Why is C++ so much faster than C in this code? - gcc

Question

My C code is:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void){
char* a = (char*)malloc(200000);
for (int i = 0;i< 100000;i++){
strcat(a,"b");
}
printf("%s",a);
}
My C++ code is
#include <iostream>
int main(void){
std::string a = "";
for (int i = 0;i< 100000;i++){
¦ a+="b";
}
std::cout<<a;
}
On my machine, the C code runs in about 5 seconds, while on my machine, the C++ code runs in 0.025! seconds.
Now, the C code doesn't check for overflows, has no C++ overhead, classes and yet is quite a few magnitudes slower than my C++ code.
Using gcc/g++ 6.2.0 compiled with -O3 on Raspberry Pi.

Answer 1

#erwin was correct.
When I change my code to
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void mystrcat(char* src,char* dest,int lenSrc){
src[lenSrc]=dest[0];
}
int main(void){
char* a = (char*)malloc(200000);
for (int i = 0;i< 100000;i++){
mystrcat(a,"b",i);
}
a[100000] = 0;
printf("%s\n",a);
}
It takes about .012s to run (mostly printing the large screen).
Shlemiel's the painter's algorithm at work!