ECPGPlus - Overview v13

EDB has enhanced ECPG (the PostgreSQL pre-compiler) to create ECPGPlus. ECPGPlus is a Pro*C-compatible version of the PostgreSQL C pre-compiler. ECPGPlus translates a program that combines C code and embedded SQL statements into an equivalent C program. As it performs the translation, ECPGPlus verifies that the syntax of each SQL construct is correct.

The following diagram charts the path of a program containing embedded SQL statements as it is compiled into an executable:

To produce an executable from a C program that contains embedded SQL statements, pass the program (my_program.pgc in the diagram above) to the ECPGPlus pre-compiler. ECPGPlus translates each SQL statement in my_program.pgc into C code that calls the ecpglib API, and produces a C program (my_program.c). Then, pass the C program to a C compiler; the C compiler generates an object file (my_program.o). Finally, pass the object file (my_program.o), as well as the ecpglib library file, and any other required libraries to the linker, which in turn produces the executable (my_program).

While the ECPGPlus preprocessor validates the syntax of each SQL statement, it cannot validate the semantics. For example, the preprocessor will confirm that an INSERT statement is syntactically correct, but it cannot confirm that the table mentioned in the INSERT statement actually exists.

Behind the Scenes

A client application contains a mix of C code and SQL code comprised of the following elements:

• C preprocessor directives
• C declarations (variables, types, functions, ...)
• C definitions (variables, types, functions, ...)
• SQL preprocessor directives
• SQL statements

For example:

1 #include <stdio.h>
2 EXEC SQL INCLUDE sqlca;
3
4 extern void printInt(char *label, int val);
5 extern void printStr(char *label, char *val);
6 extern void printFloat(char *label, float val);
7
8 void displayCustomer(int custNumber)
9 {
10  EXEC SQL BEGIN DECLARE SECTION;
11    VARCHAR custName[50];
12    float custBalance;
13    int custID = custNumber;
14  EXEC SQL END DECLARE SECTION;
15
16  EXEC SQL SELECT name, balance
17    INTO :custName, :custBalance
18    FROM customer
19    WHERE id = :custID;
20
21  printInt("ID", custID);
22  printStr("Name", custName);
23  printFloat("Balance", custBalance);
24 }

In the above code fragment:

• Line 1 specifies a directive to the C preprocessor.

  C preprocessor directives may be interpreted or ignored; the option is controlled by a command line option (-C PROC) entered when you invoke ECPGPlus. In either case, ECPGPlus copies each C preprocessor directive to the output file (4) without change; any C preprocessor directive found in the source file will appear in the output file.

• Line 2 specifies a directive to the SQL preprocessor.

  SQL preprocessor directives are interpreted by the ECPGPlus preprocessor, and are not copied to the output file.

• Lines 4 through 6 contain C declarations.

  C declarations are copied to the output file without change, except that each VARCHAR declaration is translated into an equivalent struct declaration.

• Lines 10 through 14 contain an embedded-SQL declaration section.

  C variables that you refer to within SQL code are known as host variables. If you invoke the ECPGPlus preprocessor in Pro*C mode (-C PROC), you may refer to any C variable within a SQL statement; otherwise you must declare each host variable within a BEGIN/END DECLARATION SECTION pair.

• Lines 16 through 19 contain a SQL statement.

  SQL statements are translated into calls to the ECPGPlus run-time library.

• Lines 21 through 23 contain C code.

  C code is copied to the output file without change.

Any SQL statement must be prefixed with EXEC SQL and extends to the next (unquoted) semicolon. For example:

printf(“Updating employee salaries\n”);

EXEC SQL UPDATE emp SET sal = sal * 1.25;
EXEC SQL COMMIT;

printf(“Employee salaries updated\n”);

When the preprocessor encounters the code fragment shown above, it passes the C code (the first line and the last line) to the output file without translation and converts each EXEC SQL statement into a call to an ecpglib function. The result would appear similar to the following:

printf("Updating employee salaries\n");

{
  ECPGdo( __LINE__, 0, 1, NULL, 0, ECPGst_normal,
         "update emp set sal = sal * 1.25",
         ECPGt_EOIT, ECPGt_EORT);
}

{
  ECPGtrans(__LINE__, NULL, "commit");
}

printf(“Employee salaries updated\n”);

Installation and Configuration

On Windows, ECPGPlus is installed by the Advanced Server installation wizard as part of the Database Server component. On Linux, install with the edb-asxx-server-devel RPM package or edb-as<xx>-server-dev APT package, where xx is the Advanced Server version number. By default, the executable is located in:

On Windows:

C:\Program Files\edb\as13\bin

On Linux:

/usr/edb/as13/bin

When invoking the ECPGPlus compiler, the executable must be in your search path (%PATH% on Windows, $PATH on Linux). For example, the following commands set the search path to include the directory that holds the ECPGPlus executable file ecpg.

On Windows:

set EDB_PATH=C:\Program Files\edb\as13\bin
set PATH=%EDB_PATH%;%PATH%

On Linux:

export EDB_PATH==/usr/edb/as13/bin
export PATH=$EDB_PATH:$PATH

Constructing a Makefile

A makefile contains a set of instructions that tell the make utility how to transform a program written in C (that contains embedded SQL) into a C program. To try the examples in this guide, you will need:

• a C compiler (and linker)
• the make utility
• ECPGPlus preprocessor and library
• a makefile that contains instructions for ECPGPlus

The following code is an example of a makefile for the samples included in this guide. To use the sample code, save it in a file named makefile in the directory that contains the source code file.

INCLUDES = -I$(shell pg_config --includedir)
LIBPATH = -L $(shell pg_config --libdir)
CFLAGS += $(INCLUDES) -g
LDFLAGS += -g
LDLIBS += $(LIBPATH) -lecpg -lpq

.SUFFIXES: .pgc,.pc

.pgc.c:
     ecpg -c $(INCLUDES) $?

.pc.c:
     ecpg -C PROC -c $(INCLUDES) $?

The first two lines use the pg_config program to locate the necessary header files and library directories:

INCLUDES = -I$(shell pg_config --includedir)
LIBPATH = -L $(shell pg_config --libdir)

The pg_config program is shipped with Advanced Server.

make knows that it should use the CFLAGS variable when running the C compiler and LDFLAGS and LDLIBS when invoking the linker. ECPG programs must be linked against the ECPG run-time library (-lecpg) and the libpq library (-lpq)

CFLAGS += $(INCLUDES) -g
LDFLAGS += -g
LDLIBS += $(LIBPATH) -lecpg -lpq

The sample makefile instructs make how to translate a .pgc or a .pc file into a C program. Two lines in the makefile specify the mode in which the source file will be compiled. The first compile option is:

.pgc.c:
      ecpg -c $(INCLUDES) $?

The first option tells make how to transform a file that ends in .pgc (presumably, an ECPG source file) into a file that ends in .c (a C program), using community ECPG (without the ECPGPlus enhancements). It invokes the ECPG pre-compiler with the -c flag (instructing the compiler to convert SQL code into C), using the value of the INCLUDES variable and the name of the .pgc file.

.pc.c:
    ecpg -C PROC -c $(INCLUDES) $?

The second option tells make how to transform a file that ends in .pg (an ECPG source file) into a file that ends in .c (a C program), using the ECPGPlus extensions. It invokes the ECPG pre-compiler with the -c flag (instructing the compiler to convert SQL code into C), as well as the -C PROC flag (instructing the compiler to use ECPGPlus in Pro*C-compatibility mode), using the value of the INCLUDES variable and the name of the .pgc file.

When you run make, pass the name of the ECPG source code file you wish to compile. For example, to compile an ECPG source code file named customer_list.pgc, use the command:

make customer_list

The make utility consults the makefile (located in the current directory), discovers that the makefile contains a rule that will compile customer_list.pgc into a C program (customer_list.c), and then uses the rules built into make to compile customer_list.c into an executable program.

ECPGPlus Command Line Options

In the sample makefile shown above, make includes the -C option when invoking ECPGPlus to specify that ECPGPlus should be invoked in Pro*C compatible mode.

If you include the -C PROC keywords on the command line, in addition to the ECPG syntax, you may use Pro*C command line syntax; for example:

$ ecpg -C PROC INCLUDE=/usr/edb/as13/include acct_update.c

To display a complete list of the other ECPGPlus options available, navigate to the ECPGPlus installation directory, and enter:

./ecpg --help

The command line options are: