trouble using string to open file

I'm having trouble figuring out exactly why my program is telling me it can't find files, but i've narrowed it down.

Here's a snippet of the code:

Code:

char            *filename;
char             num[10];
FILE              *imageFileP;

sprintf(num, "%d", i);  // i is an integer that represents file name before the dot
strcpy(filename, num);
strcat(filename, ".");
strcat(filename, app->FileExt); // FileExt is a char size [10]
        
printf("filename is: '%s'\n", filename); 
         
imageFileP = fopen(filename,"r"); 
                 
printf("filename is: '%s'\n", filename);

on the first print, filename looks exactly right, but on the second print the filename is empty. I believe something is wrong with filename -- even if it looks right -- because I am using char buffers of fixed size to build it. Can someone show me how to adjust my code to make this work?

How is filename declared? It needs to have enough space to store the entire filename.

Since you're already using sprintf(), consider using something like this.

Code:

sprintf(filename, "%d.%s", i, app->FileExt);

filename is a char*. I changed it to char[127] and it works great. Thanks!

I still don't understand why it works, because the printf was displaying the filename correctly. Yet, when it was passed to fopen, it couldn't do anything with it.

You had undefined behaviour. It could have worked, and sort of did -- but it could also blow up on you at any time. Don't rely on this sort of thing working, or not working, or even behaving the same way when you run the program again.

As for what happened? When the variable was a char*, it pointed to some random location in memory. This location was evidently writable, because you could assign your filename to it. But fopen() presumably used the same memory, because you didn't declare the memory as yours. It was unallocated.

Actually, on a guess, the char* might have been pointing at or near the FILE* variable. It could overwrite the data without seg faulting, because the space was actually yours, but when fopen() set the FILE* pointer to NULL, the string got truncated prematurely.