【提问】c报错 in function int main()

in function int main() 未定义strupr
怎么解决
#include<stdio.h>
#include<math.h>
#include<string.h>
int main()
{
char str[5];
int a=0,i,sum=0;
for (i=0;i<4;i++)
{
scanf("%c",&str[i]);
}
str[i]='\0';
printf("\n");
for (i=0;i<4;i++)
{
if ( str[i]>='0'&&str[i]<='9'||str[i]>=97&&str[i]<=102||str[i]>=65&&str[i]<=70);
else
printf("Original data error");
break;
}
if (i==4)
{
printf("The dec data is:");
strupr(str);
for (i=0;i<4;i++)
{
if (str[i]>64)
{
a=str[i]-55;
}
else
{
a=str[i]-48;
}
sum+=a*pow(16,3-i);
}
printf("%d",sum);
}
return 0;
}