program printing ".N=?" at beginning of string

I've written a program to read a string from the UNIX command line, and repeat or reverse it depending on the command line options it's given, and it seems to be working correctly apart from for some reason printing ".N=?" (without quotes, and question mark in a bold hexagon) at the beginning of my string.

here is my code:

Code:

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#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[]) {       

char option;                               
char string[100];                        //To store string being processed
char rstring[100];                        //To store reverse string

int n,i;                                //n for number of repeats, i for iteration of loop
opterr=0;                                //Disable standard error messages

while((option=getopt(argc,argv,"hr:b"))!=-1){
        printf("string=%s\n",string);       
        i=1;                                                        //skip parsing of program name       
        while(i<argc){                                                //parse through strings
                if(!strcmp(argv[i],"-r")){                        //if r option found...
                        i+=2;                                        //... skip option and argument
                }else{
                        if(!strcmp(argv[i],"-b")){                //if b option found...
                                i++;                                //...skip option
                        }else{
                                strcat(string,argv[i]);                //otherwise, add current string to string
                                strcat(string," ");                //add space between words of string               
                                i++;                                //increment counter, move to next string               
                        }
                }
        }       

/*then I have a switch statement to take the command line options and perform the
functions on the string, which I know work as I've tested seperately.
The printfs above are for debugging.

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This part of the code is just for reading the string from the command line (ignoring any options there may be), but for some reason it puts the ".N=?" at the beginning of the string. This happens even with the first printf before the nested if statements.

Anyone know whats wrong?

Printing the string w/o initializing it would likely print garabage as in

Code:

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printf("string=%s\n",string);

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Quote:

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Originally Posted by itCbitC

Printing the string w/o initializing it would likely print garabage as in

Code:

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printf("string=%s\n",string);

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I thought this might be the case, but thing is, i need an empty string to begin with, since I'm concatenating each word of the command line to the string.

It's kind of awkward to use getopt and then also try to get a parameter from the command line yourself. Why don't you just use getopt to get the string (so myprog -s "blah blah" -r or whatever) and then proceed from there?

Code:

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
        int opt;
        char *string;
        if (argc<2) return 0;
        while ((opt=getopt(argc,argv,"s:"))>0) switch(opt) {
                case ('?'): puts("error"); return 0;
                case ('s'): string=malloc(strlen(optarg)+1);
                        strcpy(string,optarg); break;
                default: break;
        }
        puts(string); free(string);       
        return 0;
}

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If you need an empty string[] memset() it to NULL. Doing it this way prevents garbage characters from being printed.

Declaring a variable doesn't mean it will be empty, so you have to initialize it. In the case of static strings, you can use:

Code:

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char string[100] = "";

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or simply set string[0] to '\0'

mk27, I'm not familiar with the malloc function you've used, I haven't got that far yet in my course. Also, I'm trying to read in anything following the options, but not the options, option arguments, or program names themselves. and the options could appear in any order, or may be completely absent.

I might try using the memset function. I'm assuming i just use memset(string);

Quote:

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Originally Posted by bertazoid

I thought this might be the case, but thing is, i need an empty string to begin with, since I'm concatenating each word of the command line to the string.

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One thing you can do to have an "empty" string prepped for strcat is:

Code:

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char string[256]="\0";

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This means string has a strlen of 0, and (string[0]=='\0') will be true. It won't contain any initial garbage for printf either.
'\0' is the NULL TERMINATOR.

Quote:

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Originally Posted by bertazoid

mk27, I'm not familiar with the malloc function you've used, I haven't got that far yet in my course. Also, I'm trying to read in anything following the options, but not the options, option arguments, or program names themselves. and the options could appear in any order, or may be completely absent.

I might try using the memset function. I'm assuming i just use memset(string);

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Okay, don't use malloc. Just use "char string[whatever size]" (and omit free()). But do believe what I am trying to say about getopt. It will make everything much simpler UNLESS the prof said specifically you cannot use "-s blah" to input the string. It doesn't matter to getopt what order the options are in either.

Regarding memset(), read post #8 first.

memset() is one of the several ways of initializing a char array to NULL; follow the one posted by MK27 - it's simple and it works. Here's how you would use memset() - as an fyi.

Code:

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memset(string, '\0', 256);

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Thanks for the replies, I forgot about the null terminator for strings. This is probably the easiest way for me to do what I want.

getopt will only parse the options on the command line won't it? i.e. those strings with the format -x. I'm still unsure as to how I could get it to parse the following words as well. I'd like to do this using only getopt and no additional if statements like you say, but I don't think I understand it well enough, and I can probably get it working using what I have and initialising the string.

getopt() is all you need. It will parse the command line options as well as any arguments to it. The optarg extern variable helps you along the way. See here

why is the following now giving me a blank string (rstring at end)?

Code:

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char string[]="hello world!";
char string2[100]="\0";
int i;

for(i=0;i<=strlen(string);i++){
        printf("strlen(string)-i= %d-%d = %d\n",strlen(string),i,strlen(string)-i);
        string2[i]=string[strlen(string)-i];
        }
printf("string=%s\n",string);
printf("rstring=%s\n\n",string2);

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because rstring[] is really string2[] which is set to NULL as in

Code:

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char string2[100]="\0";
...
printf("rstring=%s\n\n",string2);

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Thanks, but even if I declare string2 as char string2[100]; without initialising it, it still prints a blank string.

post the relevant code

Code:

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#include <stdio.h>
#include <string.h>

int main(){

char string[]="hello world";
char string2[100];
int i;

for(i=0;i<=strlen(string);i++){
        printf("strlen(string)-i= %d-%d = %d\n",strlen(string),i,strlen(string)-i);
        string2[i]=string[strlen(string)-i];
        }
printf("string=%s\n",string);
printf("rstring=%s\n\n",string2);

return 0;
}

---

Output:

strlen(string)-i= 14-0 = 14
strlen(string)-i= 14-1 = 13
strlen(string)-i= 14-2 = 12
strlen(string)-i= 14-3 = 11
strlen(string)-i= 14-4 = 10
strlen(string)-i= 14-5 = 9
strlen(string)-i= 14-6 = 8
strlen(string)-i= 14-7 = 7
strlen(string)-i= 14-8 = 6
strlen(string)-i= 14-9 = 5
strlen(string)-i= 14-10 = 4
strlen(string)-i= 14-11 = 3
strlen(string)-i= 14-12 = 2
strlen(string)-i= 14-13 = 1
strlen(string)-i= 14-14 = 0
string=I am a string!
rstring=

I think I got it. It's because it's putting the null termintor of string as the first element of string2.

that's exactly it

I changed the for loop to start at i=0, so it would exclude the null character at the end of the string, but my program still gives the same output - rstring is blank.

try this (starting a line 12):

Code:

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        if (i>0) string2[i-1]=string[strlen(string)-i];
        }
string2[i]='\0';

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rstring=dlrow olleh

Thanks it worked, except instead of using the if statement, I just started the loop at i=1.

IMO you are getting confused between the length of string[] and the char stored at a particular index of the array string[].

Code:

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string="I am a string!";
strlen(string)-i=14-0=14;  /* i = 0 */
strlen(string)-i=14-1=13;  /* i = 1 */
string[14] = NULL;        /* char at index 14 is NULL */
string[13] = '!';          /* char at index 13 is '!' */

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So setting i=1 is the way to go.