Sprof "Pltrel Not Found Error"

sprof PLTREL not found error

According to an unanswered question on Google Groups, it looks like you aren't the very first person with this problem.

I think pltrel means plt-relative; in some ELF design notes,

There is a .plt section created in the code segment, which is an array of function stubs used to handle the run-time resolution of library calls.

And here's yet a little more:

The next section I want to mention is the .plt section. This contains the jump table that is used when we call functions in the shared library. By default the .plt entries are all initialized by the linker not to point to the correct target functions, but instead to point to the dynamic loader itself. Thus, the first time you call any given function, the dynamic loader looks up the function and fixes the target of the .plt so that the next time this .plt slot is used we call the correct function. After making this change, the dynamic loader calls the function itself.

Sounds to me like there's an issue with how the shared library was compiled or assembled. Hopefully a few more searches to elf PLT section gets you on the right track.

How do I profile a MEX-function in Matlab

Profiling MEX files is tricky since the MEX files are shared libraries. It can not be done on Linux using standard 'gprof' approach - gprof simply does not do that. I tried to use sprof, but I get “PLTREL not found error” - sprof can not be used either. There is a previous post here, but no one gave a final answer.

Luckily, there is a way in which one can do it with valgrind on Linux. First, we need to write 'running' code that loads the mex file, provides the mexFunction symbol for us to call, and sets up the parameters of the MEX file. I have chosen to use the recommended way to do this with MATLAB - using MATLAB engine. The following code (save as test.c) loads a MEX file and finds the mexFunction symbol, loads input data from a file previously saved as 'input.mat' (can be done in MATLAB using save command), and calls the mexFunction.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <dlfcn.h>
#include "engine.h"

typedef void (*mexFunction_t)(int nargout, mxArray *pargout [ ], int nargin, const mxArray *pargin[]);

int main(int argc, const char *argv[])

{
  Engine *ep;
  char buff[1024];
  int i;

  /* matlab must be in the PATH! */
  if (!(ep = engOpen("matlab -nodisplay"))) {
    fprintf(stderr, "Can't start MATLAB engine\n");
    return -1;
  }
  engOutputBuffer(ep, buff, 1023);

  /* load the mex file */
  if(argc<2){
    fprintf(stderr, "Error. Give full path to the MEX file as input parameter.\n");
    return -1;
  }
  void *handle = dlopen(argv[1], RTLD_NOW);
  if(!handle){
    fprintf(stderr, "Error loading MEX file: %s\n", strerror(errno));
    return -1;
  }

  /* grab mexFunction handle */
  mexFunction_t mexfunction = (mexFunction_t)dlsym(handle, "mexFunction");
  if(!mexfunction){
    fprintf(stderr, "MEX file does not contain mexFunction\n");
    return -1;
  }

  /* load input data - for convenience do that using MATLAB engine */
  /* NOTE: parameters are MEX-file specific, so one has to modify this*/
  /* to fit particular needs */
  engEvalString(ep, "load input.mat");
  mxArray *arg1 = engGetVariable(ep, "Ain");
  mxArray *arg2 = engGetVariable(ep, "opts");
  mxArray *pargout[1] = {0};
  const mxArray *pargin[2] = {arg1, arg2};

  /* execute the mex function */
  mexfunction(1, pargout, 2, pargin);

  /* print the results using MATLAB engine */
  engPutVariable(ep, "result", pargout[0]);
  engEvalString(ep, "result");
  printf("%s\n", buff);

  /* cleanup */
  mxDestroyArray(pargout[0]);
  engEvalString(ep, "clear all;");
  dlclose(handle);
  engClose(ep);

  return 0;
}

The MEX file itself should also compiled with the mex -g switch. The above code has to be compiled with mex -g and using engopts.sh as compilation parameters. From MATLAB command line type

mex('-v', '-f', fullfile(matlabroot,...
    'bin','engopts.sh'),...
    'test.c');

or in a standard Linux terminal run

/path/to/matlab/bin/mex -g -f /path/to/matlab/bin/engopts.sh test.c

Profiling the MEX file with valgrind requires running the 'test' program from the command line. In the directory where both test and the MEX file reside type the command:

PATH=$PATH:/path/to/matlab/bin/ LD_LIBRARY_PATH=/path/to/matlab/bin/glnxa64/:/path/to/matlab/sys/os/glnxa64/ valgrind --tool=callgrind ./test ./mex_file.mexa64

Note that the path to MATLAB and correct architecture-dependent library paths need to be set! matlab executable must be present in the PATH, otherwise 'test' will fail.

There is one more catch. MATLAB engine requires csh to be installed on the system (you can use any shell, csh just needs to be present in /bin). So if you don't have it, you have to install it for this to work.

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