Something Loopy in Assigning Strings

MattFunke · 09-17-2007, 08:23 AM

I tried this in desperation after strcat() refused to work. I just want to concatenate three strings. Can anyone tell me why this doesn't work? I can see the creation of "10", but not of "11", though there's plenty of room on the disk for both. Moreover, if I ask for a printf of fieldn and strlen(fieldn) for all three fields, I get the right responses. field1[] is more than long enough to handle the job.

Code:

void AssembleData(char field1[], char field2[], char field3[])
{
    int i;
    int startPos;
    int stopPos;

    if (field2[0] != '\0')
    {
        startPos = strlen(field1);
        stopPos = startPos + strlen(field2);

        system("touch /var/log/10\n");

        for (i = startPos; i < stopPos; i++)
        {
            field1[i] = field2[i - startPos];
        }

        system("touch /var/log/11\n");

        if (field3[0] != '\0')
        {
            startPos += strlen(field2);
            stopPos += strlen(field3);

            for (i = startPos; i < stopPos; i++)
            {
                field1[i] = field2[i - startPos];
            }
        }
    }
}

... and here's the bit I tried with strcat() that didn't work:

Code:

void AssembleData(char field1[], char field2[], char field3[])
{
    system("touch /var/log/10\n");

    field1 = strcat(field1, field2);
    field1 = strcat(field1, field3);

    system("touch /var/log/11\n");
}

Please help. Is there anything about assigning strings or something that I'm missing here?

bigearsbilly · 09-17-2007, 10:01 AM

of course it works,
you are probably calling it wrong.

NOTE this isn't safe as we should use strncpy strncat to
prevent buffer overflows.

Code:

#include <stdio.h>
#include <stdlib.h>
#include<string.h>
void AssembleData(char field1[], char field2[], char field3[])
{

    field1 = strcat(field1, field2);
    field1 = strcat(field1, field3);


}

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

    char buffer[1024];

    strcpy(buffer,  "hello ");
    AssembleData(buffer, "old ", "chap!");
    puts(buffer);

    return 0;
}

billym.primadtpdev>1
hello old chap!

MattFunke · 09-17-2007, 10:39 AM

Found it! Here's the problem. This is what I was doing, essentially:

Code:

#define HI_THERE "hi "
#define ID10T "idiot"
char *data;

data = strcat(HI_THERE, ID10T);

... and the statement, of course, after the preprocessor was done with it, looked like this:

Code:

data = strcat("hi ", "idiot");

It was trying to concatenate literals, not string variables. This works:

Code:

char data[80] = HI_THERE;
char data2[80] = ID10T;

data = strcat(data, data2);

bigearsbilly · 09-19-2007, 02:26 AM

the pitfalls of C