Language: C

int main( void ) {

	int n, i, j, p, q, r, temp, carry, c[81];

	scanf( "%d", &n );

	for ( i = 0; i < n; i++ ) {

		scanf( "%s", a );
		scanf( "%s", b );

		p = strlen( a );
		q = strlen( b );

		if ( p > 80 || q > 80 ) {

			printf( "overflow\n" );

		} else {

			r = 0; carry = 0;

			while ( 1 ) {

				if ( r >= 80 ) {

					printf( "overflow\n" );
					break;

				} else if ( p <= 0 && q <= 0 ) {

					if ( carry == 1 ) { c[r] = 1; r++; }

					for ( j = r - 1; j >= 0; j-- ) {
					
						printf( "%d", c[j] );

					}

					printf( "\n" );

					break;

				} else {

					c[r] = carry;
					if ( p > 0 ) { c[r] += a[p-1] - '0'; p--; }
					if ( q > 0 ) { c[r] += b[q-1] - '0'; q--; }

					if ( c[r] / 10 == 0 ) { carry = 0; } else { c[r] %= 10; carry = 1; }
				
					r++;

				}

			}

		}

	}

	return 0;

}

Compiler output:
main.c: In function 'main':
main.c:5:9: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    5 |         scanf( "%d", &n );
      |         ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | int main( void ) {
main.c:5:9: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    5 |         scanf( "%d", &n );
      |         ^~~~~
main.c:5:9: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:9:30: error: 'a' undeclared (first use in this function)
    9 |                 scanf( "%s", a );
      |                              ^
main.c:9:30: note: each undeclared identifier is reported only once for each function it appears in
main.c:10:30: error: 'b' undeclared (first use in this function)
   10 |                 scanf( "%s", b );
      |                              ^
main.c:12:21: error: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
   12 |                 p = strlen( a );
      |                     ^~~~~~
main.c:1:1: note: include '<string.h>' or provide a declaration of 'strlen'
  +++ |+#include <string.h>
    1 | int main( void ) {
main.c:12:21: warning: incompatible implicit declaration of built-in function 'strlen' [-Wbuiltin-declaration-mismatch]
   12 |                 p = strlen( a );
      |                     ^~~~~~
main.c:12:21: note: include '<string.h>' or provide a declaration of 'strlen'
main.c:17:25: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   17 |                         printf( "overflow\n" );
      |                         ^~~~~~
main.c:17:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:17:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:17:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:27:41: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   27 |                                         printf( "overflow\n" );
      |                                         ^~~~~~
main.c:27:41: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:36:49: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   36 |                                                 printf( "%d", c[j] );
      |                                                 ^~~~~~
main.c:36:49: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:40:41: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   40 |                                         printf( "\n" );
      |                                         ^~~~~~
main.c:40:41: note: include '<stdio.h>' or provide a declaration of 'printf'