undefined reference to `PQclear' compiling simple 'C' program with Postgres 10 calls

Question

Simple C prog that compiles and runs on Debian Linux. I need to get it running on Windows, but get the following when I compile it. It seems to be a common problem but I haven't yet found a solution.

If I use

#include <libpq-fe.h>

I get

C:\Users\Martin\Desktop\cl>gcc -o test2 test2.c
test2.c:3:22: fatal error: libpq-fe.h: No such file or directory
 #include <libpq-fe.h>

If I use

#include "C:\Program Files\PostgreSQL\10\include\libpq-fe.h"

I get

C:\Users\Martin\Desktop\cl>gcc -o test2 test2.c
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0xd): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x18): undefined reference to `PQfinish'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x46): undefined reference to `PQconnectdb'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x56): undefined reference to `PQstatus'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x66): undefined reference to `PQerrorMessage'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x8e): undefined reference to `PQfinish'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0xae): undefined reference to `PQexec'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0xbe): undefined reference to `PQresultStatus'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0xcf): undefined reference to `PQerrorMessage'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x10b): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x11f): undefined reference to `PQexec'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x12f): undefined reference to `PQresultStatus'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x140): undefined reference to `PQerrorMessage'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x17c): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x190): undefined reference to `PQexec'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x1a0): undefined reference to `PQresultStatus'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x1b1): undefined reference to `PQerrorMessage'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x1ed): undefined reference to `PQnfields'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x20f): undefined reference to `PQfname'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x26a): undefined reference to `PQgetvalue'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x2a6): undefined reference to `PQntuples'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x2b8): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x2cc): undefined reference to `PQexec'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x2dc): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x2f0): undefined reference to `PQexec'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x300): undefined reference to `PQclear'
C:\Users\Martin\AppData\Local\Temp\cclMue3l.o:test2.c:(.text+0x30c): undefined reference to `PQfinish'
collect2.exe: error: ld returned 1 exit status

How exactly do you build? Please show the equivalent commandlines or the makefile? — Yunnosch
– Yunnosch, Commented Jun 2, 2018 at 12:48
Yes, the first time (in fact the fist time I've done any C in windows!) - plus I'm not a C programmer.... The cmdline commands are in my original post. — Martin Moore
– Martin Moore, Commented Jun 2, 2018 at 16:12
How do you build? Do you use a compiler? Do you use an IDE? Tell us what you do between writing the text which is your program and running an executable. Have you successfully compiled and run a "HelloWorld"? — Yunnosch
– Yunnosch, Commented Jun 2, 2018 at 18:02
All this looks totally normal. You need the -I option to indicate where includes are, -L for where the libpq library is located, and -lpq to tell to link with libpq. BTW the same is true with Debian Linux. — Daniel Vérité
– Daniel Vérité, Commented Jun 2, 2018 at 18:44

Stephan Schlecht · Accepted Answer · 2018-06-06 19:57:27Z

2

Here a simple program that additionally to including the header file also queries a table test in a schema test which has two fields firstname and lastname. This makes sure that not only the include file is found but also the linker finds the additional libraries:

Simple test program

#include <libpq-fe.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    int lib_ver = PQlibVersion();
    printf("PQlibVersion: %d\n", lib_ver);

    PGconn *conn = PQconnectdb("user=stephan password=mypwd dbname=postgres");
    if (PQstatus(conn) == CONNECTION_BAD) {        
        fprintf(stderr, "Can't connect to Postgres: %s\n", PQerrorMessage(conn));
        PQfinish(conn);
        exit(-1);
    }

    PGresult *res = PQexec(conn, "SELECT * FROM test.test");    
    if (PQresultStatus(res) == PGRES_TUPLES_OK) {
        printf("\n     %-10s %-10s\n", PQfname(res, 0), PQfname(res, 1));
        printf("-------------------------\n");
        int rows = PQntuples(res);    
        for(int i=0; i<rows; i++) {       
            printf("[%d]: %-10s %-10s\n", i, PQgetvalue(res, i, 0), PQgetvalue(res, i, 1));
        }   
    }       
    PQclear(res);
    PQfinish(conn);
    return 0;
}

Build

This can be build by the following command line:

gcc -Wall -Wextra -I "C:\PostgreSQL\pg10\include" -L "C:\PostgreSQL\pg10\lib" query.c -lpq -o query

Output of test run

Explanation of compiler arguments

+--------------+----------------------------------------------------------------------+
|    argumet   |                meaning                                               |
+--------------+----------------------------------------------------------------------+
|   -Wall      |  switches on many warnigs                                            |
|   -Wextra    |  switches on additional warnings like unused parameters              |
|   -I _xxx_   |  searches for additional include files in the directory _xxx_                   |
|   -L _yyy_   |  looks in directory _yyy_ for additional libraries                   |
|   -l_zz_     |  links library _zz_                                                  |
|   -o _name_  |  the resulting program is called _name_.exe instead of a.exe         |
+--------------+----------------------------------------------------------------------+

Order of libraries

The order is important when linking against libraries. To avoid undefined reference errors, the -lpq argument should come after the .c source files.

How to find include and lib directories?

The following commands are printing out the include and lib directory for Postgres:

pg_config --includedir
pg_config --libdir

Alternatively use Window File Explorer, navigate to the installation directory of Postgres and search for the include and lib directories.

edited Jun 6, 2018 at 19:57

answered Jun 2, 2018 at 22:40

Stephan Schlecht

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11 Comments

Martin Moore Over a year ago

Looks very promising, but I now get (note the forward slash) - there;s some very old threads I've found that mention it, but no answers. I thought it was just me that got confused between linux and windows... C:\Program Files\PostgreSQL\10\lib/libpq.dll: file not recognized: File format not recognized collect2.exe: error: ld returned 1 exit status

Martin Moore Over a year ago

I've omitted to say I'm using MinGW (easiest way to get GCC installed that I could find)

Stephan Schlecht Over a year ago

File format not recognized could be an indicator that there is a 32-bit/64-bit mismatch between gcc / Postgres / Windows version. Can this be the case?

Martin Moore Over a year ago

I think it's just the fwd slash instead of the backslash in the path. But I could be wrong!.

Stephan Schlecht Over a year ago

I often see in software that is mainly developed on *NIX systems, that diagnostic output often mixes backslashes and forward slashes. Therefore this is presumably not the problem. Also found this: github.com/pgmodeler/pgmodeler/issues/695 Here somebody has a 32-bit Windows and a 64-bit version of Postgres with the same error message you have encountered. So might be worth to check.

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