How can I get a file's size in C?

You need to seek to the end of the file and then ask for the position:

fseek(fp, 0L, SEEK_END);
sz = ftell(fp);

You can then seek back, e.g.:

fseek(fp, 0L, SEEK_SET);

or (if seeking to go to the beginning)

rewind(fp);

Using standard library:

Assuming that your implementation meaningfully supports SEEK_END:

fseek(f, 0, SEEK_END); // seek to end of file
size = ftell(f); // get current file pointer
fseek(f, 0, SEEK_SET); // seek back to beginning of file
// proceed with allocating memory and reading the file

Linux/POSIX:

You can use stat (if you know the filename), or fstat (if you have the file descriptor).

Here is an example for stat:

#include <sys/stat.h>
struct stat st;
stat(filename, &st);
size = st.st_size;

Win32:

You can use GetFileSize or GetFileSizeEx.

If you have the file descriptor fstat() returns a stat structure which contain the file size.

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

// fd = fileno(f); //if you have a stream (e.g. from fopen), not a file descriptor.
struct stat buf;
fstat(fd, &buf);
off_t size = buf.st_size;