printf effects vars ?

[waxby]

Hi !
As a beginner in C I use a lot of printf() in my code to verify what's happening in my app. I just wrote an app. that scans thru a directory and made some tests on the fieldname. When I was pleased with the result, I deleted all printf() except the last one.

When I deleted them, I got wrong result on my last print().
The one I deleted, contained the same var as the last one except for one.

In my app I use DIR struct. I found that if I did not printf( "%s\n", dir->d_name ) in the loop, I got another result in my var....

My question is, can the use of printf() effect the variables ?

Best regards
Micael

[MrWizard]

It all depends. If you do something like:

int num = 55;

printf( "%i", num++ );
printf( "%i", num );

Obviously you will get two different answers. However with simply outputting a string like you state in your post I don't see how it could change. Why don't you post your code and we can look more closely.

[alan_moran]

As a side remark....

Using traces (printf statements) is one was to verify what
your code is doing but you might be better off learning
how to use a debugger (e.g. gdb + ddd) or using
assertions (see assert.h) in your code - as a bonus these
will also help you understand how C works ;-)

A.

[Hammer]

>>My question is, can the use of printf() effect the variables ?
If your program has memory overlay related bugs (writing to memory locations incorrectly), use of any code can affect how it runs (or doesn't run!).

[micke]

I got it working, but I'm not sure of my conclusion..

I did :
strncpy( buff, &dir->d_name[ipos+1], 8 );

I guess &dir->d_name[ipos+1] gave me a mem address to the structure and not the value of dir->d_name and caused position skips in the memory (?)

Member d_name is the first member in struct DIR, and I guess it share the same mem position as the struct itself.

When doing a printf("%s\n", dir->d_name);
It seems like this notation resets the structs pointer to the start again ?

I made a for statement and took char by char to fill my buff[] instead of using strncpy() so I got it working.

It would be very valuble for me to understand this correct, since I guess this will not be the last time I'll bump in to this kind of a problem again :-)

Thank's all for your answer !

Best regards
Micael

[Hammer]

Post some of your actual code (using code tags), that way we can see if it's right or not.

>>It seems like this notation resets the structs pointer to the start again ?
Nothing you've shown so far will actually change the value to the pointer is holding.