Why does strcat() on a new string craps out undefined characters in the beginning while strcpy is ok?

Question

For example

sprintf(pos,"%f ",cl.snap.ps.origin[0]); //don't start with strcat
sprintf(tmp,"%f ",cl.snap.ps.origin[1]);strcat(pos, tmp);

fine.

with

sprintf(tmp,"%f ",cl.snap.ps.origin[0]);strcat(pos, tmp);
sprintf(tmp,"%f ",cl.snap.ps.origin[1]);strcat(pos, tmp);

not fine.

Answer 1

strcat concatenates strings ; which means, it merges the contents of pos and tmp. What does pos contain before you call strcat? Has it been defined?

Answer 2

The strcat() function expects that the destination argument already contains a properly null-terminated string. In your case, it sounds like pos contains some junk that looks like a null-terminated string, but isn't what you expect. strcat() is dutifully appending on to the end of that junk.

One way to fix this is to initialise pos before your code:

pos[0] = '\0';
sprintf(tmp,"%f ",cl.snap.ps.origin[0]);strcat(pos, tmp);
sprintf(tmp,"%f ",cl.snap.ps.origin[1]);strcat(pos, tmp);

Answer 3

Don't use strcat and tmp. You're writing senselessly overcomplicated and inefficient code. Instead:

pos+=sprintf(pos,"%f ",cl.snap.ps.origin[0]);
pos+=sprintf(pos,"%f ",cl.snap.ps.origin[1]);
...

Unless you're sure sprintf cannot fail, rather than directly adding the return value to pos, you should probably first store the return value in a separate int variable and check that it's not -1.

It would also be better to use snprintf to make sure you don't overflow your buffer:

size_t cnt, rem=your_buffer_size;

cnt=snprintf(pos, rem,"%f ",cl.snap.ps.origin[0]);
if (cnt>=rem) goto error;
pos+=cnt; rem-=cnt;

cnt=snprintf(pos, rem,"%f ",cl.snap.ps.origin[1]);
if (cnt>=rem) goto error;
pos+=cnt; rem-=cnt;
...

Note that cnt being an unsigned type (size_t) is critical to the error check working.