C Programming : pointers/char array

Question

I have a project that entails us as a programmer, to translate a line of assembly language into machine language and calculate the decimal. We have our own sample computer with opcodes to make it simpler to calculate. My question is if the following function does what I aim to do.

In the main function, I have an array of chars labeled char binary[3] since the value returned will be three bits. Also, in the function is a char opcode[MAXLINELENGTH] which reads in from a file properly the opcode line so I point to it with *string within the function. Will this correctly find the match and fill the binary array in main with the proper 3 digit bit code?

//function should return true if the proper binary output was successfuly copied with the matching opcode
int opcodeBinary(char *string,char *binary){
    if(strncmp(*string,"add"){
        *binary="000";
        return 1;
    }
    else if(strncmp(*string,"nand"){
        *binary="001";
        return 1;
    }
    else if(strncmp(*string,"lw"){
        *binary="010";
        return 1;
    }
    else if(strncmp(*string,"sw"){
        *binary="011";
        return 1;
    }
    else if(strncmp(*string,"beq"){
        *binary="100";
        return 1;
    }
    else if(strncmp(*string,"jalr"){
        *binary="101";
        return 1;
    }
    else if(strncmp(*string,"halt"){
        *binary="110";
        return 1;
    }
    else if(strncmp(*string,"noop"){
        *binary="111";
        return 1;
    }
    else{
        return 0;
    }
}

1) if(strncmp(*string,"add"){ strncmp() takes 3 arguments. 2) And you probably want if( !strncmp(*string,"add", 3) ){ 3) ` *binary="011";` You can't assign a char pointer to a dereferenced char pointer. — wildplasser
– wildplasser, Commented Sep 19, 2013 at 18:17
You ought to put each string/binary pair into an array, then loop through each pair in the array, comparing to your string. This makes the code much smaller. — David R Tribble
– David R Tribble, Commented Sep 19, 2013 at 20:58

Keith Thompson · Accepted Answer · 2013-09-19 18:31:05Z

2

No -- in fact, it shouldn't even compile. (Did you try compiling your code before posting it here?)

strncmp and strcmp return 0 if the strings are equal (and strncmp takes three arguments, not two!). To compare two strings for equality, you want something like:

if (strncmp(string, "nand", 4) == 0) {
    ...

Or, more simply:

if (strcmp(string, "nand") == 0) {
    ...

Since the string literal "nand" sets a limit on the number of characters to be compared, there's not much point in using strncmp rather than strcmp`.

Both arguments to strcmp (or the first two arguments to strncmp) are of type char*. string is already of type char*; assuming it points to (the first character of) a string, just pass string, not *string, as the first argument to strcmp.

*binary = "000"; shouldn't even compile. binary = "000"; would compile, but it wouldn't do what you want; it's a pointer assignment, and it only affects the value of binary, which is local to your function; there's no visible effect once the function returns.

You can either make the second argument to your function a char**; then the caller can do something like:

char *binary;
if (opcodeBinary(some_string, &binary) {
    /* binary now points to a string like "000" */
}

Or the caller can allocate space for the string:

char binary[5]; /* or whatever size you need */

and then inside your function, replace the assignment by a call to strcpy:

strcpy(binary, "000");

You can't return an array from a function; there are several ways you can accomplish the same thing indirectly, but they're all somewhat tedious.

edited Sep 19, 2013 at 18:31

answered Sep 19, 2013 at 18:22

Keith Thompson

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5 Comments

Allie Steele Over a year ago

Okay thanks, I understand now why strcmp should not take in the pointer as the parameter but string itself. I'm not too experienced with ** so I'm more so looking at your second suggestion. I like the idea of doing the string copy to binary. Would i allocate space for the string inside the function? I already allocated space outside the function for binary which is char binary[3] but I didn't think the function would have access to that

Allie Steele Over a year ago

ps I made an error, I wanna do strcmp and compare the two strings entirely

Keith Thompson Over a year ago

@AllieSteele: The two arguments to strcmp() are pointers. They're pointers to char, through which the function can then access the string to whose first character the pointer points (I hope that makes sense). If the caller declares char binary[4]; and passes binary as an argument, your function receives a pointer and is able to access the contents of the caller's binary array. Note that it has to be 4 bytes to accommodate the 3 characters of "000" plus the terminating null character '\0'.

Keith Thompson Over a year ago

@AllieSteele: The relationship between arrays and pointers can be confusing. Read section 6 of the comp.lang.c FAQ.

Allie Steele Over a year ago

thank you so much!! this really cleared up some misconceptions.

nickie · Accepted Answer · 2013-09-19 18:30:46Z

Your function opCodeBinary does not typecheck. When assigning *binary="000", the left hand side is an lvalue of type char and the right hand side is a char*. The same is true for the first operand to strncmp. You should not prepend a * when using strings in C.

What I assume you are required to do is to generate the binary opcode that corresponds to an assembly command. You should read the command from string and write the opcode to binary. There are several things that you do wrong here:

You don't have a way to move the pointers string and binary, as you consume characters from your input and produce new binary opcodes to your output. This would be useful if you were to call the function opCodeBinary repeatedly. I would suggest that you use double pointers for both parameters to opCodeBinary.
I assume you should not produce opcode 000 in ASCII representation (that is, the string "000") but one byte that contains it in binary form (or, worse, just three bits).

I'd suggest the following for your opCodeBinary function. It correctly resolves (1) and (2) above, but it assumes that each opcode goes to one or more (full) bytes in your binary.

int opCodeBinary (char **string, char **binary)
{
  if (strncmp(*string, "add", 3) == 0) {
     *string += 3;
     *(*binary)++ = '\x00';
     return 1;
  }
  ...
}

Sinn · Accepted Answer · 2013-09-19 20:48:58Z

0

using wrong strcnmp: Passing character(dereferenced char pointer) instead of char pointer(string) you need to pass the number of characters to compare, as a third argument.

Recommendation: Use a hash, to map keywords to binary codes

answered Sep 19, 2013 at 20:48

Sinn

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