Question

I’m using fmemopen to create a variable FILE* fid to pass it to a function the reads data from an open file.

Somewhere in that function it uses the following code to find out the size of the file:

fseek(fid, 0, SEEK_END);
file_size = ftell(fid);

this works well in case of regular files, but in case of file ids created by fmemopen I always get file_size = 8192

Any ideas why this happens?
Is there a method to get the correct file size that works for both regular files and files created with fmemopen?

EDIT:
my call to fmemopen:

fid = fmemopen(ptr, memSize, "r");

where memSize != 8192

EDIT2:

I created a minimal example:

#include <cstdlib>
#include <stdio.h>
#include <string.h>
using namespace std;

int main(int argc, char** argv)
{
    const long unsigned int memsize = 1000000;
    void * ptr = malloc(memsize);
    FILE *fid = fmemopen(ptr, memsize, "r");
    fseek(fid, 0, SEEK_END);
    long int file_size = ftell(fid);
    printf("file_size = %ld\n", file_size);
    free(ptr);
    return 0;
}

btw, I am currently working on another computer, and here I get file_size=0

Answer 1

In case of fmemopen , if you open using the option b then SEEK_END measures the size of the memory buffer. The value you see must be the default buffer size.