如果我使用fread会出错，而使用read则不会出错

Question

I'm trying to make some experiments on disk I/O using cache and not using it. In order to perform a read directly from the disk, I open the file with the O_DIRECT flag (defining the variable DISK_DIRECT).

Now the two branches of the if beneath, should perform the same operation, with the difference that one is helped by the cache and the other not. The files to which I try to access are stored on disk and they do not change over time. Also the two branches access to the same files.

At some point here, when I use fread I get ferror() to be true. While when I use read everything goes fine.

I'm sure they access the same files. Do you have any idea why this could happen?

EDIT

Ok, i'm posting here an minimal example. the code i use is:

#include <iostream>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#include <fstream>
#include <sstream>


using namespace std;

typedef float fftwf_complex [2] ;

void fetch_level(unsigned long long tid, unsigned short level, fftwf_complex* P_read, fftwf_complex* P_fread, int n_coeff_per_level, FILE** files_o_direct, fstream* & files) {

  int b_read;
  fseek(files_o_direct[level],(long int) (tid * sizeof(fftwf_complex)*n_coeff_per_level), SEEK_SET);


  b_read = fread(reinterpret_cast<char*>(P_fread),sizeof(fftwf_complex), n_coeff_per_level,files_o_direct[level]);

  if(b_read == 0){ 
    cerr << "nothing read\n";

  }

  files[level].seekg((streamoff) (tid * sizeof(fftwf_complex)*n_coeff_per_level), files[level].beg);

  files[level].read(reinterpret_cast<char*>(P_read), 
            sizeof(fftwf_complex) * n_coeff_per_level);

}

void open_files (fstream* & files){

  for(int i=0; i<1;i++) {
    std::ostringstream oss;
    oss << "./Test_fread_read/1.txt.bin";

    files[i].open(oss.str().c_str(),
          std::ios::in | std::ios::out | 
          std::ios::binary | std::ios::ate);
    if (!files[i])
      {
    cerr << "fstream could not open " << oss.str() << endl;
      }
  }
}

void open_files_o_direct (FILE** files_o_direct, int* fd){

  for(unsigned int i=0;i<1; i++){
    std::ostringstream oss;
    oss << "./Test_fread_read/1.txt.bin";
    fd[i]=open(oss.str().c_str(), O_RDONLY | O_DIRECT);
    files_o_direct[i] = fdopen(fd[i], "rb");

    if(!files_o_direct[i])
      cerr << "Could not open " << oss.str() << endl;

  }
}

int close_files(FILE** files_o_direct, int* fd, fstream* & files) {

  for(unsigned int i=0; i<1; i++){
    //#if defined (DISK_DIRECT)
    if(files_o_direct[i])
      close(fd[i]);
    //#else
    if(files[i].is_open())
      files[i].close();
    //#endif
  }

  return 0;
}

int main(){

  FILE**files_o_direct = new FILE* [256];
  fstream* files = new fstream [256];
  int * fd = new int [256];


  fftwf_complex * P_read =  new fftwf_complex [1];
  fftwf_complex * P_fread =  new fftwf_complex [1];

  open_files_o_direct(files_o_direct, fd);
  open_files(files);

fetch_level(2, 0, P_read, P_fread, 1, files_o_direct, files);
  cout << "P_read: " << P_read[0][0] << " P_fread: " << P_fread[0][0] << endl;
  cout << "P_read: " << P_read[0][1] << " P_fread: " << P_fread[0][1] << endl;
fetch_level(7, 0, P_read, P_fread, 1, files_o_direct, files);
  cout << "P_read: " << P_read[0][0] << " P_fread: " << P_fread[0][0] << endl;
  cout << "P_read: " << P_read[0][1] << " P_fread: " << P_fread[0][1] << endl;
fetch_level(8, 0, P_read, P_fread, 1, files_o_direct, files);
  cout << "P_read: " << P_read[0][0] << " P_fread: " << P_fread[0][0] << endl;
  cout << "P_read: " << P_read[0][1] << " P_fread: " << P_fread[0][1] << endl;


  close_files(files_o_direct, fd, files);

  delete [] P_read;
  delete [] P_fread;
  delete [] files;
  delete [] files_o_direct;

  return 0;
}

and the file which is accessed is:

0.133919 0.0458176 
1.67441 2.40805 
0.997525 -0.279977 
-2.39672 -3.076 
-0.0390913 0.854464 
-0.0176478 -1.3142 
-0.667981 -0.486272 
0.831051 0.282802 
-0.638032 -0.630943 
-0.669854 -1.49762 

which is stored in a binary format and that can be download from here: 1.txt.bin .

The output i get is:

nothing read
P_read: 0.997525 P_fread: 0
P_read: -0.279977 P_fread: 0
nothing read
P_read: 0.831051 P_fread: 0
P_read: 0.282802 P_fread: 0
nothing read
P_read: -0.638032 P_fread: 0
P_read: -0.630943 P_fread: 0

The problem persists even if i change the type of fftwf_complex from float[2] to simple float. If i remove the fseek line everything works correctly.

Answer 1

This if (b_read == 0) , will be true at the end of the file, and you will enter this branch

if(ferror(this->files_o_direct[level]))
    fseek(this->files_o_direct[level], 0, SEEK_END); //ftell here returns 4800000 
cerr << "nothing read\n";

even if ferror returns 0, the end of the file was reached anyway

fseek(this->files_o_direct[level], 0, SEEK_END);

makes no sense, and "nothing read\\n" will be output either or not ferror returns nonzero.

From the manual page

fread() does not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred.

so you have to check feof and if it is false you use ferror .

Answer 2

For who ever may have the same problem here there is the answer:

The O_DIRECT flag may impose alignment restrictions on the length and address of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by filesystem and kernel version and might be absent entirely. However there is currently no filesystem-independent interface for an application to discover these restrictions for a given file or filesystem. Some filesystems provide their own interfaces for doing so, for example the XFS_IOC_DIOINFO operation in xfsctl(3).

  Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset must all be multiples of the logical block size of the filesystem. Since Linux 2.6.0, alignment to the logical block size of the underlying storage (typically 512 bytes) suffices. The logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the command: blockdev --getss 

linux reference page