In Section 36.3 you saw how you can execute SQL statements from an embedded SQL program. Some of those statements only used fixed values and did not provide a way to insert user-supplied values into statements or have the program process the values returned by the query. Those kinds of statements are not really useful in real applications. This section explains in detail how you can pass data between your C program and the embedded SQL statements using a simple mechanism called host variables. In an embedded SQL program we consider the SQL statements to be guests in the C program code which is the host language. Therefore the variables of the C program are called host variables.
Another way to exchange values between PostgreSQL backends and ECPG applications is the use of SQL descriptors, described in Section 36.7.
Passing data between the C program and the SQL statements is particularly simple in embedded SQL. Instead of having the program paste the data into the statement, which entails various complications, such as properly quoting the value, you can simply write the name of a C variable into the SQL statement, prefixed by a colon. For example:
EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2);
This statement refers to two C variables named
v1 and v2 and also uses a
regular SQL string literal, to illustrate that you are not
restricted to use one kind of data or the other.
This style of inserting C variables in SQL statements works anywhere a value expression is expected in an SQL statement.
To pass data from the program to the database, for example as parameters in a query, or to pass data from the database back to the program, the C variables that are intended to contain this data need to be declared in specially marked sections, so the embedded SQL preprocessor is made aware of them.
This section starts with:
EXEC SQL BEGIN DECLARE SECTION;
and ends with:
EXEC SQL END DECLARE SECTION;
Between those lines, there must be normal C variable declarations, such as:
int x = 4; char foo[16], bar[16];
As you can see, you can optionally assign an initial value to the variable. The variable's scope is determined by the location of its declaring section within the program. You can also declare variables with the following syntax which implicitly creates a declare section:
EXEC SQL int i = 4;
You can have as many declare sections in a program as you like.
The declarations are also echoed to the output file as normal C variables, so there's no need to declare them again. Variables that are not intended to be used in SQL commands can be declared normally outside these special sections.
The definition of a structure or union also must be listed inside
a DECLARE section. Otherwise the preprocessor cannot
handle these types since it does not know the definition.
Now you should be able to pass data generated by your program into
an SQL command. But how do you retrieve the results of a query?
For that purpose, embedded SQL provides special variants of the
usual commands SELECT and
FETCH. These commands have a special
INTO clause that specifies which host variables
the retrieved values are to be stored in.
SELECT is used for a query that returns only
single row, and FETCH is used for a query that
returns multiple rows, using a cursor.
Here is an example:
/* * assume this table: * CREATE TABLE test1 (a int, b varchar(50)); */ EXEC SQL BEGIN DECLARE SECTION; int v1; VARCHAR v2; EXEC SQL END DECLARE SECTION; ... EXEC SQL SELECT a, b INTO :v1, :v2 FROM test;
So the INTO clause appears between the select
list and the FROM clause. The number of
elements in the select list and the list after
INTO (also called the target list) must be
equal.
Here is an example using the command FETCH:
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test;
...
do
{
...
EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2;
...
} while (...);
Here the INTO clause appears after all the
normal clauses.
When ECPG applications exchange values between the PostgreSQL server and the C application, such as when retrieving query results from the server or executing SQL statements with input parameters, the values need to be converted between PostgreSQL data types and host language variable types (C language data types, concretely). One of the main points of ECPG is that it takes care of this automatically in most cases.
In this respect, there are two kinds of data types: Some simple
PostgreSQL data types, such as integer
and text, can be read and written by the application
directly. Other PostgreSQL data types, such
as timestamp and numeric can only be
accessed through special library functions; see
Section 36.4.4.2.
Table 36.1 shows which PostgreSQL data types correspond to which C data types. When you wish to send or receive a value of a given PostgreSQL data type, you should declare a C variable of the corresponding C data type in the declare section.
Table 36.1. Mapping Between PostgreSQL Data Types and C Variable Types
| PostgreSQL data type | Host variable type |
|---|---|
smallint | short |
integer | int |
bigint | long long int |
decimal | decimal[a] |
numeric | numeric[a] |
real | float |
double precision | double |
smallserial | short |
serial | int |
bigserial | long long int |
oid | unsigned int |
character(, varchar(, text | char[, VARCHAR[[b] |
name | char[NAMEDATALEN] |
timestamp | timestamp[a] |
interval | interval[a] |
date | date[a] |
boolean | bool[c] |
bytea | char * |
[a] This type can only be accessed through special library functions; see Section 36.4.4.2. [b] declared in [c] declared in | |
To handle SQL character string data types, such
as varchar and text, there are two
possible ways to declare the host variables.
One way is using char[], an array
of char, which is the most common way to handle
character data in C.
EXEC SQL BEGIN DECLARE SECTION;
char str[50];
EXEC SQL END DECLARE SECTION;
Note that you have to take care of the length yourself. If you use this host variable as the target variable of a query which returns a string with more than 49 characters, a buffer overflow occurs.
The other way is using the VARCHAR type, which is a
special type provided by ECPG. The definition on an array of
type VARCHAR is converted into a
named struct for every variable. A declaration like:
VARCHAR var[180];
is converted into:
struct varchar_var { int len; char arr[180]; } var;
The member arr hosts the string
including a terminating zero byte. Thus, to store a string in
a VARCHAR host variable, the host variable has to be
declared with the length including the zero byte terminator. The
member len holds the length of the
string stored in the arr without the
terminating zero byte. When a host variable is used as input for
a query, if strlen(arr)
and len are different, the shorter one
is used.
VARCHAR can be written in upper or lower case, but
not in mixed case.
char and VARCHAR host variables can
also hold values of other SQL types, which will be stored in
their string forms.
ECPG contains some special types that help you to interact easily
with some special data types from the PostgreSQL server. In
particular, it has implemented support for the
numeric, decimal, date, timestamp,
and interval types. These data types cannot usefully be
mapped to primitive host variable types (such
as int, long long int,
or char[]), because they have a complex internal
structure. Applications deal with these types by declaring host
variables in special types and accessing them using functions in
the pgtypes library. The pgtypes library, described in detail
in Section 36.6 contains basic functions to deal
with those types, such that you do not need to send a query to
the SQL server just for adding an interval to a time stamp for
example.
The follow subsections describe these special data types. For more details about pgtypes library functions, see Section 36.6.
Here is a pattern for handling timestamp variables
in the ECPG host application.
First, the program has to include the header file for the
timestamp type:
#include <pgtypes_timestamp.h>
Next, declare a host variable as type timestamp in
the declare section:
EXEC SQL BEGIN DECLARE SECTION; timestamp ts; EXEC SQL END DECLARE SECTION;
And after reading a value into the host variable, process it
using pgtypes library functions. In following example, the
timestamp value is converted into text (ASCII) form
with the PGTYPEStimestamp_to_asc()
function:
EXEC SQL SELECT now()::timestamp INTO :ts;
printf("ts = %s\n", PGTYPEStimestamp_to_asc(ts));
This example will show some result like following:
ts = 2010-06-27 18:03:56.949343
In addition, the DATE type can be handled in the same way. The
program has to include pgtypes_date.h, declare a host variable
as the date type and convert a DATE value into a text form using
PGTYPESdate_to_asc() function. For more details about the
pgtypes library functions, see Section 36.6.
The handling of the interval type is also similar
to the timestamp and date types. It
is required, however, to allocate memory for
an interval type value explicitly. In other words,
the memory space for the variable has to be allocated in the
heap memory, not in the stack memory.
Here is an example program:
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_interval.h>
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
interval *in;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
in = PGTYPESinterval_new();
EXEC SQL SELECT '1 min'::interval INTO :in;
printf("interval = %s\n", PGTYPESinterval_to_asc(in));
PGTYPESinterval_free(in);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
The handling of the numeric
and decimal types is similar to the
interval type: It requires defining a pointer,
allocating some memory space on the heap, and accessing the
variable using the pgtypes library functions. For more details
about the pgtypes library functions,
see Section 36.6.
No functions are provided specifically for
the decimal type. An application has to convert it
to a numeric variable using a pgtypes library
function to do further processing.
Here is an example program handling numeric
and decimal type variables.
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_numeric.h>
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
numeric *num;
numeric *num2;
decimal *dec;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
num = PGTYPESnumeric_new();
dec = PGTYPESdecimal_new();
EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec;
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));
/* Convert decimal to numeric to show a decimal value. */
num2 = PGTYPESnumeric_new();
PGTYPESnumeric_from_decimal(dec, num2);
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));
PGTYPESnumeric_free(num2);
PGTYPESdecimal_free(dec);
PGTYPESnumeric_free(num);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
As a host variable you can also use arrays, typedefs, structs, and pointers.
There are two use cases for arrays as host variables. The first
is a way to store some text string in char[]
or VARCHAR[], as
explained in Section 36.4.4.1. The second use case is to
retrieve multiple rows from a query result without using a
cursor. Without an array, to process a query result consisting
of multiple rows, it is required to use a cursor and
the FETCH command. But with array host
variables, multiple rows can be received at once. The length of
the array has to be defined to be able to accommodate all rows,
otherwise a buffer overflow will likely occur.
Following example scans the pg_database
system table and shows all OIDs and names of the available
databases:
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
int dbid[8];
char dbname[8][16];
int i;
EXEC SQL END DECLARE SECTION;
memset(dbname, 0, sizeof(char)* 16 * 8);
memset(dbid, 0, sizeof(int) * 8);
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
/* Retrieve multiple rows into arrays at once. */
EXEC SQL SELECT oid,datname INTO :dbid, :dbname FROM pg_database;
for (i = 0; i < 8; i++)
printf("oid=%d, dbname=%s\n", dbid[i], dbname[i]);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
This example shows following result. (The exact values depend on local circumstances.)
oid=1, dbname=template1 oid=11510, dbname=template0 oid=11511, dbname=postgres oid=313780, dbname=testdb oid=0, dbname= oid=0, dbname= oid=0, dbname=
A structure whose member names match the column names of a query result, can be used to retrieve multiple columns at once. The structure enables handling multiple column values in a single host variable.
The following example retrieves OIDs, names, and sizes of the
available databases from the pg_database
system table and using
the pg_database_size() function. In this
example, a structure variable dbinfo_t with
members whose names match each column in
the SELECT result is used to retrieve one
result row without putting multiple host variables in
the FETCH statement.
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
long long int size;
} dbinfo_t;
dbinfo_t dbval;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, dbval.size);
}
EXEC SQL CLOSE cur1;
This example shows following result. (The exact values depend on local circumstances.)
oid=1, datname=template1, size=4324580 oid=11510, datname=template0, size=4243460 oid=11511, datname=postgres, size=4324580 oid=313780, datname=testdb, size=8183012
Structure host variables “absorb” as many columns
as the structure as fields. Additional columns can be assigned
to other host variables. For example, the above program could
also be restructured like this, with the size
variable outside the structure:
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
} dbinfo_t;
dbinfo_t dbval;
long long int size;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval, :size;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, size);
}
EXEC SQL CLOSE cur1;
Use the typedef keyword to map new types to already
existing types.
EXEC SQL BEGIN DECLARE SECTION;
typedef char mychartype[40];
typedef long serial_t;
EXEC SQL END DECLARE SECTION;
Note that you could also use:
EXEC SQL TYPE serial_t IS long;
This declaration does not need to be part of a declare section.
You can declare pointers to the most common types. Note however that you cannot use pointers as target variables of queries without auto-allocation. See Section 36.7 for more information on auto-allocation.
EXEC SQL BEGIN DECLARE SECTION;
int *intp;
char **charp;
EXEC SQL END DECLARE SECTION;
This section contains information on how to handle nonscalar and user-defined SQL-level data types in ECPG applications. Note that this is distinct from the handling of host variables of nonprimitive types, described in the previous section.
Multi-dimensional SQL-level arrays are not directly supported in ECPG. One-dimensional SQL-level arrays can be mapped into C array host variables and vice-versa. However, when creating a statement ecpg does not know the types of the columns, so that it cannot check if a C array is input into a corresponding SQL-level array. When processing the output of a SQL statement, ecpg has the necessary information and thus checks if both are arrays.
If a query accesses elements of an array
separately, then this avoids the use of arrays in ECPG. Then, a
host variable with a type that can be mapped to the element type
should be used. For example, if a column type is array of
integer, a host variable of type int
can be used. Also if the element type is varchar
or text, a host variable of type char[]
or VARCHAR[] can be used.
Here is an example. Assume the following table:
CREATE TABLE t3 (
ii integer[]
);
testdb=> SELECT * FROM t3;
ii
-------------
{1,2,3,4,5}
(1 row)
The following example program retrieves the 4th element of the
array and stores it into a host variable of
type int:
EXEC SQL BEGIN DECLARE SECTION;
int ii;
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii ;
printf("ii=%d\n", ii);
}
EXEC SQL CLOSE cur1;
This example shows the following result:
ii=4
To map multiple array elements to the multiple elements in an array type host variables each element of array column and each element of the host variable array have to be managed separately, for example:
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[1], ii[2], ii[3], ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii_a[0], :ii_a[1], :ii_a[2], :ii_a[3];
...
}
Note again that
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* WRONG */
EXEC SQL FETCH FROM cur1 INTO :ii_a;
...
}
would not work correctly in this case, because you cannot map an array type column to an array host variable directly.
Another workaround is to store arrays in their external string
representation in host variables of type char[]
or VARCHAR[]. For more details about this
representation, see Section 8.15.2. Note that
this means that the array cannot be accessed naturally as an
array in the host program (without further processing that parses
the text representation).
Composite types are not directly supported in ECPG, but an easy workaround is possible. The available workarounds are similar to the ones described for arrays above: Either access each attribute separately or use the external string representation.
For the following examples, assume the following type and table:
CREATE TYPE comp_t AS (intval integer, textval varchar(32)); CREATE TABLE t4 (compval comp_t); INSERT INTO t4 VALUES ( (256, 'PostgreSQL') );
The most obvious solution is to access each attribute separately.
The following program retrieves data from the example table by
selecting each attribute of the type comp_t
separately:
EXEC SQL BEGIN DECLARE SECTION;
int intval;
varchar textval[33];
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch each element of the composite type column into host variables. */
EXEC SQL FETCH FROM cur1 INTO :intval, :textval;
printf("intval=%d, textval=%s\n", intval, textval.arr);
}
EXEC SQL CLOSE cur1;
To enhance this example, the host variables to store values in
the FETCH command can be gathered into one
structure. For more details about the host variable in the
structure form, see Section 36.4.4.3.2.
To switch to the structure, the example can be modified as below.
The two host variables, intval
and textval, become members of
the comp_t structure, and the structure
is specified on the FETCH command.
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int intval;
varchar textval[33];
} comp_t;
comp_t compval;
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
EXEC SQL CLOSE cur1;
Although a structure is used in the FETCH
command, the attribute names in the SELECT
clause are specified one by one. This can be enhanced by using
a * to ask for all attributes of the composite
type value.
...
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).* FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
...
This way, composite types can be mapped into structures almost seamlessly, even though ECPG does not understand the composite type itself.
Finally, it is also possible to store composite type values in
their external string representation in host variables of
type char[] or VARCHAR[]. But that
way, it is not easily possible to access the fields of the value
from the host program.
New user-defined base types are not directly supported by ECPG.
You can use the external string representation and host variables
of type char[] or VARCHAR[], and this
solution is indeed appropriate and sufficient for many types.
Here is an example using the data type complex from
the example in Section 38.12. The external string
representation of that type is (%f,%f),
which is defined in the
functions complex_in()
and complex_out() functions
in Section 38.12. The following example inserts the
complex type values (1,1)
and (3,3) into the
columns a and b, and select
them from the table after that.
EXEC SQL BEGIN DECLARE SECTION;
varchar a[64];
varchar b[64];
EXEC SQL END DECLARE SECTION;
EXEC SQL INSERT INTO test_complex VALUES ('(1,1)', '(3,3)');
EXEC SQL DECLARE cur1 CURSOR FOR SELECT a, b FROM test_complex;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :a, :b;
printf("a=%s, b=%s\n", a.arr, b.arr);
}
EXEC SQL CLOSE cur1;
This example shows following result:
a=(1,1), b=(3,3)
Another workaround is avoiding the direct use of the user-defined types in ECPG and instead create a function or cast that converts between the user-defined type and a primitive type that ECPG can handle. Note, however, that type casts, especially implicit ones, should be introduced into the type system very carefully.
For example,
CREATE FUNCTION create_complex(r double, i double) RETURNS complex LANGUAGE SQL IMMUTABLE AS $$ SELECT $1 * complex '(1,0')' + $2 * complex '(0,1)' $$;
After this definition, the following
EXEC SQL BEGIN DECLARE SECTION; double a, b, c, d; EXEC SQL END DECLARE SECTION; a = 1; b = 2; c = 3; d = 4; EXEC SQL INSERT INTO test_complex VALUES (create_complex(:a, :b), create_complex(:c, :d));
has the same effect as
EXEC SQL INSERT INTO test_complex VALUES ('(1,2)', '(3,4)');
The examples above do not handle null values. In fact, the retrieval examples will raise an error if they fetch a null value from the database. To be able to pass null values to the database or retrieve null values from the database, you need to append a second host variable specification to each host variable that contains data. This second host variable is called the indicator and contains a flag that tells whether the datum is null, in which case the value of the real host variable is ignored. Here is an example that handles the retrieval of null values correctly:
EXEC SQL BEGIN DECLARE SECTION; VARCHAR val; int val_ind; EXEC SQL END DECLARE SECTION: ... EXEC SQL SELECT b INTO :val :val_ind FROM test1;
The indicator variable val_ind will be zero if
the value was not null, and it will be negative if the value was
null.
The indicator has another function: if the indicator value is positive, it means that the value is not null, but it was truncated when it was stored in the host variable.
If the argument -r no_indicator is passed to
the preprocessor ecpg, it works in
“no-indicator” mode. In no-indicator mode, if no
indicator variable is specified, null values are signaled (on
input and output) for character string types as empty string and
for integer types as the lowest possible value for type (for
example, INT_MIN for int).