我写了一个包含 pari.h 头文件的 cpp 源代码：
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>
#include<stdlib.h>
#include<time.h>
#include<iterator> // for ostream_iterator
#include<strings.h>
#include<string.h>
#include<sstream>
#include <pari/pari.h> // for PARI/GP library
#include<Rcpp.h> // for sourceCpp to work, this line must be uncommented
// Enable C++11 via this plugin (Rcpp 0.10.3 or later)
// [[Rcpp::plugins(cpp11)]]
using namespace std;
using namespace Rcpp; // for sourceCpp to work, this line must be uncommented
// [[Rcpp::export]]
int main() {
long maxp = 1000000; // max value
pari_init(500000,2); // initiate pari
size_t rsize = 500000; // set stack size variables
size_t vsize = 100000000;
void paristack_setsize(size_t rsize, size_t vsize); // declare stack function
paristack_setsize(rsize, vsize); // set stack size
gp_allocatemem(stoi(100000000)); // allocate memory
GEN p1; // declare PARI variable
p1 = cgetg(maxp, t_VEC); // make the PARI variable a vector
long j; // declare the variable for the number to be checked. one above the vector iterator
for (long i = 0; i <= maxp; ++i) { // iterate over PARI vector
j = i + 1; // decrement index for number
gel(p1, i) = sumdiv(stoi(j)); // calculate the sum of divisors and update the vector
}
vector<long> p2(maxp); // empty vector of native type
GEN x; // declare a PARI variable to subset PARI vector
for (long i = 0; i < maxp; i++) { // for2, across vector indices
x = gel(p1, i); // subset one item of vector
p2[i] = gtolong(x); // convert PARI to native long integer and update long vector item
} // close for2
for (long z = 0; z < maxp; z++) { // for3, to iterate for stdout
cout << p2[z] << '\n'; // return the result. the vector items are printed separately
} // close for3
} // close function
（请注意，可能会有不必要的标题，我通常会跨源复制所有标题，但这不是问题）。没有 pari.h 标头的类似源文件可以使用 Rcpp 很好地编译，其中包含必要的部分（例如标头、命名空间、导出行等）。
源代码，当 Rcpp 相关参考被注释时，编译良好并且在直接使用 g++ 编译时没有问题，具有以下标志：
g++ -lpari -fpermissive -Wall -Wextra -lm -fno-strict-aliasing -fomit-frame-pointer -o sumdivisors.o sumdivisors.cpp
我也将这些标志导入到 R 中：
Sys.setenv("PKG_CXXFLAGS"="-lpari -fpermissive -Wall -Wextra -lm -fno-strict-aliasing -fomit-frame-pointer")
我还在/usr/local/lib64/R/library/Rcpp/include/.pari 目录下创建了一个符号链接/usr/include。
但是 sourceCpp 命令的输出是这样的：
> sourceCpp("sumdivisors.cpp")
In file included from /usr/local/lib64/R/library/Rcpp/include/Rcpp/r/headers.h:48:0,
from /usr/local/lib64/R/library/Rcpp/include/RcppCommon.h:29,
from /usr/local/lib64/R/library/Rcpp/include/Rcpp.h:27,
from sumdivisors.cpp:15:
/usr/local/lib64/R/library/Rcpp/include/Rcpp/platform/compiler.h:47:0: warning: "GCC_VERSION" redefined
#define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
In file included from /usr/local/lib64/R/library/Rcpp/include/pari/pari.h:16:0,
from sumdivisors.cpp:14:
/usr/local/lib64/R/library/Rcpp/include/pari/paricfg.h:19:0: note: this is the location of the previous definition
#define GCC_VERSION "gcc version 6.2.1 20160830 (GCC)"
Error in dyn.load("/tmp/Rtmpc9edZe/sourceCpp-x86_64-pc-linux-gnu-0.12.8/sourcecpp_188e46b44088/sourceCpp_2.so") :
unable to load shared object '/tmp/Rtmpc9edZe/sourceCpp-x86_64-pc-linux-gnu-0.12.8/sourcecpp_188e46b44088/sourceCpp_2.so':
/tmp/Rtmpc9edZe/sourceCpp-x86_64-pc-linux-gnu-0.12.8/sourcecpp_188e46b44088/sourceCpp_2.so: undefined symbol: pari_mainstack
我复制了包含或不包含 C++11 启用行的步骤，没有任何变化。我也更改了 gcc 标志，但没有结果。看来gcc版本定义和pari_mainstack的定义有问题。
我相信问题不在于源代码的编写方式。下面的两个例子中，上面的 cpp 代码被转换为一个返回向量并且函数不是 main 的。还提供了一个可以很好地与 Rcpp 一起编译的类似且简单的代码：
#include<stdio.h>
#include<numeric> // for "accumulate"
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>
#include<stdlib.h>
#include<time.h>
#include<iterator> // for ostream_iterator
#include<strings.h>
#include<string.h>
#include<sstream>
#include <pari/pari.h> // for PARI/GP library
#include<Rcpp.h> // for sourceCpp to work, this line must be uncommented
// Enable C++11 via this plugin (Rcpp 0.10.3 or later)
// [[Rcpp::plugins(cpp11)]]
using namespace std;
using namespace Rcpp; // for sourceCpp to work, this line must be uncommented
// [[Rcpp::export]]
vector<long> sumdivisors() {
long maxp = 1000000; // max value
pari_init(500000,2); // initiate pari
size_t rsize = 500000; // set stack size variables
size_t vsize = 100000000;
void paristack_setsize(size_t rsize, size_t vsize); // declare stack function
paristack_setsize(rsize, vsize); // set stack size
gp_allocatemem(stoi(100000000)); // allocate memory
GEN p1; // declare PARI variable
p1 = cgetg(maxp, t_VEC); // make the PARI variable a vector
long j; // declare the variable for the number to be checked. one above the vector iterator
for (long i = 0; i <= maxp; ++i) { // iterate over PARI vector
j = i + 1; // decrement index for number
gel(p1, i) = sumdiv(stoi(j)); // calculate the sum of divisors and update the vector
}
vector<long> p2(maxp); // empty vector of native type
GEN x; // declare a PARI variable to subset PARI vector
for (long i = 0; i < maxp; i++) { // for2, across vector indices
x = gel(p1, i); // subset one item of vector
p2[i] = gtolong(x); // convert PARI to native long integer and update long vector item
} // close for2
return(p2);
/*
for (long z = 0; z < maxp; z++) { // for3, to iterate for stdout
cout << p2[z] << '\n'; // return the result. the vector items are printed separately
} // close for3
*/
} // close function
.
#include<stdio.h>
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>
#include<math.h>
#include<time.h>
#include<Rcpp.h>
using namespace std;
using namespace Rcpp;
//#include "std_lib_facilities.h"
// [[Rcpp::export]]
int pe001Cpp(int x) { // define a function pe001 with one in$teger input
int sum35 = 0; // define a scalar for the sum. start value is 0
for (int i=1; i<x; ++i) { // for 1 loop for counting up to x
if (i % 3 == 0 || i % 5 == 0) { // if 1, divisible by 3 or 5
sum35 += i; // update sum
} // close if 1
} // close for 1
return sum35; // return the final value
} // close function
// [[Rcpp::export]]
int pe001Cppb(int x) { // efficient method
int sumdivisible(int x, int y); // declare the below function in this scope
return sumdivisible(x, 3) + sumdivisible(x, 5) - sumdivisible(x, 15); // return the total sum
} // close function pe001Cppb
int sumdivisible(int x, int y) { // sum of terms divisibile by y
int ny = floor ((x-1) / y); // number of terms less than x and divisible by y
return ny * (ny + 1) / 2 * y; // return the sum
} // close function sumdivisible
执行直接编译的二进制文件过滤后的 strace 输出如下：
open("/usr/lib/libpari-gmp-tls.so.5", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libstdc++.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libgcc_s.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libgmp.so.10", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_compat.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnsl.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_nis.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 3
正如我们从这里看到的https://github.com/rstats-db/RPostgres/issues/80，问题可能是链接库的错误版本，可以通过符号链接解决。所以我必须知道 Rcpp 试图链接哪些库文件。
更新：
Scanelf 输出显示有问题的符号位于 /usr/lib/libpari-gmp-tls.so.2.9.1 中。
[s@SS ~]$ scanelf -l -s pari_mainstack | grep pari_mainstack
ET_DYN pari_mainstack /usr/lib/libpari-gmp-tls.so.2.9.1
g++ 编译文件的 strace 输出显示可执行文件链接到 /usr/lib/libpari-gmp-tls.so.5，它本身是 2.9.1 版本的符号链接：
[s@SS library]$ strace ./sumdivisors3.o |& grep so | grep -v "No such file"
execve("./sumdivisors3.o", ["./sumdivisors3.o"], [/* 79 vars */]) = 0
open("/usr/lib/libpari-gmp-tls.so.5", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libstdc++.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libgcc_s.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libgmp.so.10", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
open("/usr/lib/libnss_compat.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnsl.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_nis.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 3
sourceCpp 命令创建的 sourceCpp_4.so 文件的 ldd 输出如下：
[s@SS library]$ ldd /tmp/Rtmpau9YqY/sourceCpp-x86_64-pc-linux-gnu-0.12.8/sourcecpp_3a105ad2bdba/sourceCpp_4.so
linux-vdso.so.1 (0x00007ffc28f9d000)
libR.so => not found
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00007f5077111000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f5076e0d000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f5076bf6000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f5076858000)
/usr/lib64/ld-linux-x86-64.so.2 (0x0000564489276000)
我用 ldd 跟踪了所有这些文件，并且没有指向 /usr/lib/libpari-gmp-tls.so.2.9.1 或 /usr/lib/libpari-gmp-tls.so.5 库的链接。所以问题是为什么 sourceCpp 不能链接到这些文件，因为包含了必要的头文件（而 g++ 可以）？
更新：
sourceCpp 的详细输出显示以下命令：
g++ -I/usr/local/lib64/R/include -DNDEBUG -I/usr/local/include -I"/usr/local/lib64/R/library/Rcpp/include" -I"/home/s/codes/cpp/projecteuler/library" -lpari -fpic -g -O2 -c sumdivisors2.cpp -o sumdivisors2.o
g++ -shared -L/usr/local/lib64/R/lib -L/usr/local/lib64 -o sourceCpp_5.so sumdivisors2.o -L/usr/local/lib64/R/lib -lR
我设置了标志（实际上 -lpari 就足够了：
Sys.setenv("PKG_CXXFLAGS"="-lpari")
根据 gp2c 的输出，-lpari 标志也应该包含在链接阶段，但这里的链接命令没有它。会不会是问题的根源？或者在此之前，为什么 sourceCpp_5.so 文件没有链接到必要的 pari 库？
和结局：
链接的依赖库也应该通过以下方式显式声明：
Sys.setenv("PKG_LIBS"="-lm -lpari -lc")
库标志由 gp2c 输出给出。顺便说一下 gcc 版本问题，我没有创建指向原始 pari 标头目录的符号链接，而是在 R 库路径中创建了一个副本并注释掉了该行：
//#define GCC_VERSION "gcc 版本 6.2.1 20160830 (GCC)"
现在编译成功了，一个R可以在R的数论计算中享受PARI/GP的速度，感谢Rcpp！