Help with an strange error?

[Zildjian]

Hello,

I'm currently writing a program for my C course at University where I have to read in lines of input from stdin, replace a specified word with another word (from command line arguments), and ouput to stdout. I have that part working with the following code:

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

main(int argc, char *argv[]) {
    char buf[BUFSIZ];
    char *tok;
    
    // read lines from stdin
    while ( fgets(buf, BUFSIZ, stdin) ) {
        // tokenize line using strtok
        for (tok = strtok(buf," "); tok != NULL; tok = strtok(NULL," ")) {
            if (strcmp(tok,argv[1]) == 0) printf("%s ",tok = argv[2]);
            else if (strchr(tok,'\n')) printf("%s",tok);
            else printf("%s ",tok);
        }
    }
}

What I also have to do, though, is if the word I'm looking for has the newline character in it, I have to output a message to stderr. What I want to do is make a new string that has the newline character concatenated onto argv[1] as follows:

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

main(int argc, char *argv[]) {
    char buf[BUFSIZ];
    char *tok;
    char *error = strcat(argv[1],"\n");
    
    // read lines from stdin
    while ( fgets(buf, BUFSIZ, stdin) ) {
        // tokenize line using strtok
        for (tok = strtok(buf," "); tok != NULL; tok = strtok(NULL," ")) {
        
            if (strcmp(tok,error) == 0) {
                fprintf(stderr, "Invalid input.\n");
                exit(1);
            }
            
            if (strcmp(tok,argv[1]) == 0) printf("%s ",tok = argv[2]);
            else if (strchr(tok,'\n')) printf("%s",tok);
            else printf("%s ",tok);
        }
    }
}

After I compile and run this code, argv[1] does not get replaced by argv[2]. I think it's because when I create the error string, I change what argv[1] is. Is there something to stop this from happening? I'm thinking that there must be a way to make a copy of argv[1], but I don't know how to do this. Any help would be greatly appreciated.

[Hammer]

>>char *error = strcat(argv[1],"\n");
This is incorrect. You shouldn't amend argv[x] memory. You'd do better with a new array:

char error[length];
sprintf(error, "%s\n", argv[1]);

Just make sure that length is long enough to hold the users input but the extra character. This isn't an idiot proof way of doing it, there are better ways, but for now, try this.

Or, how about this:
Stick with the concept of your original program, but instead of using strcmp(), use strncmp(), checking only the first few bytes of the data from the stdin token. Something like this:

len = strlen(argv[1]);
if (strncmp(tok, argv[1], len) == 0 && tok[len] == '\0')
{
    /* Found a match */
}

[XSquared]

Plus, you should use int main( ).

[Zildjian]

Hey, thanks for the help, guys. I really appreciate it. Things are working great now!

[Hammer]

Originally posted by XSquared
Plus, you should use int main( ).

And how will the application access the command line args then?
http://faq.cprogramming.com/cgi-bin/...&id=1043284376