Error C2059: syntax error : 'string' - c++

Question

I have looked at other posts and to be honest I am still not sure what is causing the problem. I am programming in Visual Studio and
I have the following code: (this is a C main)
int main(int arc, char **argv) {
struct map mac_ip;
char line[MAX_LINE_LEN];
char *arp_cache = (char*) calloc(20, sizeof(char)); //yes i know the size is wrong - to be changed
char *mac_address = (char*) calloc(17, sizeof(char));
char *ip_address = (char*) calloc(15, sizeof(char));
arp_cache = exec("arp -a", arp_cache);
It uses the following cpp code:
#include "arp_piping.h"
extern "C" char *exec(char* cmd, char* arp_cache, FILE* pipe) {
pipe = _popen(cmd, "r");
if (!pipe) return "ERROR";
char buffer[128];
while(!feof(pipe)) {
if(fgets(buffer, 128, pipe) != NULL) {
strcat(arp_cache, buffer);
}
}
_pclose(pipe);
return arp_cache;
}
With the matching header file:
#ifndef ARP_PIPING_H
#define ARP_PIPING_H
#endif
#ifdef __cplusplus
#define EXTERNC extern "C"
#else
#define EXTERNC
#endif
#include <stdio.h>
#include <string.h>
extern "C" char *exec(char* cmd, char* arp_cache, FILE* pipe);
#undef EXTERNC
But I keep on getting the following errors:
1>d:\arp_proto\arp_proto\arp_piping.h(14): error C2059: syntax error : 'string'
1>main.c(22): warning C4013: 'exec' undefined; assuming extern returning int
1>main.c(22): warning C4047: '=' : 'char *' differs in levels of indirection from 'int'
Please can I get some help, I have looked at other posts regarding the c2059 but am still getting nowhere

Answer 1

Change your exec declaration to use the EXTERNC macro you have taken pains to define.
EXTERNC char *exec(char* cmd, char* arp_cache, FILE* pipe);

Answer 2

I ran into this compilation error when adding an enum to a project. It turned out that one of the values in the enum definition had a name clash with a preprocessor #define.
The enum looked something like the following:
// my_header.h
enum Type
{
kUnknown,
kValue1,
kValue2
};
And then elsewhere there was a #define with the following:
// ancient_header.h
#define kUnknown L"Unknown"
Then, in a .cpp somewhere else in the project, both of these headers were included:
// some_file.cpp
#include "ancient_header.h"
#include "my_header.h"
// other code below...
Since the name kUnknown was already #define'd, when the compiler came to the kUnknown symbol in my enum, it generated an error since the symbol was already used to define a string. This caused the cryptic syntax error: 'string' that I saw.
This was incredibly confusing since everything appears to be correct in the enum definition and compiles just fine on it's own.
It didn't help that this was in a very large C++ project, and that the #define was being transitively included in a completely separate compilation unit and was written by someone 15 years ago.
Obviously, the right thing to do from here is rename that terrible #define to something less common than kUnknown, but until then, just renaming the enum value to something else works as a fix, e.g.:
// my_header.h
enum Type
{
kSomeOtherSymbolThatIsntDefined,
kValue1,
kValue2
};
Anyway, hopefully this answer is helpful for someone else, since the cause of this error stumped me for a good day and a half.

Answer 3

extern "C" is used to tell the compiler to make it as C grammer, but your mean is to declear a extern function called exec. you just make fusion to the differ of this. so rewrite your code like this in arp_piping.h:
/*extern "C"*/ char *exec(char* cmd, char* arp_cache, FILE* pipe);
and then del the preffix of extern "C" in cpp file.
if you want to comiler them with C grammer, just setting in the cpp which call for the function exec, so write like this:
extern "C" {
#include "arp_piping.h"
}