Ways to get the file size in C

In this article, I will discuss two approaches for getting the file size using the C programming language.

Approach 1

When we have to get the file size first approach that comes to our mind is to open the file, seek the cursor to the end of the file, and then get the position of the cursor which is nothing but the size of the file.

Let's understand how can we achieve this.

• First, open a file using fopen. The first argument is the file path and the second argument is mode and it returns the file descriptor. Since we don't have to modify the file open it in read mode using "r".
• Now move the cursor to the end of the file using fseek which takes the following arguments respectively.
  • fd file descriptor of the file opened.
  • offset how much to offset w.r.t the third argument position.
  • whence from where the cursor should consider moving from. We have three options for it, SEEK_SET relative to the start of the file, SEEK_CUR relative to the current position, and SEEK_END relative to the end of the file.

Since we have to move to the end of the file we take relative to the end of the file and offset it as zero, as we have to move zero from the end. fseek return -1 if it fails.

• Now use ftell to get the current position which takes fd as an argument and returns the current position which is our file size. On failing it returns -1.

NOTE: If the cursor move by 1 unit we consider it as 1 byte because the size of char is 1 byte and fopen reads the file character by character only.

Code

#include <stdio.h>

// function get file size
long get_file_size(char *);

int main() {
    char *filename = "a.out";
    printf(
        "Size of file `%s` is %ld\n", 
        filename, 
        get_file_size(filename)
    );

    return 0;
}

long get_file_size(char *filename) {
    FILE *fp = fopen(filename, "r");

    if (fp==NULL)
        return -1;

    if (fseek(fp, 0, SEEK_END) < 0) {
        fclose(fp);
        return -1;
    }

    long size = ftell(fp);
    // release the resources when not required
    fclose(fp);
    return size;
}

Output

Size of file `a.out` is 51880 bytes

Approach 2

In this approach, we use stat API to get the file information which also includes file size.
For this, we simply call the stat function which takes a file path as the first argument and pointer to the stat struct variable as the second argument and returns -1 on failure.

To get the file size we can simply use the st_size attribute.

Code

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

// inherit the above base code here

long get_file_size(char *filename) {
    struct stat file_status;
    if (stat(filename, &file_status) < 0) {
        return -1;
    }

    return file_status.st_size;
}

Output

Size of file `a.out` is 51880 bytes

Using this we can get some other information about a file such as permission, creation time, user id, group id, etc. Also, there are some other functions defined that can be used based on requirements. Check out its man page for more info.

Which one should be used?

The second approach is more recommended because of the following reason.

• It's fast as we don't have to open a file which in the first approach has to be done. Some compilers may buffer a file on opening which makes the first approach slower.
• The code is more clear to other developers that we are trying to get file info.

But this code is not portable. As other APIs are developed on OS like windows to get file info.

Whereas the first approach code is portable. As same code works normally without any problem.

So, in conclusion, which strategy we should take is entirely dependent on our requirements.

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This article is highly inspired by the content provided by Jacob Sorber on his youtube channel.

❤️Thank you so much for reading this article. I'm a passionate engineering student learning new things so If you find any mistakes or have any suggestions please let me know in the comments.

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