Unclear pointer and array Post: 302884187

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Top Forums Programming Unclear pointer and array Post 302884187 by yifangt on Saturday 18th of January 2014 07:46:49 AM

01-18-2014

Registered User

Here it is:

Code:

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

char *substring(size_t start, size_t stop, const char *src, char *dst)
{
    unsigned int count = stop - start;
    size_t size = strlen(dst);

    if (count >= --size) {
    count = size;
    }
    sprintf(dst, "%.*s", count, src + start);
    return dst;
}

int main(int argc, char **argv)
{
    char *text=(char*)malloc(100*sizeof(char));  //Line 40, change to text[100] will work
    char *a=(char*)malloc(100*sizeof(char));    //Line 41, change to a[100] will work
    int start, end;

    if (argc != 4) {
    printf("Error! Usage:\n\t \
argv[0]=program;\n\t \
argv[1]=input string\n\t \
argv[2]=start_position of string\n\t \
argv[3]=end_postion of string\n");

    return 1;
    }

    strcpy(text, argv[1]);
    start = atoi(argv[2]);
    end = atoi(argv[3]);

    printf("substring = \"%s\"\n", substring(start, end, text, a));

    return 0;
}

Rewrote the substring() function as compared with the original version which may only handle string array specifically.
I came to the dead corner of my knowledge on string pointer in C: How to get the string length when you dynamically allocate the memory space for it? Or after you allocate the memory of a string pointer, how to get the string length?

Last edited by yifangt; 01-18-2014 at 09:13 AM.. Reason: Add more information.

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LEARN ABOUT LINUX

qsort_r

QSORT(3)						     Linux Programmer's Manual							  QSORT(3)

NAME

       qsort - sorts an array

SYNOPSIS

       #include <stdlib.h>

       void qsort(void *base, size_t nmemb, size_t size,
		  int(*compar)(const void *, const void *));

DESCRIPTION

       The qsort() function sorts an array with nmemb elements of size size.  The base argument points to the start of the array.

       The  contents  of the array are sorted in ascending order according to a comparison function pointed to by compar, which is called with two
       arguments that point to the objects being compared.

       The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be  respec-
       tively less than, equal to, or greater than the second.	If two members compare as equal, their order in the sorted array is undefined.

RETURN VALUE

       The qsort() function returns no value.

CONFORMING TO

       SVr4, 4.3BSD, C89, C99.

NOTES

       Library	routines  suitable  for  use as the compar argument include alphasort(3) and versionsort(3).  To compare C strings, the comparison
       function can call strcmp(3), as shown in the example below.

EXAMPLE

       For one example of use, see the example under bsearch(3).

       Another example is the following program, which sorts the strings given in its command-line arguments:

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

       static int
       cmpstringp(const void *p1, const void *p2)
       {
	   /* The actual arguments to this function are "pointers to
	      pointers to char", but strcmp(3) arguments are "pointers
	      to char", hence the following cast plus dereference */

	   return strcmp(* (char * const *) p1, * (char * const *) p2);
       }

       int
       main(int argc, char *argv[])
       {
	   int j;

	   if (argc < 2) {
	    fprintf(stderr, "Usage: %s <string>...
", argv[0]);
	    exit(EXIT_FAILURE);
	   }

	   qsort(&argv[1], argc - 1, sizeof(argv[1]), cmpstringp);

	   for (j = 1; j < argc; j++)
	       puts(argv[j]);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO

       sort(1), alphasort(3), strcmp(3), versionsort(3)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

								    2009-09-15								  QSORT(3)