#include <stdio.h>

int prime(int a);
int main ()
{
    int n,p;
    printf("plaese enter n=");
    scanf("%d",&n);
    n=prime(n);
    if(p=0)
         printf("%d不是素数n",n);
    else
         printf("%d是素数n",n);
    return 0;
}

int prime(int a)
{
    int n,i,p;
    for(i=2;i<n;i++)
   
        if(n%i==0)
            break;
   
    if(i<n)
        p=0;
    else
        p=1;
    return n;
}

plaese enter n=5
-858993460是素数
Press any key to continue

编译的时候，编译器已经告诉你错误原因了

#include <stdio.h>
#include <stdbool.h>

bool isprime( unsigned n );

int main( void )
{
    printf( "please enter a positive integer: ");
    unsigned n;
    if( scanf("%u",&n) != 1 )
        return 1;

    printf( "%u %s a prime number.", n, isprime(n)?"is":"isn't" );
}

bool isprime( unsigned n )
{
    for( unsigned k=2; k*k<=n; ++k )
        if( n%k == 0 )
            return false;
    return n>1;
}