is this right way to use malloc.

This is a discussion on is this right way to use malloc. within the C Programming forums, part of the General Programming Boards category; i would like to ask if this is the right way to use malloc or if i am doing anything ...

  1. #1
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    is this right way to use malloc.

    i would like to ask if this is the right way to use malloc or if i am doing anything wrong.
    i just turned the page about structures a few days ago in the book i am using.
    thanks for the time to comment.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <string.h>
    
    struct contents
    {
    	int chars;
    	int digits;
    	int others;
    };
    void PrintUsage(const char *progname);
    int main(int argc, char** argv)
    {
    	int c;
    	int i;
    	int ch,digit,other;
    	    ch=digit=other=0;
    
    	FILE *fp;
    	
    	  if( argc == 1 )
    	  {
    		   PrintUsage(argv[0]);
    		   exit(EXIT_FAILURE);
    	  } 
    		  
    		  struct contents *count=malloc(sizeof(count));
    		  
    		  if(count == NULL)
    		  { 
    			 errno=EINVAL;
    			 perror(argv[0]);
    		        exit(1);
                     }
    		  
    		 for(i=1;i< argc;i++)
                   {
    	                if((fp=fopen(argv[i],"r")) == NULL)
    	                {
                                errno=EINVAL;
    		                    perror(argv[0]);
    	                        exit(EXIT_FAILURE);
    	                }
    
    	              while((c=fgetc(fp))!= EOF)
    	             {
    		             switch(c)
    		            {
    		            	  case '1':case '2':case '3':case '4':case '5':
    			              case '6':case '7':case '8':case '9':case '0':
    			              digit++;  
    				      count->digits++;
    			              case ' ': case '\n': case '\t':
    			              other++;
    				     count->others++;
    			              default:
    			              ch++;
    				     count->chars++;
    		            }
    
                 	}  
    	/* it prints total of the files if more than 1 file is counted. for now its considered a feature :) */
       fprintf(stdout,"nchar %d ndigit %d nother %d %s\n",ch,digit,other,argv[i]);
       fprintf(stdout,"nchar %d ndigit %d nohter %d %s\n",count->chars,count->digits,count->others,argv[i]);
    
              } /* end for loop*/
    		  
    	     fclose(fp);
    		free(count);
    	     count=NULL;
    	
    	return 0;
    }
    
    void PrintUsage(const char *progname)
    {
    	fprintf(stdout," usage: %s [file]              \n"
    	               "counts chars digits and others \n",progname);
    }

  2. #2
    C++ Witch laserlight's Avatar
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    This is not correct:
    Code:
    struct contents *count=malloc(sizeof(count));
    It should be:
    Code:
    struct contents *count = malloc(sizeof(*count));
    However, in this case it would be better not to use malloc at all since you do not need dynamic memory allocation.

    By the way, you need to indent more consistently. Perhaps you are mixing tabs and spaces?
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  3. #3
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    Also, you appear to be missing some break statements in your switch block. If you don't use break, each case will fall through to the next.
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  4. #4
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    thanks.

    i completely forgot the break statements. it was just a rather hello world like project i started out last night so i just posted what i got from before i went to bed.

    sorry the indent is a bit off. i fine tuned it before posting.

  5. #5
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    Quote Originally Posted by laserlight View Post
    This is not correct:
    Code:
    struct contents *count=malloc(sizeof(count));
    It should be:
    Code:
    struct contents *count = malloc(sizeof(*count));
    However, in this case it would be better not to use malloc at all since you do not need dynamic memory allocation.

    By the way, you need to indent more consistently. Perhaps you are mixing tabs and spaces?
    i dissagree
    it should be
    struct contents *count=malloc(sizeof(struct contents));

    count is just a variable its not a type

  6. #6
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    No, laserlight is correct. You can do sizeof with a variable just as well as a type. Using the variable has the added benefit that if you ever change the variable type, the code still works as is.

    For example:
    Code:
    char* var = malloc(sizeof(char));
    vs
    char* var = malloc(sizeof(*var));
    // If you ever change the type of var from char to int, you do not need to change what gets passed to malloc()
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  7. #7
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by kakaroto
    i dissagree
    it should be
    struct contents *count=malloc(sizeof(struct contents));
    That is certainly one way of doing it, but it has the disadvantage that if the type of count is changed, the update has to be made in two places, and it can be the case that failure to do so results in a run time error (or no error at all, until further down the road after maintenance).

    Quote Originally Posted by kakaroto
    count is just a variable its not a type
    Yes, and sizeof(*count) results in the size of an object that count would point to.
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  8. #8
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    ahhhh aint that the coolest thing

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