Question

#include<stdlib.h>
#include<string.h>
#include<stdio.h>
int pstrcmp( char **p,char **q)
{  return strcmp(*p,*q) ;
}
int comlen(char *p,char *q)
{
int i=0;
while(*p && (*p++=*q++))
 i++;
 return i;

}
#define M 1
#define MAXN 5000000
char c[MAXN],*a[MAXN];
int main()
{
int i,ch,n=0,maxi,maxlen=-1;
while((ch=getchar())!=EOF){
a[n]=&c[n];
c[n++]=ch;


}
c[n]=0;
qsort(a,n,sizeof(char *),pstrcmp);
for(i=0;i<n-M;i++)
if(comlen(a[i],a[i+M])>maxlen){
maxlen=comlen(a[i],a[i+M]);
maxi=i;


}
printf("%.*s\n",maxlen,a[maxi]);
return 0;
}

in this code compiler shows me error

Error   1   error C2664: 'qsort' : cannot convert parameter 4 from 'int (__cdecl *)(char **,char **)' to 'int (__cdecl *)(const void *,const void *)'   d:\fe\longest_duplicated\longest_duplicated\longest_duplicated.cpp  33  longest_duplicated

i know that,have to convert from void type to char type,but how to do it dont know and please help me

Answer 1

Change your function to

int pstrcmp(const void* p, const void* q)
{
    return strcmp(*(const char**)p, *(const char**)q);
}

and the error should be gone. qsort expects a int(*compar)(const void *, const void *) as 4th parameter while you are passing it a function that takes two char** arguments.

P.S.: I did just analyse the error message, not your program, since it is so badly formatted. Maybe improving the formatting would help here.

UPDATE: You are not creating strings, you are storing pointers in a that point into some location in c. A string needs to be terminated with a 0 in order to work. Although the whole design of your program looks strange, you could do

a[n]=&c[2*n];
c[2*n]=ch;
c[2*n+1]=0;
n++;