Question

I’m trying to read information from a file and process that information in a certain way. I need to make an array of all the words on the very left hand side of the file that don’t have white space in front of them. I keep however getting really odd output when I try to display the contents of that char array.

Here is the sample input:

# Sample Input

    LA 1,3
    LA 2,1
TOP    NOP
    ADDR 3,1
    ST 3, VAL
    CMPR 3,4
    JNE TOP
    P_INT 1,VAL
    P_REGS
    HALT
VAL     INT 0
TAN     LA  2,1

So for instance when I run my program, my output should be:

TOP
VAL
TAN

Instead I’m getting:

a
aTOP
aVAL
aTAN
a
a

I’m not sure why this is happening. Any minor changes I make don’t actually help, they just change what’s in front of my expected output. Sometimes it’s ASCII value 0 or 20 characters. Hopefully someone can help me fix this because it’s driving me crazy.

Here’s my code:

#include <string>
#include <iostream>
#include <cstdlib>
#include <string.h>
#include <fstream>
#include <stdio.h>



using namespace std;


int main(int argc, char *argv[])
{
// If no extra file is provided then exit the program with error message
if (argc <= 1)
{
    cout << "Correct Usage: " << argv[0] << " <Filename>" << endl;
    exit (1);
}

// Array to hold the registers and initialize them all to zero
int registers [] = {0,0,0,0,0,0,0,0};

string memory [16000];

string symTbl [1000][1000];

char line[100];
char label [9];
char opcode[9];
char arg1[256];
char arg2[256];
char* pch;

// Open the file that was input on the command line
ifstream myFile;
myFile.open(argv[1]);


if (!myFile.is_open())
{
    cerr << "Cannot open the file." << endl;
}

int counter = 0;
int i = 0;

while (myFile.good())
{
    myFile.getline(line, 100, '\n');

    // If the line begins with a #, then just get the next line
    if (line[0] == '#')
    {
        continue;
    }


    // If there is a label, then this code will run

    if ( line[0] != '\t' && line[0]!=' ')
    {
        if( pch = strtok(line-1," \t"));
            {
                strcpy(label,pch);
                cout << label << endl;
            }

        if (pch = strtok(NULL, " \t"))
        {
            strcpy(opcode,pch);
        }

        if (pch = strtok(NULL, " \t,"))
        {
            strcpy(arg1,pch);
        }

        if (pch = strtok(NULL, ","))
        {
            strcpy(arg2, pch);
        }
    }


}



return 0;
}

Answer 1

You are passing line-1 to strtok, which will cause it to return a pointer to the character before the start of the string; accessing line[-1] will produce undefined behaviour. strtok takes a pointer to the start of a string.

You’ve also got a ; at the end of your if( pch = strtok(line-1," \t")) statement, which nullifies the if test and causes the block to run even if pch is NULL.