include <sldio. h>

int main（）
{void cpy(char C ],char [ ]);
char str[80],cC[80];
printf(" input string:");
gets(str);
cpy(str,c);
printf( The vowel letters are:%s\n",c);
return 0;}
void cpy(char s[ ],char c[ ])
{ int i,j;
for (i=0,j=0;s[i]!= '\0';i++)
if (s[i]= ='l|s[i]= ='A'lls[i]≈='ells[i]= ='E' |s[i]= ='1l s[i]= ='lIs[i]=='o'1ls[i]=='lls[i]=='l|s[i]=='U')
{c[j]=s[i];
j++;}
c[i]='\0';}

#include<stdio.h>
int main()
{ 
    void cpy(char s[], char c[]);
    char str[80], c[80];
    printf(" input string:");
    gets(str);
    cpy(str, c);
    printf("The vowel letters are : % s\n",c);
    return 0;
}
void cpy(char s[], char c[])
{
    int i, j;
    for (i = 0, j = 0; s[i] != '\0'; i++)
        if (s[i] == 'a'||s[i]=='A'||s[i]=='e'|s[i] == 'E' || s[i] == 'i'||s[i]=='I'||s[i] == 'o'||s[i] == 'O'||s[i]=='u'|| s[i] == 'U')
        {
            c[j] = s[i];
            j++;
        }
    c[j] = '\0';
}

修改如下，供参考：

#include <stdio.h>
#include <string.h>
int main()
{
    void cpy(char [],char []);
    char str[80],c[80];
    printf(" input string:");
    gets(str);
    cpy(str,c);
    printf("The vowel letters are:%s ,%d\n",c,strlen(c));
    return 0;
}
void cpy(char s[],char c[])
{
    int i,j;
    for (i=0,j=0;s[i]!= '\0';i++)
        if (s[i]=='a' || s[i]=='A' ||
            s[i]=='e' || s[i]=='E' ||
            s[i]=='i' || s[i]=='I' ||
            s[i]=='o' || s[i]=='O' ||
            s[i]=='u' || s[i]=='U')
        {
           c[j]=s[i];
           j++;
        }
    c[j]='\0';
}