c dynamic array size based on fstat

Question

I use fstat to get the file size. I want to use this size to declare an array and then change the size with another fstat and re-use the same array. Example:

fstat(file1, &fileStat);
fsize = filestat.st_size;
char filebuffer[size-of-file1];
/* do something */
fstat(file2, &fileStat);
fsize = filestat.st_size;
char filebuffer[size-of-file2];
/* do something */

Obviously i cannot re-declare the filebuffer array, i have to declare a new one. But if i want to re-use the same array with a different size how can i do it??
Thanks!!

EDIT:

filebuffer = malloc(fsize);
if(filebuffer == NULL){
    perror("malloc");
    onexit(sockd, 0, fd, 4);
}

and

tmpfilebuf = realloc(filebuffer, fsize);
if(tmpfilebuf){
    filebuffer = tmpfilebuf;
}
else{
    perror("realloc");
    free(filebuffer);
    onexit(sockd, 0, fd, 4);
}

but now i got a segfault :(

Answer 1

Don't use variable length arrays. Use malloc the first time and then realloc as needed.

char *filebuffer;    
filebuffer = malloc(...);

tmp = realloc(filebuffer, ...);
if (tmp)
    filebuffer = tmp;

What you are using now is an interesting and somewhat dangerous feature called "variable length arrays". That is, you declare an array with a length computed at runtime. The problem with this approach is that it uses the stack and has no means to inform you if there's insufficient space.