C Concatenate string in while loop [duplicate]

Question

I cant seem to use the following function to concatenate a string in the while loop. Any idea what Im doing wrong?

void do_file(FILE *in, FILE *out, OPTIONS *options)
{
    char ch;
    int loop = 0;
    int sz1,sz2,sz3;

    int seeker = offsetof(struct myStruct, contents.datas);

    //find total length of file
    fseek(in, 0L, SEEK_END);
    sz1 = ftell(in);

    //find length to struct beginning and minus that from total length
    fseek(in, seeker, SEEK_SET);
    sz2 = sz1 - ftell(in);

    sz3 = (sz2 + 1) * 2;//Allocate enough size for 2x array length
    char buffer[sz3];//set size of buffer to be copied to
    char msg[sz3];//set size of msg

    buffer[0] = '\0';

    while (loop < sz2)
    {
        if (loop == sz2)
        {
            break;
        }

        fread(&ch, 1, 1, in);
        //sprintf(msg, "%02X", (ch & 0x00FF));
        strcat(buffer, msg);

        ++loop;
    }
    printf("%s\n", buffer);
}

Repeating the post continuously with little (and wrong/bad) modification is not the way to solve your problems... — LPs
– LPs, Commented Sep 13, 2016 at 6:52

GMichael · Accepted Answer · 2016-09-13 08:06:35Z

0

Your only strcat appends the char msg[sz3]; which is never filled.

EDIT The simplest fix would be to add

msg[1] = '\0';

after

char msg[sz3];//set size of msg

and to replace

fread(&ch, 1, 1, in);

with

fread(&msg[0], 1, 1, in);

edited Sep 13, 2016 at 8:06

answered Sep 13, 2016 at 6:48

GMichael

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5 Comments

Ke. Over a year ago

Hi GMichael, I'm wondering how do I fill it. I know it probably sounds silly, but I've been trying to fill it for a long time and still cant figure it out, maybe you can help me?

Zan Lynx Over a year ago

@Ke. Comparing this to your older question...did you completely forget that you commented out your sprintf line that writes into msg?

Ke. Over a year ago

I'm not really sure how that sprintf is supposed to work. I was commenting it out because I thought it was printing into the window for each loop, whereas I wanted it to print one loop at the end. Is the sprintf not really printing into the console then until the end?

LPs Over a year ago

printf and sprintf don't differ only by a s but work differently. As told you many times: look at the man!

GMichael Over a year ago

@Ke Please see the update in the answer

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