c语言求字符串a从第m个字符起长度为k的子串

#include <stdio.h>
#include <string.h>
char *substr(char a[],int m,int k)
{
    char *p;
    if(m>strlen(a))
        return a+strlen(a);
    p=a+m-1;
    if(m+k-1<strlen(a))
        *(a+m+k-1)='\0';
    return p;
}
int main(int argc, const char * argv[]) {
    int m,k;
    char a[40];
    printf("input string:\n");
    scanf("%s",a);
    printf("input m,k:\n");
    scanf("%d%d",&m,&k);
    printf("the result is:%s\n",substr(a,m,k));
    return 0;
}

output:

input string:
spurs_ping
input m,k:
3 5
the result is:urs_p