atoi来自char *在C中返回0

Question

Very new to C so please bear with me. I have a function that takes in 3 char * variables, opcode , arg1 and arg2 .

arg1 and arg2 can either be (1) a string containing the name of a variable holding an unsigned int, or (2) an actual integer.

Either way, I need to get the actual integer out. I am trying to use atoi so far but it seems to be returning 0 for the first case.

ie

sscanf(instruction, "%s %s %s", opcode, arg1, arg2);

sum = atoi(arg1) + atoi(arg2);

I can't post the entire code as it doesn't belong to me, but hopefully the above demonstration helps a little?

Answer 1

As I said in my comment, you can't expect to retrieve the value of a named variable at run-time, in C. Variable names are not something that is kept when the code runs, then it's pure machine code and all variables have been "cooked down" to memory addresses. The names are no longer around.

The atoi() function converts a string holding an integer, such as "4711" , into the actual value, 4711 . That's all it does, it cannot retreive values "by name" in any way.

If you want to have a mechanism to map variable names to values when the program runs, you are going to have to create that mechanism yourself, since the language does not have one . For instance, you could create some kind of array of structures that let you associate names with values, but that would of course require you to initialize that array.

Something like this:

int foo = 12;
char *vname = "foo";

print("the value of %s is %d\n", vname, get_variable_value(vname));

simply is not going to happen in C, the function get_variable_value() cannot exist .

Answer 2

Variable names are used only at compile-time. Once the program is compiled, all information regarding what names you used are lost ¹ . Therefore, you have no way of accessing a variable contents by a "string" containing its name at runtime. ²

¹ more precisely, they are available to the linker or possibly debugger, but that's not something you could use (or count on)

² I can think of a (terrible) way to do so with macros or use a map of all your variable names to their addresses, but I believe your program needs to be fixed by design.

Answer 3

You must create a data structure in which you put the variables and the values of them. Look for "hash map" in the libraries that you use.

Then you can use isdigit(arg[0]) to check whether the argument is a number or a variable name. Pseudocode:

sum = toInt(arg1) + toInt(arg2)

int toInt(char * arg) {
    return isdigit(arg[0]) ? atoi(arg) : map->lookupValue(arg);
}