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

int main( void )
{
    char *basec;
    double base;

    basec = malloc( 10 * sizeof( char ) );

    printf( "Ingresar base: " );
    basec = fgets( basec, 10, stdin );

    if ( !strncmp( basec, "e", 1 ) )
        base = 2.718281;
    else
        base = atof( basec );

    printf( "%f\n", base );

    free( basec );

    return 0;
}

int main()
{
	void crear_fich();
	void deuda_mayor();
...

struct cliente cli;
...
cli[i].deuda
...

double deuda_mayor = 0.0;
...
cli[deuda_mayor].nombre, ...