How to pass a vector of strings to execv

Question

I have found that the easiest way to build my program argument list is as a vector of strings. However, execv expects an array of chars for the second argument. What's the easiest way to get it to accept of vector of strings?

Answer 1

execv() accepts only an array of string pointers. There is no way to get it to accept anything else. It is a standard interface, callable from every hosted language, not just C++.

I have tested compiling this:

std::vector<string> vector;
const char *programname = "abc";

const char **argv = new const char* [vector.size()+2];   // extra room for program name and sentinel
argv [0] = programname;         // by convention, argv[0] is program name
for (int j = 0;  j < vector.size()+1;  ++j)     // copy args
        argv [j+1] = vector[j] .c_str();

argv [vector.size()+1] = NULL;  // end of arguments sentinel is NULL

execv (programname, (char **)argv);

Answer 2

The prototype for execv is:

int execv(const char *path, char *const argv[]);

That means the argument list is an array of pointers to null-terminated c strings.

You have vector<string>. Find out the size of that vector and make an array of pointers to char. Then loop through the vector and for each string in the vector set the corresponding element of the array to point to it.

Answer 3

I stumbled over the same problem a while ago.

I ended up building the argument list in a std::basic_string<char const*>. Then I called the c_str() method and did a const_cast<char* const*> on the result to obtain the list in a format that execv accepts.

For composed arguments, I newed strings (ordinary strings made of ordinary chars ;) ), took their c_str() and let them leak.

The const_cast is necessary to remove an additional const as the c_str() method of the given string type returns a char const* const* iirc. Typing this, I think I could have used std::basic_string<char*> but I guess I had a reason...

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I am well aware that the const-casting and memory leaking looks a bit rude and is indeed bad practise, but since execv replaces the whole process it won't matter anyway.