C :

#include<stdio.h>
#define N 80 
int appendfile (const char *srcName,const char *dstName);
int main()  
{
char srcFilename[N]	;
char dstFilename[N];
scanf("%s",srcFilename );
scanf("%s",dstFilename);
if(appendfile(srcFilename,dstFilename)) printf("Append succeed!\n");
else 
printf("Append failed!\n");
return 0;
}
int appendfile (const char *srcName,const char *dstName)
{
	
	FILE *fpSrc=NULL,*fpDst=NULL;
	int ch,rval=1;
	if((fpSrc=fopen(srcName,"r"))==NULL) goto ERROR;
	if((fpDst=fopen(dstName,"w"))==NULL) goto ERROR;
	while((ch=fgetc(fpSrc))!=EOF)
	{
	if(fputc(ch,fpDst)==EOF) goto ERROR;	
	}
	fflush(fpDst);
	goto EXIT;
	ERROR:
		rval=0;
		EXIT:
			if(fpSrc!=NULL)fclose(fpSrc);
			if(fpDst!=NULL)fclose(fpDst);
			return rval;
}

C++ :

#include<cstdio>
#include<iostream>
using namespace std;
int main()
{
    char a[100]={},b[100]={};
    cin >> a >> b;
    FILE *fa,*fb;
    if ((fa = fopen(a,"r")) == NULL)
    {
        printf("Append failed!");
        return 0;
    }
    if ((fb = fopen(b,"a")) == NULL)
    {
        printf("Append failed!");
        return 0;
    }
    char tmp;
    while (!feof(fa))
    {
        tmp = fgetc(fa);
        fputc(tmp, fb);
    }
    cout << "Append succeed!";
    return 0;
}