C Language, Dynamic Alloc

Dynamic Alloc - Arrays

Author	Message
Douglas A. Gwy #1 / 3	Dynamic Alloc - Arrays Quote: > realloc(fieldsize,sizeof(unsigned short int)*no_fields); That's not even close to correct. You should use realloc only with a pointer you already got from a previous call to malloc (or realloc), and it is essential to capture the pointer returned as the value of the malloc or realloc function call, since that is the only handle you will have for locating where the allocator placed the new object. Look at several existing, working applications that use dynamic allocation to see how the malloc/realloc/free subsystem is used. --
Mon, 03 Oct 2005 14:52:06 GMT

Barry Schwar #2 / 3	Dynamic Alloc - Arrays Quote: >I am new to C and trying to write a little utility programme to >manipulate records in a file. I have written some code to get input >on the number of fields in a record and their size. I want to >dynamically declare an array to hold the field sizes and have used >realloc() in the code below but have a strong feeling that I am doing >something wrong (i.e. I could be writing over other data). Could do It is worse than that. You need to find a text book and learn what the parameters and return value from realloc signify. Quote: >with an opinion. (I have removed the indents to avoid too many split >lines hope this does not cause a problem. >/********************************************/ >#include <stdio.h> >#include <stdlib.h> >int main(int argc,char argv[]) { >unsigned short int i, no_fields, fieldsize[1], x = 0; // x is for >debuging only >/if(argc != 3){ >printf("\nIncorrect usage:\n\nCorrect usage = exe filename >[InputFilename] [OutputFilename]\n\nTry again please\n"); >exit(0); >}/ >printf("\nHow many fields are there in a record?"); >scanf("%d",&no_fields); printf("\n"); >realloc(fieldsize,sizeof(unsigned short int)no_fields); fieldsize is an array. It is never legal to pass an array name to realloc. The first parameter must be a pointer which contains the value returned from a previous allocation or NULL. You never use the value returned by realloc. You seem to think that somehow fieldsize will be changed to point to the newly allocated area. Remember that c passes all function arguments by value so it is not possible for a function to change an argument. Why are you using realloc instead of malloc? - Hide quoted text - - Show quoted text - Quote: >for (i = 0; i < no_fields; i++) { // Get all the field sizes >printf("Please enter the size of field %d ",i+1); >scanf("%d",&fieldsize[i]); >} >/************ Debug code **************/ >printf("\nThe input file is %s and output is to >%s\n",argv[1],argv[2]); >for (i = 0; i < no_fields; i++) printf("\nField %d has %d >charcters\n\n",i+1,fieldsize[i]); >scanf("%d",x); >/**********************************************/ >return 0; >} >. <<Remove the del for email>> --
Mon, 03 Oct 2005 14:52:42 GMT

Mark Gordo #3 / 3	Dynamic Alloc - Arrays On 16 Apr 2003 00:23:54 GMT Quote: > I am new to C and trying to write a little utility programme to > manipulate records in a file. I have written some code to get input > on the number of fields in a record and their size. I want to > dynamically declare an array to hold the field sizes and have used > realloc() in the code below but have a strong feeling that I am doing > something wrong (i.e. I could be writing over other data). Could do > with an opinion. (I have removed the indents to avoid too many split > lines hope this does not cause a problem. Please don't remove all indenting, it makes it MUCH harder to read. In lines being split is a problem then reformat the code manually splitting the lines as required, make sure it still compilers, then submit it. Quote: > /********************************************/ > #include <stdio.h> > #include <stdlib.h> > int main(int argc,char argv[]) { > unsigned short int i, no_fields, fieldsize[1], x = 0; // x is for ^^^^^^^^^ unsigned short temp, fieldsize=NULL; /* You can't realloc an array, only space that was allocated by realloc or malloc / Quote: > debuging only // style comments are a pain when posting to Usenet. Quote: > /if(argc != 3){ > printf("\nIncorrect usage:\n\nCorrect usage = exe filename > [InputFilename] [OutputFilename]\n\nTry again please\n"); > exit(0); > }/ > printf("\nHow many fields are there in a record?"); > scanf("%d",&no_fields); printf("\n"); > realloc(fieldsize,sizeof(unsigned short int)no_fields); /* Realloc returns a pointer to the memory on success or NULL on failure, it does not EVER modify its parameters. / temp=realloc(fieldsize, fields sizeof fieldsize); if (temp==NULL) { / handless failed realloc / Quote: } fieldsize=temp; / I've not checked the rest. However, I suggest you read up further on memory allocation. */ // <snip rest of code> -- Mark Gordon Paid to be a Geek & a Senior Software Developer Currently looking for a new job commutable from Slough, Berks, U.K. Although my email address says spamtrap, it is real and I read it. --
Mon, 03 Oct 2005 14:53:36 GMT

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