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

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

#include <stdio.h>

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

This question is related to c file io filesize

The answer is

Here's a simple and clean function that returns the file size.

long get_file_size(char *path)
    FILE *fp;
    long size = -1;
    /* Open file for reading */
    fp = fopen(path, "r");
    fseek(fp, 0, SEEK_END);
    size = ftell(fp); 


The POSIX standard has its own method to get file size.
Include the sys/stat.h header to use the function.


  • Get file statistics using stat(3).
  • Obtain the st_size property.


Note: It limits the size to 4GB. If not Fat32 filesystem then use the 64bit version!

#include <stdio.h>
#include <sys/stat.h>

int main(int argc, char** argv)
    struct stat info;
    stat(argv[1], &info);

    // 'st' is an acronym of 'stat'
    printf("%s: size=%ld\n", argv[1], info.st_size);
#include <stdio.h>
#include <sys/stat.h>

int main(int argc, char** argv)
    struct stat64 info;
    stat64(argv[1], &info);

    // 'st' is an acronym of 'stat'
    printf("%s: size=%ld\n", argv[1], info.st_size);

ANSI C (standard)

The ANSI C doesn't directly provides the way to determine the length of the file.
We'll have to use our mind. For now, we'll use the seek approach!


  • Seek the file to the end using fseek(3).
  • Get the current position using ftell(3).


#include <stdio.h>

int main(int argc, char** argv)
    FILE* fp = fopen(argv[1]);
    int f_size;

    fseek(fp, 0, SEEK_END);
    f_size = ftell(fp);
    rewind(fp); // to back to start again

    printf("%s: size=%ld", (unsigned long)f_size);

If the file is stdin or a pipe. POSIX, ANSI C won't work.
It will going return 0 if the file is a pipe or stdin.

Opinion: You should use POSIX standard instead. Because, it has 64bit support.

You can open the file, go to 0 offset relative from the bottom of the file with

#define SEEKBOTTOM   2

fseek(handle, 0, SEEKBOTTOM)  

the value returned from fseek is the size of the file.

I didn't code in C for a long time, but I think it should work.

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 ;)

**Don't do this (why?):

Quoting the C99 standard doc that i found online: "Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.**

Change the definition to int so that error messages can be transmitted, and then use fseek() and ftell() to determine the file size.

int fsize(char* file) {
  int size;
  FILE* fh;

  fh = fopen(file, "rb"); //binary mode
  if(fh != NULL){
    if( fseek(fh, 0, SEEK_END) ){
      return -1;

    size = ftell(fh);
    return size;

  return -1; //error

I 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).

I have a function that works well with only stdio.h. I like it a lot and it works very well and is pretty concise:

size_t fsize(FILE *File) {
    size_t FSZ;
    fseek(File, 0, 2);
    FSZ = ftell(File);
    return FSZ;

I used this set of code to find the file length.

//opens a file with a file descriptor
FILE * i_file;
i_file = fopen(source, "r");

//gets a long from the file descriptor for fstat
long f_d = fileno(i_file);
struct stat buffer;
fstat(f_d, &buffer);

//stores file size
long file_length = buffer.st_size;

If you're fine with using the std c library:

#include <sys/stat.h>
off_t fsize(char *file) {
    struct stat filestat;
    if (stat(file, &filestat) == 0) {
        return filestat.st_size;
    return 0;

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

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

Try this --

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

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! :-)

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);
    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.

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