如何利用C程序有规律的合并两个文本文件！

#include <stdio.h>
#include <stdbool.h>
#include <string.h>

bool insert_line_to( FILE* src, FILE* dst )
{
    for( bool bwrote=false; ; )
    {
        char buf[1024+1]; // 行缓冲，确保其不小于2
        char* p = fgets( buf, sizeof buf, src );
        if( !p )
        {
            if( bwrote )
            {
                fputc( '\n', dst );
                return true;
            }
            return false;
        }

        fputs( buf, dst );
        bwrote = true;

        if( buf[strlen(buf)-1] == '\n' )
            return true;
    }
}

size_t insert_lines_to( FILE* src, FILE* dst, size_t count )
{
    for( size_t r=0; r!=count; ++r )
        if( !insert_line_to(src,dst) )
            return r;
    return count;
}

int main( void )
{
    FILE* fa = fopen( "a.txt", "r" );
    FILE* fb = fopen( "b.txt", "r" );
    FILE* fc = fopen( "c.txt", "w" );

    for( ; ; )
    {
        size_t a = insert_lines_to( fa, fc, 1 );
        size_t b = insert_lines_to( fb, fc, 5 );
        if( a!=1 && b!=5 )
            break;
    }

    fclose( fc );
    fclose( fb );
    fclose( fa );
}