Create and compile a program with PostgreSQL libpq is not so straightforward. I created a sample program like “testlibpq.c” in PostgreSQL documentation to test it.

When i compile it, i hit following error

libpq-fe.h: No such file or directory
PGconn’ undeclared (first use in this function)

mkyong@mkyong-desktop:~/Desktop/index$ gcc -o test.o test.c
test.c:8:22: error: libpq-fe.h: No such file or directory
test.c:11: error: expected ‘)’ before ‘*’ token
test.c: In function ‘main’:
test.c:21: error: ‘PGconn’ undeclared (first use in this function)
test.c:21: error: (Each undeclared identifier is reported only once
test.c:21: error: for each function it appears in.)
test.c:21: error: ‘conn’ undeclared (first use in this function)
test.c:22: error: ‘PGresult’ undeclared (first use in this function)
test.c:22: error: ‘res’ undeclared (first use in this function)
test.c:41: error: ‘CONNECTION_OK’ undeclared (first use in this function)
test.c:57: error: ‘PGRES_COMMAND_OK’ undeclared (first use in this function)
test.c:83: error: ‘PGRES_TUPLES_OK’ undeclared (first use in this function)

It do need to include -I/usr/include/postgresql/ in order to compile it, please check your database or Linux administrator, or just type “pg_config –includedir” to find out where is PostgreSQL include file located.

However I still hit error like follwing

undefined reference to `PQfinish’
undefined reference to `PQconnectdb’

mkyong@mkyong-desktop:~/Desktop/index$ gcc -I/usr/include/postgresql/  -o test.o test.c
/tmp/ccoOJzAT.o: In function `exit_nicely':
test.c:(.text+0xd): undefined reference to `PQfinish'
/tmp/ccoOJzAT.o: In function `main':
test.c:(.text+0x57): undefined reference to `PQconnectdb'
test.c:(.text+0x65): undefined reference to `PQstatus'
test.c:(.text+0x74): undefined reference to `PQerrorMessage'
test.c:(.text+0xac): undefined reference to `PQexec'
test.c:(.text+0xba): undefined reference to `PQresultStatus'
test.c:(.text+0xca): undefined reference to `PQerrorMessage'
test.c:(.text+0xef): undefined reference to `PQclear'
test.c:(.text+0x105): undefined reference to `PQclear'
test.c:(.text+0x118): undefined reference to `PQexec'

Library is missing. Include PostgreSQL library pathwith -L/usr/lib/postgresql/8.3/lib/, this all PostgreSQL configuration information can get it from “pg_config”.

But I still hit the same error again

undefined reference to `PQfinish’
undefined reference to `PQconnectdb”

mkyong@mkyong-desktop:~/Desktop/index$ gcc -I/usr/include/postgresql/ -L/usr/lib/postgresql/8.3/lib/ -o test.o test.c
/tmp/ccgWnRJg.o: In function `exit_nicely':
test.c:(.text+0xd): undefined reference to `PQfinish'
/tmp/ccgWnRJg.o: In function `main':
test.c:(.text+0x57): undefined reference to `PQconnectdb'
test.c:(.text+0x65): undefined reference to `PQstatus'
test.c:(.text+0x74): undefined reference to `PQerrorMessage'
test.c:(.text+0xac): undefined reference to `PQexec'
test.c:(.text+0xba): undefined reference to `PQresultStatus'
test.c:(.text+0xca): undefined reference to `PQerrorMessage'
test.c:(.text+0xef): undefined reference to `PQclear'
test.c:(.text+0x105): undefined reference to `PQclear'
test.c:(.text+0x118): undefined reference to `PQexec'
test.c:(.text+0x126): undefined reference to `PQresultStatus'
test.c:(.text+0x136): undefined reference to `PQerrorMessage'
test.c:(.text+0x15b): undefined reference to `PQclear'
test.c:(.text+0x171): undefined reference to `PQclear'

It do need to specified exactly library with -lpq to include libpg library, correct command is like following

gcc -I/usr/include/postgresql/ -L/usr/lib/postgresql/8.3/lib/ -lpq -o test.o test.c

Done, compile without error.

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