signed to unsigned conversion in C - is it always safe?

unsigned int u = 1234;
int i = -5678;

unsigned int result = u + i;

#include <stdio.h>

int main(void)
{
    unsigned int u = 1;
    int i = -1;

    if(u < i)
        printf("1 < -1");
    else
        printf("boring");

    return 0;
}

unsigned int umax = -1; // umax set to UINT_MAX

int iValue, iResult;
unsigned int uValue, uResult;

unsigned int u = 1234;
int i = -5678;

unsigned int result = u + i;