRASS Development Team (https://grass.osgeo.org)

Subpages

This chapter is divided as follows:

Introduction

The GIS Library is the primary programming library provided with the GRASS system. Programs must use this library to access the GIS database. It contains the routines which locate, create, open, rename, and remove GRASS database files. It contains the routines which read and write raster files. It contains routines which interface the user to the database, including prompting the user, listing available files, validating user access, etc. It also has some general purpose routines (string manipulation, user information, etc.) which are not tied directly to database processing.

It is assumed that the reader has read File structure of GRASS Location for a general description of GRASS databases, Raster_File_Processing for details about raster map layers in GRASS, and Region_and_Mask (???) which discusses regions and masks. The routines in the GIS Library are presented in functional groupings, rather than in alphabetical order. The order of presentation will, it is hoped, provide a better understanding of how the library is to be used, as well as show the interrelationships among the various routines. Note that a good way to understand how to use these routines is to look at the source code for GRASS modules which use them. Most routines in this library require that the header file grass/gis.h be included in any code using these routines. Therefore, programmers should always include this file when writing code using routines from this library:

#include <grass/gis.h>

Note: All routines and global variables in this library, documented or undocumented, start with the prefix G_. To avoid name conflicts, programmers should not create variables or routines in their own modules which use this prefix.

Library Initialization

It is mandatory that the system be initialized before any other library routines are called.

G_gisinit() initialize GIS library.

This routine reads the user's GRASS environment file into memory and makes sure that the user has selected a valid database and mapset. It also initializes hidden variables used by other routines. If the user's database information is invalid, an error message is printed and the module exits. The program_name is stored for later recall by G_program_name(). It is recommended that argv[0] be used for the program_name:

int main (int argc, char **argv)
{
G_gisinit(argv[0]);
}

Diagnostic Messages

The following routines are used by other routines in the library to report warning and error messages. They may also be used directly by GRASS programs.

These routines report errors to the user. The normal mode is to write the message to the screen (on the standard error output). G_warning() will return and G_fatal_error() will exit.

If the standard error output is not a tty device, then the message is mailed to the user instead.

If the file GIS_ERROR_LOG exists (with write permission), in either the user's home directory or in the $GISBASE directory, the messages will also be logged to this file.

While most applications will find the normal error reporting quite adequate, there will be times when different handling is needed. For example, graphics modules may want the messages displayed graphically instead of on the standard error output. If the programmer wants to handle the error messages differently, the following routines can be used to modify the error handling:

This routine provides a different error handler for G_fatal_error() and G_warning(). The handler routine must be defined as follows:

int handler(char *message, int fatal)

where message is the message to be handled and fatal indicates the type of error:

  • 1 (fatal error)
  • or 0 (warning).

Note: The handler only provides a way to send the message somewhere other than to the error output. If the error is fatal, the module will exit after the handler returns.

This routine resets the error handling for G_fatal_error() and G_warning() back to the default action.

Environment and Database Information

The following routines return information about the current database selected by the user. Some of this information is retrieved from the user's GRASS environment file. Some of it comes from files in the database itself. See Environment_Variables for a discussion of the GRASS environment.

The following four routines can be used freely by the programmer:

Returns the name of the current database location. This routine should be used by modules that need to display the current location to the user. See Locations for an explanation of locations.

Returns the name of the current mapset in the current location. This routine is often used when accessing files in the current mapset. See Mapsets for an explanation of mapsets.

Returns a one line title for the database location. This title is read from the file MYNAME in the PERMANENT mapset. See also Permanent_Mapset for a discussion of the PERMANENT mapset.

Returns the full path name of the top level directory for GRASS programs. This directory will have subdirectories which will contain modules and files required for the running of the system. Some of these directories are:

bin  commands run by the user
etc  modules and data files used by GRASS commands
html help files

The use of G_gisbase() to find these subdirectories enables GRASS modules to be written independently of where the GRASS system is actually installed on the machine. For example, to run the module sroff in the GRASS etc directory:

char command[200];
sprintf(command, "%s/etc/sroff", G_gisbase());
G_spawn(command, "sroff", NULL);

The following two routines return full path UNIX directory names. They should be used only in special cases. They are used by other routines in the library to build full UNIX file names for database files. The programmer should not use the next two routines to bypass the normal database access routines.

Returns the full UNIX path name of the directory which holds the database locations. See GISDBASE for a full explanation of this directory.

Returns the full UNIX path name of the current database location. For example, if the user is working in location spearfish in the /home/user/grassdata database directory, this routine will return a string which looks like /home/user/grassdata/spearfish.

These next routines provide the low-level management of the information in the user's GRASS environment file. They should not be used in place of the higher level interface routines described above.

These routines look up the variable name in the GRASS environment and return its value (which is a character string). If name is not set, G_getenv() issues an error message and calls exit(). G_setenv_nogisrc() just returns the NULL pointer.

These routines set the the GRASS environment variable name to value. If value is NULL, the name is unset.

Both routines set the value in module memory, but only G_setenv() writes the new value to the user's GRASS environment file.

Fundamental Database Access Routines

In general, a user can only access data in the current location. For most modules, it's meaningless to access data from a different location as the projection will be wrong. A handful of modules (e.g. r.proj, v.proj) support accessing data from a different location, and in such cases there's no need for the user to own the source mapset, or even to own any mapset in the source location.

Since r54264, it is possible to bypass the ownership check by setting GRASS_SKIP_MAPSET_OWNER_CHECK to any non-empty string. This was added mostly for accessing VFAT/NTFS-format external drives or network shares where the ownership checks don't behave correctly. It should not be used on multi-user systems, nor set by programs.

But other than that, a user cannot make a mapset the current mapset (i.e. the one a user can modify) unless the user owns it. The user can access data from any mapset for which they have the relevant filesystem permissions (i.e. read permission for files, execute permission for the ancestor directories, read permission for any directories which need to be enumerated).

Originally this restriction was related to locking (the lock file was created in the user's home directory, so being able to set the current mapset to a directory which the user didn't own meant that they could have multiple sessions modifying a mapset concurrently).

That's no longer an issue (the lock file is created in the mapset directory). However, the other issue is that allowing someone other than the owner to modify the directory can result in problems for its owner.

Creating a new map typically creates a subdirectory of cell_misc for that map, and some files within it (e.g. a "null" file). Any new files and subdirectories will be owned by their creator (not the mapset's owner) and will have permissions determined by the current umask. The result is that there's no guarantee that tha mapset's owner will be able to delete (or even access) such files or directories.

In short, it's a safety mechanism against people trying to share directories by making them group-writeable without fully understanding the implications.

The routines described in this section provide the low-level interface to the GRASS database. They search the database for files, prompt the user for file names, open files for reading or writing, etc. The programmer should never bypass this level of database interface. These routines must be used to access the GRASS database unless there are other higher level library routines which perform the same function. For example, routines to process raster files (see Raster_File_Processing), vector files (see Vector_File_Processing), etc., should be used instead.

In the descriptions below, the term database element is used. Elements are subdirectories within a mapset and are associated with a specific GRASS data type. For example, raster files live in the "cell" and "fcell" element. See Elements for more details.

Fully Qualified File Names

All GRASS routines which access database files must be given both the file name and the mapset where the file resides. Often the name and the mapset are 2 distinct character strings. However, there is a need for a single character string which contains both the name and the mapset (e.g., for interactive interfacing to command-line programs). This form of the name is known as the fully qualified file name and is built by the following routine:

Returns a fully qualified name for the file name in mapset Currently this string is in the form name, but the programmer should pretend not to know this and always call this routine to get the fully qualified name.

The following example shows how an interactive version of d.rast interfaces with the command-line version of d.rast:

#include <stdlib.h>
#include <grass/gis.h>
int main(int argc, char *argv[])
{
char name[GNAME_MAX], *mapset, *fqn;
char command[1024];
G_gisinit(argv[0]);
mapset = G_ask_cell_old("", name);
if (mapset == NULL)
exit(EXIT_SUCCESS);
fqn = G_fully_qualified_name(name, mapset);
sprintf(command, "d.rast map='%s'", fqn);
G_spawn(command, "d.rast", NULL);
exit(EXIT_SUCCESS);
}

Finding Files in the Database

Command line driven module requires a database file name as one of the command arguments. In this case, the programmer must search the database to find the mapset where the file resides.

The following routines search the database for files:

Look for the file name under the specified element in the database. The mapset parameter can either be the empty string "", which means search all the mapsets in the user's current mapset search path, or it can be a specific mapset, which means. Look for the file only in this one mapset (for example, in the current mapset).

If found, the mapset where the file lives is returned. If not found, the NULL pointer is returned.

If the user specifies a fully qualified file name, (i.e, a name that also contains the mapset; see Fully Qualified File Names) then G_find_file() modifies name by eliminating the mapset from the name.

For example, to find a "paint/labels" file anywhere in the database:

char name[GNAME_MAX];
char *mapset;
if ((mapset = G_find_file("paint/labels", name, "")) == NULL)
/* not found */

To check that the file exists in the current mapset:

char name[GNAME_MAX];
if (G_find_file("paint/labels", name, G_mapset()) == NULL)
/* not found */

Legal File Names

Not all names that a user may enter will be legal files for the GRASS databases. The routines which create new files require that the new file have a legal name. If the name is obtained from the command line, the programmer must check that the name is legal. The following routine checks for legal file names:

Opening an Existing Database File for Reading

The following routines open the file name in mapset from the specified database element for reading (but not for writing). The file name and mapset can be obtained using G_find_file().

The database file name under the element in the specified mapset is opened for reading (but not for writing).

The UNIX open() routine is used to open the file. If the file does not exist, -1 is returned. Otherwise the file descriptor from the open() is returned.

The UNIX fopen() routine, with "r" read mode, is used to open the file. If the file does not exist, the NULL pointer is returned. Otherwise the file descriptor from the fopen() is returned.

Opening an Existing Database File for Update

The following routines open the file name in the current mapset from the specified database element for writing. The file must exist. Its name can be obtained using G_find_file().

The database file name under the element in the current mapset is opened for reading and writing.

The UNIX open() routine is used to open the file. If the file does not exist, -1 is returned. Otherwise the file is positioned at the end of the file and the file descriptor from the open() is returned.

The UNIX fopen() routine, with "a" append mode, is used to open the file. If the file does not exist, the NULL pointer is returned. Otherwise the file is positioned at the end of the file and the file descriptor from the fopen() is returned.

Creating and Opening a New Database File

The following routines create the new file name in the current mapset (GRASS does not allow files to be created outside the current mapset; see Database_Access_Rules) under the specified database element and open it for writing. The database element is created, if it does not already exist.

The file name is obtained noninteractively (e.g., from the command line), G_legal_filename() should be called first to make sure that name is a valid GRASS file name. Warning: It is not an error for name to already exist. However, the file will be removed and recreated empty. G_find_file() could be used to see if name exists.

The database file name under the element in the current mapset is created and opened for writing (but not reading).

The UNIX open() routine is used to open the file. If the file does not exist, -1 is returned. Otherwise the file is positioned at the end of the file and the file descriptor from the open() is returned.

The UNIX fopen() routine, with "w" write mode, is used to open the file. If the file does not exist, the NULL pointer is returned. Otherwise the file is positioned at the end of the file and the file descriptor from the fopen() is returned.

Database File Management

The following routines allow the renaming and removal of database files in the current mapset (These functions only apply to the current mapset since GRASS does permit users to modify things in mapsets other than the current mapset; see Database_Access_Rules).

Note: These functions only apply to the specific element and not to other "related" elements. For example, if element is "cell", then the specified raster file will be removed (or renamed), but the other support files, such as "cellhd" or "cats", will not. To remove these other files as well, specific calls must be made for each related element.

Region

The region concept is explained in Region. It can be thought of as a two-dimensional matrix with known boundaries and rectangular cells.

There are logically two different regions. The first is the database region that the user has set in the current mapset. The other is the region that is active in the module. This active module region is what controls reading and writing of raster file data. The vector map export does not take care for the active region settings.

The routines described below use a GRASS data structure Cell_head to hold region information. This structure is defined in the "gis.h" header file. It is discussed in detail under GIS Library Data Structures.

Database Region

Reading and writing the user's database region are done by the following routines:

Note: Previous versions of GRASS called this the "window". Due to overuse of this term (database window, graphics window, etc.), the term was changed to "region". However, to maintain compatibility with existing programs, library routine names were not changed - hence the term "window" is used in the routine name (where "region" should probably be used instead).

Reads the database region as stored in the WIND file in the user's current mapset into region.

An error message is printed and exit() is called if there is a problem reading the region.

Note: GRASS applications that read or write raster files should not use this routine since its use implies that the active module region will not be used. Programs that read or write raster file data (or vector data) can query the active module region using G_window_rows() and G_window_cols().

Writes the database region file (WIND) in the user's current mapset from region.

Warning: Since this routine actually changes the database region, it should only be called by modules which the user knows will change the region. It is probably fair to say that only the g.region should call this routine.

There is another database region. This region is the default region for the location. The default region provides the user with a "starting" region, i.e., a region to begin with and return to as a reference point. The GRASS modules g.region allow the user to set their database region from the default region (see Permanent_Mapset for a discussion of the default region). The following routine reads this region:

Active Module Region

The active module region is the one that is used when reading and writing raster file data. This region determines the resampling when reading raster data. It also determines the extent and resolution of new raster files.

Initially the active module region and the user's database region are the same, but the programmer can make them different. The following routines manage the active module region.

  • G_window_rows() number of rows in active region
  • G_window_cols() number of columns in active region

These routines return the number of rows and columns (respectively) in the active module region. Before raster files can be read or written, it is necessary to known how many rows and columns are in the active region. For example:

int nrows, cols;
int row, col;
nrows = G_window_rows();
ncols = G_window_cols();
for (row = 0; row < nrows; row++) {
/* read row ... */
for (col = 0; col < ncols; col++) {
/* process col ... */
}
}

This routine sets the active region from given region. Setting the active region does not change the WIND file in the database. It simply changes the region for the duration of the module.

However, the new region setting is not retained across the UNIX exec() call. This implies that G_set_window() cannot be used to set the region for a module to be executed using the system() or popen() routines.

Note: This routine overrides the region as set by the user. Its use should be very limited since it changes what the user normally expects to happen. If this routine is not called, then the active region will be the same as what is in the user's WIND file.

Warning: Calling this routine with already opened raster files has some side effects. If there are raster files which are open for reading, they will be read into the newly set region, not the region that was active when they were opened. However, CELL buffers allocated for reading the raster files are not automatically reallocated. The module must reallocate them explicitly. Also, this routine does not change the region for raster files which are open for writing. The region that was active when the open occurred still applies to these files.

Gets the values of the currently active region into region. If G_set_window() has been called, then the values set by that call are retrieved. Otherwise the user's database region is retrieved.

Note: For modules that read or write raster data, and really need the full region information, this routine is preferred over G_get_window(). However, since G_window_rows() and G_window_cols() return the number of rows and columns in the active region, the programmer should consider whether or not the full region information is really needed before using this routine.

  • G_align_window() align two regions

Modifies the input region to align to the reference region. The resolutions in region are set to match those in refefence region and the region edges (north, south, east, west) are modified to align with the grid of the reference region.

The region may be enlarged if necessary to achieve the alignment. The north is rounded northward, the south southward, the east eastward and the west westward.

  • G_col_to_easting()

Converts a column relative to a region to an easting.

Note: col is a double: col+0.5 will return the easting for the center of the column; col+0.0 will return the easting for the western edge of the column; and col+1.0 will return the easting for the eastern edge of the column.

  • G_row_to_northing()

Converts a row relative to a region to a northing.

Note: row is a double: row+0.5 will return the northing for the center of the row; row+0.0 will return the northing for the northern edge of the row; and row+1.0 will return the northing for the southern edge of the row.

  • G_easting_to_col()

Converts an easting relative to a region to a column.

Note: The result is a double. Casting it to an integer will give the column number.

  • G_northing_to_row()

Converts a northing relative to a region to a row.

Note: the result is a double. Casting it to an integer will give the row number.

Projection Information

The following routines return information about the cartographic projection and zone. See Region for more information about these values.

This routine returns a code indicating the projection for the active region. The current values are:

  • PROJECTION_XY - unreferenced x,y (imagery data)
  • PROJECTION_UTM - UTM
  • PROJECTION_SP - State Plane
  • PROJECTION_LL - Latitude-Longitude
  • PROJECTION_OTHER - Other (more than 121 projections are supported)
  • G_database_projection_name()

Returns a pointer to a string which is a printable name for projection code (as returned by G_projection()).

Returns a string describing the database grid units.

Returns a factor which converts the grid unit to meters (by multiplication). If the database is not metric (eg. imagery) then 0.0 is returned.

This routine returns the zone for the active region. The meaning for the zone depends on the projection. For example zone 18 for projection type 1 would be UTM zone 18.

Latitude-Longitude GIS Databases

GRASS supports databases in a longitude-latitude grid using a projection where the x coordinate is the longitude and the y coordinate is the latitude. This projection is called the Equidistant Cylindrical Projection (also known as Plate Carree). ECP has the property that where am I and row-column calculations are identical to those in planimetric grids (like UTM, Universal Transverse Mercator Projection). This implies that normal GRASS registration and overlay functions will work without any special considerations or modifications to existing code. However, the projection is not planimetric. This means that distance and area calculations are no longed Euclidean.

Also, since the world is round, maps may not have edges in the east-west direction, especially for global databases. Maps may have the same longitude at both the east and west edges of the display. This feature, called global wraparound, must be accounted for by GRASS modules (particularly vector based functions, like plotting). What follows is a description of the GIS Library routines that are available to support latitude-longitude databases.

Coordinates

Latitudes and longitudes are specified in degrees. Northern latitudes range from 0 to 90 degrees, and southern latitudes from 0 to -90. Longitudes have no limits since longitudes ±360 degrees are equivalent.

Coordinates are represented in ASCII using the format dd:mm:ssN or dd:mm:ssS for latitudes, ddd:mm:ssE or ddd.mm.ssW for longitudes, and dd.mm.ss for grid resolution. For example, 80:30:24N represents a northern latitude of 80 degrees, 30 minutes, and 24 seconds. 120:15W represents a longitude 120 degrees and 15 minutes west of the prime meridian. 30:15 represents a resolution of 30 degrees and 15 minutes. These next routines convert between ASCII representations and the machine representation for a coordinate. They work both with latitude-longitude projections and planimetric projections.

Note: In each subroutine, the programmer must specify the projection number. If the projection number is PROJECTION_LL (defined in "gis.h"), then latitude-longitude ASCII format is invoked. Otherwise, a standard floating-point to ASCII conversion is made.

Converts the double representation of the given coordinate to its ASCII representation.

Converts the double representation of the resolution to its ASCII representation.

Converts the ASCII coordinate string in to its double representation.

Converts the ASCII "resolution" string to its double representation (into resolution).

The following are examples of how these routines are used.

double north;
char buf[50];
G_scan_northing(buf, north, G_projection()); /* ASCII to double */
G_format_northing(north, buf, G_projection()); /* double to ASCII */
G_format_northing(north, buf, -1); /* double to ASCII */
/* This last example forces floating-point ASCII format */

Global Wraparound

These next routines provide a mechanism for determining the relative position of a pair of longitudes. Since longitudes of ±360 are equivalent, but GRASS requires the east to be bigger than the west, some adjustment of coordinates is necessary.

Returns east larger than west. If the region projection is PROJECTION_LL, then this routine returns an equivalent east that is larger, but no more than 360 degrees larger, than the coordinate for the western edge of the region. Otherwise no adjustment is made and the original east is returned.

This routine returns an equivalent east that is larger, but no more than 360 larger than the west coordinate.

This routine should be used only with latitude-longitude coordinates.

Returns shortest way between eastings.

Miscellaneous

This routine returns a pointer to a string containing the name for the ellipsoid in the GRASS ellipsoid table. It can be used as follows:

int n;
char *name;
for(n = 0; name = G_ellipsoid_name(n); n++)
fprintf(stdout, "%s\n", name);

This routine returns the semi-major axis (in meters) and eccentricity squared for the named ellipsoid.

This routine returns the semi-major axis (in meters) and the eccentricity squared for the ellipsoid associated with the database. If there is no ellipsoid explicitly associated with the database, it returns the values for the WGS 84 ellipsoid.

Returns the meridional radius of curvature at a given longitude:

$ \rho = \frac{a (1-e^2)}{(1-e^2\sin^2 lon)^{3/2}} $

Returns the transverse radius of curvature at a given longitude:

$ \nu = \frac{a}{(1-e^2\sin^2 lon)^{1/2}} $

Returns the radius of the conformal sphere tangent to ellipsoid at a given longitude:

$ r = \frac{a (1-e^2)^{1/2}}{(1-e^2\sin^2 lon)} $

For latitude-longitude coordinates, this routine determines if the polygon contains one of the poles.

Calculations

Raster Area Calculations

The following routines perform area calculations for raster maps. They are based on the fact that while the latitude-longitude grid is not planimetric, the size of the grid cell at a given latitude is constant. The first routines work in any projection.

This routine must be called once before any call to G_area_of_cell_at_row(). It can be used in either planimetric projections or the latitude-longitude projection.

This routine returns the area in square meters of a cell in the specified row. This value is constant for planimetric grids and varies with the row if the projection is latitude-longitude.

Initializes raster area calculations for an ellipsoid.

Returns the area between latitudes scaled by the factor passed to G_begin_zone_area_on_ellipsoid().

Initializes raster area calculations for a sphere.

Returns the area between latitudes.

Polygonal Area Calculations

These next routines provide area calculations for polygons. Some of the routines are specifically for latitude-longitude, while others will function for all projections.

However, there is an issue for latitude-longitude that does not occur with planimetric grids. Vector/polygon data is described as a series of x,y coordinates. The lines connecting the points are not stored but are inferred. This is a simple, straight-forward process for planimetric grids, but it is not simple for latitude-longitude. What is the shape of the line that connects two points on the surface of a globe?

One choice (among many) is the shortest path from x1,y1 to x2,y2, known as the geodesic. Another is a straight line on the grid. The area routines described below assume the latter. Routines to work with the former have not yet been developed.

This initializes the polygon area calculation routines. It is used both for planimetric and latitude-longitude projections.

Returns the area in square meters of the polygon. It is used both for planimetric and latitude-longitude projections.

Note. If the database is planimetric with the non-meter grid, this routine performs the required unit conversion to produce square meters.

Return the area in map units of the polygon,

This initializes the polygon area calculations for the ellipsoid.

Returns the area in square meters of the polygon for latitude-longitude grids.

Note: This routine assumes grid lines on the connecting the vertices (as opposed to geodesics).

Distance Calculations

Two routines perform distance calculations for any projection.

Initializes the distance calculations. It is used both for the planimetric and latitude-longitude projections.

This routine computes the distance between two points in meters.

Initializes the distance calculations for the ellipsoid. It is used only for the latitude-longitude projection.

Calculates the geodesic distance between two points in meters.

The calculation of the geodesic distance is fairly costly. These next three routines provide a mechanism for calculating distance with two fixed latitudes and varying longitude separation.

Calculates the geodesic distance between two points set by G_set_geodesic_distance_latl() and G_set_geodesic_distance_lat2().

General Plotting Routines

The following routines form the foundation of a general purpose line and polygon plotting capability.

Draws a line from one point to another using Bresenham's algorithm. A routine to plot points must be provided, as is defined as: point(x, y) plot a point at x,y.

This routine does not require a previous call to G_setup_plot() to function correctly, and is independent of all following routines.

Initializes the plotting capability. This routine must be called once before calling the G_plot_*() routines described below.

Plots line between latlon coordinates. This routine handles global wrap-around for latitude-longitude databases. See G_setup_plot() for the required coordinate initialization procedure.

Plots filled polygon with n vertices. See G_setup_plot() for the required coordinate initialization procedure.

Plots multiple polygons. Like G_plot_polygon(), except it takes a set of polygons, each with n vertices, where the number of polygons is specified with the rings argument. It is especially useful for plotting vector areas with interior islands.

The pixel coordinates x,y are converted to map coordinates east,north. See G_setup_plot() for the required coordinate initialization procedure.

The map coordinates east,north are converted to pixel coordinates x,y. See G_setup_plot() for the required coordinate initialization procedure.

Temporary Files

Often it is necessary for modules to use temporary files to store information that is only useful during the module run. After the module finishes, the information in the temporary file is no longer needed and the file is removed. Commonly it is required that temporary file names be unique from invocation to invocation of the module. It would not be good for a fixed name like "/tmp/mytempfile" to be used. If the module were run by two users at the same time, they would use the same temporary file. In addition systematic use of the /tmp directory could leave the system vulnerable to symlink attacks. The following routine generates temporary file names which are unique within the module and across all GRASS programs.

This routine returns a pointer to a string containing a unique file name that can be used as a temporary file within the module. Successive calls to G_tempfile() will generate new names.

Only the file name is generated. The file itself is not created. To create the file, the module must use standard UNIX functions which create and open files, e.g., creat() or fopen().

The programmer should take reasonable care to remove (unlink) the file before the module exits. However, GRASS database management will eventually remove all temporary files created by G_tempfile() that have been left behind by the modules which created them.

Note: The temporary files are created in the GRASS database rather than under /tmp. This is done for two reasons. The first is to increase the likelihood that enough disk is available for large temporary files since /tmp may be a very small file system. The second is so that abandoned temporary files can be automatically removed (but see the warning below).

Warning: The temporary files are named, in part, using the process id of the module. GRASS database management will remove these files only if the module which created them is no longer running. However, this feature has a subtle trap. Programs which create child processes (using the UNIX fork(), see also G_fork() routine) should let the child call G_tempfile(). If the parent does it and then exits, the child may find that GRASS has removed the temporary file since the process which created it is no longer running.

String Manipulation Functions

This section describes some routines which perform string manipulation. Strings have the usual C meaning: a NULL terminated array of characters.

These next 3 routines remove unwanted white space from a single string.

Leading and trailing white space is removed from the string and internal white space which is more than one character is reduced to a single space character. White space here means spaces, tabs, linefeeds, newlines, and formfeeds.

Leading and trailing white space is removed from the string. White space here means only spaces and tabs. There is no return value.

Chop leading and trailing white spaces: space, &#92;f, &#92;n, &#92;r, &#92;t, &#92;v.

The next routines replaces character(s) from string.

Replace all occurrences of character in string with new.

This next routine copies a string to allocated memory.

This routine allocates enough memory to hold the string, and returns a pointer to the allocated memory.

The next 2 routines convert between upper and lower case.

Upper case letters in the string are converted to their lower case equivalent.

Lower case letters in the string are converted to their upper case equivalent.

This routine remove trailing zeros from decimal number for example: 23.45000 would come back as 23.45.

  • G_index()
  • G_rindex()

Get position of delimiter.

String compare ignoring case (upper or lower).

Return a pointer to the first occurrence of subString in mainString, or NULL if no occurrences are found.

  • G_strdup()

Returns a pointer to a string that is a duplicate of the string given to G_strdup. The duplicate is created using malloc. If unable to allocate the required space, NULL is returned.

Enhanced UNIX Routines

A number of useful UNIX library routines have side effects which are sometimes undesirable. The routines here provide the same functions as their corresponding UNIX routine, but with different side effects.

Running in the Background

The standard UNIX fork() routine creates a child process which is a copy of the parent process. The fork() routine is useful for placing a module into the background. For example, a module that gathers input from the user interactively, but knows that the processing will take a long time, might want to run in the background after gathering all the input. It would fork() to create a child process, the parent would exit() allowing the child to continue in the background, and the user could then do other processing.

However, there is a subtle problem with this logic. The fork() routine does not protect child processes from keyboard interrupts even if the parent is no longer running. Keyboard interrupts will also kill background processes that do not protect themselves.

Note: Programmers who use Bash know that programs run in the background (using & on the command line) are not automatically protected from keyboard interrupts. To protect a command that is run in the background, Bash users must do nohup command &. Programmers who use the C Shell (or other variants) do not know, or forget that the C-shell automatically protects background processes from keyboard interrupts.

Thus a module which puts itself in the background may never finish if the user interrupts another module which is running at the keyboard.

Partially Interruptible System Call

The UNIX system() call allows one program, the parent, to execute another UNIX command or module as a child process, wait for that process to complete, and then continue. The problem addressed here concerns interrupts. During the standard system() call, the child process inherits its responses to interrupts from the parent. This means that if the parent is ignoring interrupts, the child will ignore them as well. If the parent is terminated by an interrupt, the child will be also.

Note that code which uses system() or popen() tends to be error-prone, particularly on Windows. We strongly discourage anyone from using either system() or popen() in the GRASS code. If you don't need to communicate with the child process via a pipe, you can use G_spawn() or G_spawn_ex(). These provide similar functionality to system() but without the shell getting in the way (using the shell creates problems if filenames or other arguments contain spaces or other characters which are significant to the shell). If you need to communicate via a pipe, you can either use G_spawn_ex() and manage the pipe yourself, or use popen(). See also G_popen_read() and G_popen_write() which do most of the work for you. Alternatively, you can use a temporary file rather than a pipe for communicating with the child process.

ENDIAN test

To test if the user's machine is little or big ENDIAN, the following function is provided:

Test if machine is little or big endian.

Miscellaneous

A number of general purpose routines have been provided.

Returns a pointer to a string which is the current date and time. The format is the same as that produced by the UNIX date command.

Returns a pointer to a string which is the full path name of the user's home directory.

This routine prints a percentage complete message to stderr. Example:

#include<grass/gis.h>
#include<grass/glocale.h>
int row;
int nrows;
nrows = 1352; /* 1352 is not a special value - example only */
G_message(_("Percent complete:"));
for (row = 0; row < nrows; row++)
G_percent(row, nrows, 10);

This will print completion messages at 10% increments; i.e., 10%, 20%, 30%, etc., up to 100%. Each message does not appear on a new line, but rather erases the previous message. After 100%, a new line is printed.

Routine returns the name of the module as set by the call to G_gisinit().

Returns a pointer to a string which is the user's login name.

GIS Library Data Structures

Some of the data structures, defined in the "gis.h" header file and used by routines in this library, are described in the sections below.

struct Cell_head

The raster header data structure is used for two purposes. It is used for raster header information for raster map. It also used to hold region values.

See documentation of Cell_head structure for details.

The format and compressed fields apply only to raster headers. The format field describes the number of bytes per raster data value and the compressed field indicates if the raster file is compressed or not. The other fields apply both to raster headers and regions. The geographic boundaries are described by north, south, east and west. The grid resolution is described by ew_res and ns_res. The cartographic projection is described by proj and the related zone for the projection by zone. The rows and cols indicate the number of rows and columns in the raster map, or in the region. See Raster Header File for more information about raster headers, and Region for more information about regions.

The routines described in Raster Header File use this structure.

struct Categories

The Categories structure contains a title for the map layer, the largest category in the map layer, an automatic label generation rule for missing labels, and a list of category labels.

struct Categories
{
CELL ncats; /* total number of categories */
CELL num; /* the highest cell values. Only exists
for backwards compatibility = (CELL)
max_fp_values in quant rules */
char *title; /* name of data layer */
char *fmt; /* printf-like format to generate labels */
float m1; /* multiplication coefficient 1 */
float a1; /* addition coefficient 1 */
float m2; /* multiplication coefficient 2 */
float a2; /* addition coefficient 2 */
struct Quant q; /* rules mapping cell values to index in
list of labels */
char **labels; /* array of labels of size num */
int *marks; /* was the value with this label was used? */
int nalloc;
int last_marked_rule;
};

This structure should be accessed using the routines described in Raster Category File.

struct Colors

The color data structure holds red, green, and blue color intensities for raster categories. The structure has become so complicated that it will not be described in this manual.

struct Colors
{
int version; /* set by read_colors: -1=old, 1=new */
DCELL shift;
int invert;
int is_float; /* defined on floating point raster data? */
int null_set; /* the colors for null are set? */
unsigned char null_red;
unsigned char null_grn;
unsigned char null_blu;
int undef_set; /* the colors for cells not in range are set? */
unsigned char undef_red;
unsigned char undef_grn;
unsigned char undef_blu;
struct _Color_Info_ fixed;
struct _Color_Info_ modular;
DCELL cmin;
DCELL cmax;
int organizing;
};

The routines described in Raster Color Table must be used to store and retrieve color information using this structure.

struct History

The History structure is used to document raster files. The information contained here is for the user. It is not used in any operational way by GRASS.

See documentation of History structure for details.

The mapid and mapset are the raster file name and mapset, title is the raster file title, creator is the user who created the file, maptype is the map type (which should always be "raster"), datasrc_1 and datasrc_2 describe the original data source, keywrd is a one-line data description and edhist contains edlinecnt lines of user comments.

The routines described in Raster History File use this structure. However, there is very little support for manipulating the contents of this structure. The programmer must manipulate the contents directly.

Note: Some of the information in this structure is not meaningful. For example, if the raster file is renamed, or copied into another mapset, the mapid and mapset will no longer be correct. Also the title does not reflect the true raster file title. The true title is maintained in the category file.

Warning: This structure has remained unchanged since the inception of GRASS. There is a good possibility that it will be changed or eliminated in future releases.

struct Range

The Range (FPRange for floating point raster maps) structure contains the minimum and maximum values which occur in a raster file.

struct Range
{
CELL min;
CELL max;
int first_time; /* whether or not range was updated */
};
struct FPRange
{
DCELL min;
DCELL max;
int first_time; /* whether or not range was updated */
};

The routines described in Raster Range File should be used to access this structure.

Loading the GIS Library

The library is loaded by specifying in the Makefile. The following example is a complete Makefile which compiles code that uses this library:

Makefile for 

MODULE_TOPDIR = ../..

PGM = r.info

LIBES = $(GISLIB)
DEPENDENCIES = $(GISDEP)

include $(MODULE_TOPDIR)/include/Make/Module.make

default: cmd

See Compiling and Installing GRASS Modules for a complete discussion of Makefiles.

Timestamp functions

This structure is defined in "gis.h", but there should be no reason to access its elements directly:

struct TimeStamp {
DateTime dt[2]; /* two datetimes */
int count;
};

Using the G_*_timestamp() routines reads/writes a timestamp file in the cell_misc/rastername mapset element.

A TimeStamp can be one DateTime, or two DateTimes representing a range. When preparing to write a TimeStamp, the programmer should use one of:

Use to copy the TimeStamp information into Datetimes, so the members of struct TimeStamp shouldn't be accessed directly.

  • count=0 means no datetimes were copied
  • count=1 means 1 datetime was copied into dt1
  • count=2 means 2 datetimes were copied
  • G_init_timestamp()

Sets ts->count = 0, to indicate no valid DateTimes are in TimeStamp.

Copies a single DateTime to a TimeStamp in preparation for writing (overwrites any existing information in TimeStamp).

Copies two DateTimes (a range) to a TimeStamp in preparation for writing (overwrites any existing information in TimeStamp).

Only timestamp files in current mapset can be removed.

  • G_read_grid3_timestamp()

See DateTime_Library for a complete discussion of GRASS datetime routines.

Memory Allocation

The following routines provide memory allocation capability. They are simply calls to the UNIX suite of memory allocation routines malloc(), realloc() and calloc(), except that if there is not enough memory, they print a diagnostic message to that effect and then call exit().

  • G_malloc ()

Allocates a block of memory at least size bytes which is aligned properly for all data types. A pointer to the aligned block is returned.

Changes the size of a previously allocated block of memory at ptr and returns a pointer to the new block of memory. The size may be larger or smaller than the original size. If the original block cannot be extended "in place", then a new block is allocated and the original block copied to the new block.

Note: If ptr is NULL, then this routine simply allocates a block of size bytes. This routine works around broken realloc() routines, which do not handle a NULL ptr.

Allocates a properly aligned block of memory n*size bytes in length, initializes the allocated memory to zero, and returns a pointer to the allocated block of memory.

Use the G_free() routine to release memory allocated by these routines.