C++ - Convert FILE* to CHAR*

Question

I found a C++ source file which calculates expressions from a command line argument (argv[1]), however I now want to change it to read a file.

double Utvardering(char* s) {
srcPos = s;
searchToken();
return PlusMinus();
}

int main(int argc, char* argv[]) {
if (argc > 1) {
    FILE* fFile = fopen(argv[1], "r");
    double Value = Utvardering(fopen(argv[1], "r"));
    cout << Value << endl;
}else{
    cout << "Usage: " << argv[0] << " FILE" << endl;
}
cin.get();
return 0;
}

However the Utvardering function requires a char* parameter. How can I convert the data read from a file, fopen to a char*?

Answer 1

I have no idea what "Utvardering" expects, or how it's using the information.

There are two possibilities -

1) Utvardering may be defined using char*, but expecting a FILE* (in effect, treating char* like void*). I've seen this before, even though it's pretty awful practice. In that case, just cast fFile to char* and pass it in.

2) Utvardering may be expecting a null terminated string (char*) as input. If you're using fopen like this, you can use fread to read the file contents into a buffer (char[]), and pass it to your function that takes a char*.

Answer 2

Use the fread() function to read data from the FILE* into a buffer. Send that buffer into Utvardering().

Answer 3

It looks like you need to write code to read the file into a character array and pass that to Utvardering.

Just passing the return value of fopen will cause the address of the opaque data structure pointed to by that pointer to be passed to Utvardering. Utvardering will happily treat those bytes as character data when they are not. Not good.

Answer 4

The function fopen just opens a file. To get a string from there, you need to read the file. There are different ways to to this. If you know the max size of your string in advance, this would do:

const int MAX_SIZE = 1024;
char buf[MAX_SIZE];
if (!fgets(buf, MAX_SIZE, fFile) {
  cerr << "Read error";
  exit(1);
}
double Value = Utvardering(buf);

Note: this method is typical for C, not for C++. If you want more idiomatic C++ code, you can use something like this (instead of FILE and fopen):

ifstream in;    
in.open(argv[1]); 
if (!in) { /* report an error */ }
string str;
in >> str;

Answer 5

Good example of reading data from a file here: http://www.cplusplus.com/reference/clibrary/cstdio/fread/ then pass the buffer to your function