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

POSIX

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

Synopsis

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

Examples

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

#include 
#include 

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 
#include 

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!

Synopsis

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

Example

#include 

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.