Tutorial: C + MySQL + MPI

--D. Thiebaut (talk) 16:30, 13 October 2013 (EDT)

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References

• http://www.cyberciti.biz/tips/linux-unix-connect-mysql-c-api-program.html

Verify that MPI works

• test your installation with the classic hello world program.

Source

// mpi_hello.c

#include <mpi.h>
#include <stdio.h>
 
int main (int argc, char* argv[])
{
  int rank, size;
 
  MPI_Init (&argc, &argv);      /* starts MPI */
  MPI_Comm_rank (MPI_COMM_WORLD, &rank);        /* get current process id */
  MPI_Comm_size (MPI_COMM_WORLD, &size);        /* get number of processes */
  printf( "Hello world from process %d of %d\n", rank, size );
  MPI_Finalize();
  return 0;
}

Compile & Run

mpicc -o hello mpi_hello.c
mpirun -np 2 ./hello
Hello world from process 0 of 2
Hello world from process 1 of 2

Verify that the MySQL API works

Using the example and tricks provided at http://www.cyberciti.biz/tips/linux-unix-connect-mysql-c-api-program.html, we can easily test whether we can access a MySQL database from our program:

Source

// mysqlTest.c
// Taken from http://www.cyberciti.biz
// Lists all the tables found in a MySQL database whose name is stored
// in the char[] database variable.

#include <mysql.h>
#include <stdio.h>
#include <stdlib.h>

main() {
   MYSQL *conn;
   MYSQL_RES *res;
   MYSQL_ROW row;
   char *server = "localhost";
   char *user = "352a";
   char *password = "xxxxxx";  
   char *database = "enwiki_images";
   conn = mysql_init(NULL);

   /* Connect to database */
   if (!mysql_real_connect(conn, server,
         user, password, database, 0, NULL, 0)) {
      fprintf(stderr, "%s\n", mysql_error(conn));
      exit(1);
   }

   /* send SQL query */
   if (mysql_query(conn, "show tables")) {
      fprintf(stderr, "%s\n", mysql_error(conn));
      exit(1);
   }

   res = mysql_use_result(conn);
   /* output table name */
   printf("MySQL Tables in mysql database:\n");
   while ((row = mysql_fetch_row(res)) != NULL)
      printf("%s \n", row[0]);

   /* close connection */
   mysql_free_result(res);
   mysql_close(conn);
}

Compile & Run

• Once you have installed mysql_config on your system, you can easily get the library switches and compiler switches corresponding to your installation. They are given by
• mysql_config --cflags
• mysql_config --libs

  mysql_config --cflags
  -I/usr/include/mysql -DBIG_JOINS=1  -fno-strict-aliasing  -g -DNDEBUG
 
  mysql_config --libs
  -L/usr/lib/x86_64-linux-gnu -lmysqlclient -lpthread -lz -lm -ldl

To compile, simply pass on the output of the mysql_config commands, or substitute their output in the compile line:

 gcc -o mysqlTest $(mysql_config --cflags) mysqlTest.c $(mysql_config --libs)

To run:

 ./mysqlTest
 images                    <--- the only table was images

Calling System Applications from C

In our application we need to process a large collection of images and get their geometry information. To get the width and height of images, we simply call imageMagick from the C program and grab the width and height from the command.

Source

// getImageInfo.c
// D. Thiebaut
// Syntax:  
//         getImageInfo  imageFileName
//
// Takes the image file and get identify (part of the ImageMagick tools) to
// get the image width and height in pixels.
//
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main( int argc, char *argv[] ) {
  FILE* fp;
  char command[100];
  char buffer[1000];
  int width, height;

  //--- display syntax info if user does not specify image name---
  if ( argc <= 1 ) {
    fprintf( stderr, "Syntax: getImageInfo imageFilename\n\n" );
    exit( 1 );
  }

  //--- create the command with the file name (passed in argv) ---
  strcpy( command, "/usr/bin/identify -format \"%w %h\" " );
  strcat( command, argv[1] );
  
  //--- open a pipe, make the command run, and return the information ---
  //--- in the pipe. ---
  if ( ( fp = popen( command, "r" ) ) != NULL ) {
    while ( fgets( buffer, 1000, fp ) != NULL ) {
	printf( "%s", buffer );
	char *p = buffer;
	while ( *p != ' ' ) p++;
	*p = '\0';

	//printf( "width = %s height = %s\n", buffer, p );
	width = atoi( buffer );
	height = atoi( p+1 );
	printf( "width = %d heiht = %d\n", width, height );
    }
  }

  //--- close pipe ---
  pclose( fp );
  return 0;
}

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