How do you determine the size of a file in C?

Question

How can I figure out the size of a file, in bytes?

#include 

unsigned int fsize(char* file){
  //what goes here?
}

Answer 1

Looking at the question, ftell can easily get the number of bytes.

  long size = ftell(FILENAME);
  printf("total size is %ld bytes",size);

Answer 2

Don't use int. Files over 2 gigabytes in size are common as dirt these days

Don't use unsigned int. Files over 4 gigabytes in size are common as some slightly-less-common dirt

IIRC the standard library defines off_t as an unsigned 64 bit integer, which is what everyone should be using. We can redefine that to be 128 bits in a few years when we start having 16 exabyte files hanging around.

If you're on windows, you should use GetFileSizeEx - it actually uses a signed 64 bit integer, so they'll start hitting problems with 8 exabyte files. Foolish Microsoft! :-)

Answer 3

Try this --

fseek(fp, 0, SEEK_END);
unsigned long int file_size = ftell(fp);
rewind(fp);

What this does is first, seek to the end of the file; then, report where the file pointer is. Lastly (this is optional) it rewinds back to the beginning of the file. Note that fp should be a binary stream.

file_size contains the number of bytes the file contains. Note that since (according to climits.h) the unsigned long type is limited to 4294967295 bytes (4 gigabytes) you'll need to find a different variable type if you're likely to deal with files larger than that.

Answer 4

A quick search in Google found a method using fseek and ftell and a thread with this question with answers that it can't be done in just C in another way.

You could use a portability library like NSPR (the library that powers Firefox) or check its implementation (rather hairy).

Answer 5

Matt's solution should work, except that it's C++ instead of C, and the initial tell shouldn't be necessary.

unsigned long fsize(char* file)
{
    FILE * f = fopen(file, "r");
    fseek(f, 0, SEEK_END);
    unsigned long len = (unsigned long)ftell(f);
    fclose(f);
    return len;
}

Fixed your brace for you, too. ;)

Update: This isn't really the best solution. It's limited to 4GB files on Windows and it's likely slower than just using a platform-specific call like GetFileSizeEx or stat64.

Answer 6

And if you're building a Windows app, use the GetFileSizeEx API as CRT file I/O is messy, especially for determining file length, due to peculiarities in file representations on different systems ;)