char pointer's segmentation fault

Question

Hello,

I'm trying to define a path at compile time by passing:

-DDCROOTDEF='"/path/to/stuff"'

on the compile line. I then try to get use this in the code like:

char * ptr_path;  
strcpy(ptr_path, DCROOTDEF);
strcat(ptr_path,"/MainCommons/CommonLib/fonts/Arial.ttf");
char *pftf=ptr_path;
gdImageStringFT(pimg,brect,iclr,pftf,pts,ang,ixp,iyp, (char *)cbuf);

Which gives me a segmentation fault. However, if I try to print the string first:

char * ptr_path;
strcpy(ptr_path, DCROOTDEF);
strcat(ptr_path,"/MainCommons/CommonLib/fonts/Arial.ttf");
char *pftf=ptr_path;
printf("%s\n",pftf);
gdImageStringFT(pimg,brect,iclr,pftf,pts,ang,ixp,iyp, (char *)cbuf);

It works just fine. What intricacy of char pointer's am I missing here?

Thanks

Answer 1

char * ptr_path;
strcpy(ptr_path, DCROOTDEF);

You never initialize ptr_path.

It doesn't work in the second code snippet, you are just getting unlucky and it appears to work. You're still using an uninitialized pointer and trying to write to who knows where in memory.

You need to initialize ptr_path to point to an array of char that is at least strlen(DCROOTDEF) + 1 in length. You also need to check the length of DCROOTDEF before copying its contents into the array to be sure that it is not too long. You can do so manually using strlen or you can use a length-checked copy function like strlcpy.

Answer 2

The pointer ptr_path is not initialized to point at writable memory, which is why dereferencing it using strcpy() is crashing.

You need to call e.g. malloc() to get the space, first:

char * ptr_path = malloc(PATH_MAX);

Or something like that.

Answer 3

In

char * ptr_path;
strcpy(ptr_path, DCROOTDEF);
strcat(ptr_path,"/MainCommons/CommonLib/fonts/Arial.ttf");

the pointer is not bound to a legally allocated block of memory, so your program runs into undefined behavior. You need to allocate a buffer first - for example by using malloc(). Be sure that the buffer is large enough to hold the resulting string together with the terminating null character.