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How or from where does df get its information?

I am writing a C program and I want I need to know the root / filesystem size, amount used, and amount or percent free.

Anyone would normally just do

df -h

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc2       550G  168G  355G  33% /
udev            253G  216K  253G   1% /dev
tmpfs           253G  1.9M  253G   1% /dev/shm
/dev/sdc1       195M   13M  183M   7% /boot/efi
/dev/sda1       5.0T  4.9T   26G 100% /data
/dev/sdb1       559G  286G  273G  52% /scratch

I only need the values for /. Currently in C I do

system("df -h > somefile.txt");
fp = fopen( somefile.txt, "r");
/* read contents of file and parse out root file system values */

This is a problem because the df command does not always work, sometimes it hangs indefinitely which causes my C program to hang.

Is there some file under /proc or /sys that contains the size, used, and use% information? Is there any other way to get this information other than df ?

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1 Answer 1

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int statfs(const char *path, struct statfs *buf); int fstatfs(int fd, struct statfs *buf);

DESCRIPTION The function statfs() returns information about a mounted file system. path is the pathname of any file within the mounted file system. buf is a pointer to a statfs structure defined approximately as follows:

      struct statfs {
          long    f_type;      type of file system (see below)
          long    f_bsize;     optimal transfer block size
          long    f_blocks;    total data blocks in file system
          long    f_bfree;     free blocks in fs
          long    f_bavail;    free blocks avail to non-superuser
          long    f_files;     total file nodes in file system
          long    f_ffree;     free file nodes in fs
          fsid_t  f_fsid;      file system id
#include <stdio.h>
#include <stdlib.h>
#include <sys/statfs.h>

int main ( int argc, char *argv[] )
{
   char path[80];
   struct statfs buffer;
   int n;
   long int block_size;
   long int total_bytes;
   long int free_bytes;
   double total_gb;

   sprintf( path, "/bin/pwd" );   /* this file should always exist */

   n = statfs( path, &buffer );

   block_size = buffer.f_blocks;
   total_bytes = buffer.f_bsize * block_size;
   total_gb = total_bytes / 1024.0 / 1024.0 / 1024.0;

   printf("total gb =  %lf\n", total_gb );

   return 0;
}

total gb = 549.952682

output from my df -h

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc2       550G  168G  355G  33% /

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