警告：取消引用 'void *' 指针错误 - C

Question

With a C program written for solving a heat problem I am getting following error:

saying

In function 'main': 
       warning: dereferencing 'void *' pointer [enabled by default]
          if (!fp) *NULL;

The program was meant for this:

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <unistd.h>
#include <mpi.h>

#define COLS 1000
#define ROWS 1000

#define WHITE    "15 15 15 "
#define RED      "15 00 00 "
#define ORANGE   "15 05 00 "
#define YELLOW   "15 10 00 "
#define LTGREEN  "00 13 00 "
#define GREEN    "05 10 00 "
#define LTBLUE   "00 05 10 "
#define BLUE     "00 00 10 "
#define DARKTEAL "00 05 05 "
#define BROWN    "03 03 00 "
#define BLACK    "00 00 00 "


void copyNewToOld(float grid_a[ROWS][COLS], float grid_b[ROWS][COLS], int x_lower, int x_upper) {
  int x, y;
  for (x = x_lower; x < x_upper; ++x) {
    for (y = 0; y < COLS; ++y) {
      grid_b[x][y] = grid_a[x][y];
    }
  }
}


void calculateNew(
  float grid_a[ROWS][COLS], 
  float grid_b[ROWS][COLS],
  int x_lower, 
  int x_upper
) {
  // Adjust bounds
  if (x_lower == 0) x_lower = 1;
  if (x_upper == ROWS) x_upper = ROWS - 1;
  int x, y;
  for (x = x_lower; x < x_upper - 1; ++x) {
    for (y = 1; y < COLS - 1; ++y) {
      grid_a[x][y] = 0.25 * (grid_b[x-1][y] + grid_b[x+1][y] + grid_b[x][y-1] + grid_b[x][y+1]);
    }
  }
}


void printGridtoFile(FILE* fp, float grid[ROWS][COLS], int x_lower, int x_upper) {
  int x, y;
  for (x = x_lower; x < x_upper; ++x) {
    for (y = 0; y < COLS; ++y) {
      if (grid[x][y] > 250) {
        fprintf(fp, "%s ", RED );
      } else if (grid[x][y] > 180) {
        fprintf(fp, "%s ", ORANGE );
      } else if (grid[x][y] > 120) {
        fprintf(fp, "%s ", YELLOW );
      } else if (grid[x][y] > 80) {
        fprintf(fp, "%s ", LTGREEN );
      } else if (grid[x][y] > 60) {
        fprintf(fp, "%s ", GREEN );
      } else if (grid[x][y] > 50) {
        fprintf(fp, "%s ", LTBLUE );
      } else if (grid[x][y] > 40) {
        fprintf(fp, "%s ", BLUE );
      } else if (grid[x][y] > 30) {
        fprintf(fp, "%s ", DARKTEAL );
      } else if (grid[x][y] > 20) {
        fprintf(fp, "%s ", BROWN );
      } else {
        fprintf(fp, "%s ", BLACK );
      }
    }
    fprintf(fp, "\n");
  }
}



int main(int argc, char **argv) {
  int h, w, cycles, heat;
  float grid_a[ROWS][COLS];
  float grid_b[ROWS][COLS];

  if (argc != 2) {
    printf("Usage: ./program <number of timestamps>\n");
    exit(0);
  }
  cycles = atoi(argv[1]);


  MPI_Init(NULL, NULL);

  int mpi_size;
  MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);

  int mpi_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);

  int x_lower =  mpi_rank    * ROWS / mpi_size;
  int x_upper = (mpi_rank+1) * ROWS / mpi_size;



  /*fprintf(stderr, "CPU=%d/%d x_lower=%d x_upper=%d\n", mpi_rank, mpi_size, x_lower, x_upper);*/


  for (h = x_lower; h < x_upper; ++h) {
    for (w = 0; w < COLS; ++w) {
      grid_a[w][h] = 20;
    }
  }

  for (heat = 299; heat < 700; ++heat) {
    grid_a[0][heat] = 300;
  }

  for (cycles; cycles > 0; --cycles) {
    /*fprintf(stderr, "[%d] Cycle=%d\n", mpi_rank, cycles);*/
    copyNewToOld(grid_a, grid_b, x_lower, x_upper);
    if (mpi_rank != 0) MPI_Sendrecv(
      grid_a[x_lower-1],  // sendbuf
      COLS,               // sendcount
      MPI_FLOAT,          // sendtype
      mpi_rank-1,         // dest
      0,                  // sendtag

      grid_b[x_lower-1],  // recvbuf
      COLS,               // recvcount
      MPI_FLOAT,          // recvtype
      mpi_rank-1,         // source
      0,                  // recvtag

      MPI_COMM_WORLD,     // communicator
      MPI_STATUS_IGNORE   // status
    );


    if (mpi_rank+1 != mpi_size) MPI_Sendrecv(
      grid_a[x_upper+1],  // sendbuf
      COLS,               // sendcount
      MPI_FLOAT,          // sendtype
      mpi_rank+1,         // dest
      0,                  // sendtag

      grid_b[x_upper+1],  // recvbuf
      COLS,               // recvcount
      MPI_FLOAT,          // recvtype
      mpi_rank+1,         // source
      0,                  // recvtag

      MPI_COMM_WORLD,     // communicator
      MPI_STATUS_IGNORE   // status
    );

    calculateNew(grid_a, grid_b, x_lower, x_upper);
  }

  FILE* fp;
  if (mpi_rank == 0) {
    fp = fopen("c.pnm", "w");
    fprintf(fp, "P3\n%d %d\n15\n", COLS, ROWS);
    fclose(fp);
  } 

  char dummy = 1;
  if (mpi_rank != 0) MPI_Recv(
    &dummy,             // buf
    1,                  // count
    MPI_BYTE,           // type
    mpi_rank-1,         // source
    0,                  // tag
    MPI_COMM_WORLD,     // communicator
    MPI_STATUS_IGNORE   // status
  );

  /*fprintf(stderr, "[%d] Open file\n", mpi_rank);*/
  fp = fopen("c.pnm", "a");
  if (!fp) *NULL;
  /*fprintf(stderr, "[%d] Start printing\n", mpi_rank);*/
  printGridtoFile(fp, grid_a, x_lower, x_upper);

  /*fprintf(stderr, "[%d] Done printing\n", mpi_rank);*/
  fclose(fp);

  if (1) MPI_Send(
    &dummy,                   // buf
    1,                        // count
    MPI_BYTE,                 // type
    (mpi_rank+1) % mpi_size,  // source
    0,                        // tag
    MPI_COMM_WORLD            // communicator
  );

  if (mpi_rank == 0) {
    MPI_Recv(
      &fp,                // buf
      1,                  // count
      MPI_UNSIGNED_LONG,  // type
      mpi_size-1,         // source
      0,                  // tag
      MPI_COMM_WORLD,     // communicator
      MPI_STATUS_IGNORE   // status
    );
    system("convert c.pnm c.png");
  }


  MPI_Finalize();

  return 0;
}

Any help with this issue would be great I'm fairly new to C. Hoping to use this code and for mpirun on Bridges supercomputer with multiple varied processes.

Answer 1

The problem is this piece of code:

  /*fprintf(stderr, "[%d] Open file\n", mpi_rank);*/
  fp = fopen("c.pnm", "a");
  if (!fp) *NULL;
  /*fprintf(stderr, "[%d] Start printing\n", mpi_rank);*/
  printGridtoFile(fp, grid_a, x_lower, x_upper);

  /*fprintf(stderr, "[%d] Done printing\n", mpi_rank);*/
  fclose(fp);

You are trying to dereference NULL , which according to the C standard results in undefined behaviour. In your case this will most likely not cause any ill effects, as this expression has no side-effects and the compiler is likely to optimize it away. The compiler you are using is clever enough to spot this potential issue and is warning you in case you made a mistake.

In your case I guess you simply want to handle failures when opening a file. This is how you'd do it in a better way:

  #include <errno.h>

...

  /*fprintf(stderr, "[%d] Open file\n", mpi_rank);*/
  errno = 0;
  fp = fopen("c.pnm", "a");
  if (NULL == fp)
  {
    fprintf(stderr, "Failed to open file due to %d\n", errno);
    exit(errno);
  }

  /*fprintf(stderr, "[%d] Start printing\n", mpi_rank);*/
  printGridtoFile(fp, grid_a, x_lower, x_upper);

  /*fprintf(stderr, "[%d] Done printing\n", mpi_rank);*/
  fclose(fp);