Determine the size of a file in C portably

Question

Before flagging this as a duplicate, yes, I have read this:

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

I need to determine the size of files. The thing is - I need to do it portably in C. The answers in the above question use the POSIX stat function.

Also, I cannot use fseek(f, 0, SEEK_END) together with ftell(f) as suggested. According to the C11 standard this is undefined behavior.

7.21.9.2:

A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

The undefined behavior actually shows up on Windows (I didn't test it on Linux), and I learned this the hard way, in a bigger program. When running the same binary program on the same file, ftell() sometimes returns -1, sometimes the correct size and sometimes garbage values.

A solution I thought about is reading the entire file (until EOF), byte by byte, and counting the number of bytes read. This could work, but it has some disadvantages that I would like to not have to deal with. Firstly, read errors could occur, and this would mess up the whole calculation. Secondly, reading a file byte by byte is slow, and normally, an operating system should be able to tell you the size of a file without even looking at its contents.

Is there any way to get the size of a file portably in C11?

Answer 1

As it turns out, this is impossible to do in pure C11. There are only 2 options:

1. Reading the file byte by byte

   #include <stdio.h>
   #include <stdint.h>
   
   int64_t fsize(const char filename[])
   {
       int64_t n = 0;
       FILE* f = fopen(filename, "rb");
       if (!f)
           return -1;
       int c = fgetc(f);
       while (c != EOF)
       {
           n++;
           c = fgetc(f);
       }
       if (ferror(f))
           n = -1;
       fclose(f);
       return n;
   }
   

2. Preprocessor directives

   #if defined(__unix__) || defined(__APPLE__)
       #include <unistd.h>
   #elif defined(_WIN32)
       #include <windows.h>
   #else
       #error "Unknown operating system"
   #endif
   #include <stdint.h>
   
   int64_t fsize(const char filename[])
   {
   #if defined(__unix__) || defined(__APPLE__)
       struct stat64 st;
       if (stat64(filename, &st) == 0)
           return st.st_size;
       return -1;
   #elif defined(_WIN32)
       int32_t low, high;
       HANDLE f = CreateFileA(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
       low = GetFileSize(f, &high);
       int err = GetLastError();
       CloseHandle(f);
       if (low == INVALID_FILE_SIZE && err != NO_ERROR)
           return -1;
       return ((int64_t)high << 32) | low;
   #endif
   }