String pointer does not work Post: 302886308

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Top Forums Programming String pointer does not work Post 302886308 by yifangt on Thursday 30th of January 2014 02:40:39 PM

01-30-2014

Registered User

Assigning a pointer does not assign its contents. I had thought I understand this, but actually I do not! Here is another example I thought I understood, but I am so unsure even it compiles fine and runs well as I expected.

Code:

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

//_____________Version 1_______________
void *strrev1(char* begin){
    char temp;
    char* end;
    end = begin + strlen(begin)-1;

    while(end>begin){
        temp = *end;
        *end = *begin;
        *begin = temp;
        end--;
        begin++;
    } 
}

//_____________Version 2_______________
char *strrev2( char *s)
  {
  char  c;
  char* s0 = s - 1;
  char* s1 = s;
 
  /* Find the end of the string */
  while (*s1) ++s1;
 
  /* Reverse it */
  while (s1-- > ++s0)
    {
    c   = *s0;
    *s0 = *s1;
    *s1 =  c;
    }
 
  return s;
  }

//_____________Version 3_______________
char *strrev3(char* seq){
  if( seq == NULL || !(*seq) ) 
            return NULL;

    int i, j = strlen(seq)-1;
    char *seq_rv;

    seq_rv = malloc(sizeof(char) * (j+1));

   for(i=0; j>=0; i++, j--) 
        *(seq_rv+i) = *(seq+j);

    return seq_rv;
//    free(seq_rv);                        //Line 57: Is this needed?
}

int main(){
    char *string, *copy1, *copy2, *copy3;            //using pointer for strings
    string=malloc(sizeof(char)*100);
    copy1=malloc(sizeof(char)*100);
    copy2=malloc(sizeof(char)*100);
    copy3=malloc(sizeof(char)*100);

 scanf("%s",string);
    strcpy(copy1, string);
    strcpy(copy2, string);
    strcpy(copy3, string);

    strrev1(string);

    printf("\n%s\n", copy1);
    printf("%s\n", string);
    printf("%s\n", strrev2(copy2));
    printf("%s\n", strrev3(copy3));

free(string);
free(copy1);
free(copy2);
free(copy3);

return 0;

}

The first two functions are adapted from stackoverflow, and the third one by myself. Now my questions are:
1)Both strrev1() and strrev2() reverse the string in place, but strrev1() does not have return value as the latter which returns a pointer; I tried to integrate to my original program for seq->qual.s, neither one worked. Why?
2) strrev3() malloc memory and has return value too. Does it take double amount of memory as compared with strrev2()? If I have 100 millions of sequence(Yes, that's normal with my files!) Then replace the original string in place would be a good idea, isn't it?
3) When malloc is used in function with returned value, should I free the memory in the function after the value is returned (Line 57) ? Memory should automatically be released on the stack once the function call is done, is this right?

Last edited by yifangt; 02-01-2014 at 10:00 AM.. Reason: typo

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LEARN ABOUT PHP

gearman_job_status

GEARMAN_JOB_STATUS(3)							 1						     GEARMAN_JOB_STATUS(3)

GearmanClient::jobStatus - Get the status of a background job

       Object oriented style (method):

SYNOPSIS

       public array GearmanClient::jobStatus (string  $job_handle)

DESCRIPTION

	Gets  the status for a background job given a job handle. The status information will specify whether the job is known, whether the job is
       currently running, and the percentage completion.

PARAMETERS

	      o $job_handle
		- The job handle assigned by the Gearman server

RETURN VALUES

	An array containing status information for the job corresponding to the supplied job handle. The first array element is a boolean indicat-
       ing  whether  the job is even known, the second is a boolean indicating whether the job is still running, and the third and fourth elements
       correspond to the numerator and denominator of the fractional completion percentage, respectively.

EXAMPLES

       Example #1

	      Monitor the status of a long running background job

	      <?php

	      /* create our object */
	      $gmclient= new GearmanClient();

	      /* add the default server */
	      $gmclient->addServer();

	      /* run reverse client */
	      $job_handle = $gmclient->doBackground("reverse", "this is a test");

	      if ($gmclient->returnCode() != GEARMAN_SUCCESS)
	      {
		echo "bad return code
";
		exit;
	      }

	      $done = false;
	      do
	      {
		 sleep(3);
		 $stat = $gmclient->jobStatus($job_handle);
		 if (!$stat[0]) // the job is known so it is not done
		    $done = true;
		 echo "Running: " . ($stat[1] ? "true" : "false") . ", numerator: " . $stat[2] . ", denomintor: " . $stat[3] . "
";
	      }
	      while(!$done);

	      echo "done!
";

	      ?>

	      The above example will output something similar to:

	      Running: true, numerator: 3, denomintor: 14
	      Running: true, numerator: 6, denomintor: 14
	      Running: true, numerator: 9, denomintor: 14
	      Running: true, numerator: 12, denomintor: 14
	      Running: false, numerator: 0, denomintor: 0
	      done!

SEE ALSO

       GearmanClient::doStatus.

PHP Documentation Group 												     GEARMAN_JOB_STATUS(3)

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