Macros in system calls

Question

In c++, is it possible and safe to use macros in system calls? Take the following code for example:

#define WINX 54
#define WINY 30

int main()
{
system("mode con lines=WINY cols=WINX");
...

Would that work and be safe to use in code? Or would I have to manually construct a string?

Answer 1

A macro will not expand inside a string literal. Instead, you can use another macro to expand a macro into a string literal, and use string literal concatenation to create the desired string:

#define STR2(x) STR(x)
#define STR(x) #x

const char *cmd = "mode con lines=" STR2(WINY) " cols=" STR2(WINX);
system(cmd);

STR2 expands the provided argument (e.g. WINY) into what it is defined to be and then passes it to STR. STR just uses the stringifying macro operator, and its result is a string literal. Adjacent string literals are concatenated into a single string by the compiler before the code is tokenized and compiled into object code.

If the macros are something more complex than simple numbers, then you need to manually construct a string. In C++, the easiest way is to use ostringstream (from <sstream>):

std::ostringstream oss;
oss << "mode con lines=" << WINY << " cols=" << WINX;
system(oss.str().c_str());

Answer 2

Macros certainly don't expand in strings. So, this

system("mode con lines=WINY cols=WINX");  

won't expand into

system("mode con lines=30 cols=54");

Answer 3

If you don't need the actual decimal value of WINX and WINY, you can concatenate static strings and save resources during execution time:

#define WINX "54"
#define WINY "30"

int main()
{
system("mode con lines=" WINY " cols=" WINX);

Answer 4

You'll have to manually construct a string.

For example:

char command[100];
int result;
sprintf(command, "mode con lines=%d cols=%d", WINY, WINX);
result = system(command);

Don't forget the required include directives:

#include <stdio.h>
#include <stdlib.h>

Consult your system's documentation for the meaning of the value returned by system.

Make sure the command array is big enough to hold the full command -- or, probably better, use std::string instead. jxh's answer shows how to do this.