找到C标头，但函数仍然未定义

Question

我在Debian 10上，我想为PostgreSQL开发一个C应用程序。这就是为什么我安装了一个软件包postgresql和libpq-dev。之后在系统文件夹中明确安装了头文件:

┌───┐
│ $ │ ziga > ziga--workstation > 001--hello_world
└─┬─┘
└─> find /usr/include/ | grep libpq
/usr/include/postgresql/libpq-fe.h
/usr/include/postgresql/libpq
/usr/include/postgresql/libpq/libpq-fs.h
/usr/include/postgresql/libpq-events.h
/usr/include/postgresql/internal/libpq
/usr/include/postgresql/internal/libpq/pqcomm.h
/usr/include/postgresql/internal/libpq-int.h

但是当我编译这个简单的C程序时:

#include <stdio.h>
#include <postgresql/libpq-fe.h>
int main(int argc, char * argv[]){
int v = PQlibVersion();
printf("Version of libpq: %dn", v);
return 0;
}
compiler says:
gcc -std=c17 -Wall -Wpedantic -g -gdwarf-2 -o main.elf main.c
/usr/bin/ld: /tmp/ccVzig6T.o: in function `main':
/home/ziga/Dropbox/workspace/racunalnistvo/projects--pistam/2021-03-03--postgressql_c/001--hello_world/main.c:6: undefined reference to `PQlibVersion'
collect2: error: ld returned 1 exit status
make: *** [makefile:59: main.elf] Error 1

我不明白。头文件在/usr/include里面，他们应该找到…图书馆找不到还是怎么了?

┌───┐
│ $ │ ziga > ziga--workstation > 001--hello_world
└─┬─┘
└─> find /usr/lib | grep libpq
/usr/lib/x86_64-linux-gnu/libpq.a
/usr/lib/x86_64-linux-gnu/libpq.so
/usr/lib/x86_64-linux-gnu/libpq.so.5
/usr/lib/x86_64-linux-gnu/pkgconfig/libpq.pc
/usr/lib/x86_64-linux-gnu/libpq.so.5.11
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/pqsignal.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/be-secure-openssl.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/auth.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/pqcomm.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/be-fsstubs.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/pqformat.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/ifaddr.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/be-secure.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/pqmq.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/crypt.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/be-secure-common.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/hba.bc
/usr/lib/postgresql/11/lib/bitcode/postgres/libpq/auth-scram.bc
/usr/lib/postgresql/11/lib/libpqwalreceiver.so

谁能告诉我如何最好地解决这个问题?

Answer 1

正如日志所说，链接器无法找到对PQlibVersion的任何引用。既然链接器正在运行，显然这不是编译的问题，而是链接的问题。

/usr/bin/ld: /tmp/ccVzig6T.o: in function `main':
/home/ziga/Dropbox/workspace/racunalnistvo/projects--pistam/2021-03-03--postgressql_c/001--hello_world/main.c:6: undefined reference to `PQlibVersion'
collect2: error: ld returned 1 exit status

我认为你应该添加链接器选项来链接到postgress库。在编译器参数中添加-lpq选项。要获得更完整的解释，请阅读构建libpq程序。