34.21. Building libpq Programs

34.21. Building libpq Programs
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34.21. Building libpq Programs #

+ To build (i.e., compile and link) a program using + libpq you need to do all of the following + things: + +

+ Include the libpq-fe.h header file: +

+#include <libpq-fe.h>
+

+ If you failed to do that then you will normally get error messages + from your compiler similar to: +

+foo.c: In function `main':
+foo.c:34: `PGconn' undeclared (first use in this function)
+foo.c:35: `PGresult' undeclared (first use in this function)
+foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
+foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
+foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
+

+ Point your compiler to the directory where the PostgreSQL header + files were installed, by supplying the + -Idirectory option + to your compiler. (In some cases the compiler will look into + the directory in question by default, so you can omit this + option.) For instance, your compile command line could look + like: +
```
+cc -c -I/usr/local/pgsql/include testprog.c
+
```
+ If you are using makefiles then add the option to the + CPPFLAGS variable: +
```
+CPPFLAGS += -I/usr/local/pgsql/include
+
```
+
+ If there is any chance that your program might be compiled by + other users then you should not hardcode the directory location + like that. Instead, you can run the utility + pg_config to find out where the header + files are on the local system: +
```
+$ pg_config --includedir
+/usr/local/include
+
```
+
+ If you + have pkg-config installed, you can run instead: +
```
+$ pkg-config --cflags libpq
+-I/usr/local/include
+
```
+ Note that this will already include the -I in front of + the path. +
+ Failure to specify the correct option to the compiler will + result in an error message such as: +
```
+testlibpq.c:8:22: libpq-fe.h: No such file or directory
+
```
+
+ When linking the final program, specify the option + -lpq so that the libpq + library gets pulled in, as well as the option + -Ldirectory to point + the compiler to the directory where the + libpq library resides. (Again, the + compiler will search some directories by default.) For maximum + portability, put the -L option before the + -lpq option. For example: +
```
+cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
+
```
+
+ You can find out the library directory using + pg_config as well: +
```
+$ pg_config --libdir
+/usr/local/pgsql/lib
+
```
+
+ Or again use pkg-config: +
```
+$ pkg-config --libs libpq
+-L/usr/local/pgsql/lib -lpq
+
```
+ Note again that this prints the full options, not only the path. +
+ Error messages that point to problems in this area could look like + the following: +
```
+testlibpq.o: In function `main':
+testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
+testlibpq.o(.text+0x71): undefined reference to `PQstatus'
+testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
+
```
+ This means you forgot -lpq. +
```
+/usr/bin/ld: cannot find -lpq
+
```
+ This means you forgot the -L option or did not + specify the right directory. +

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