'THE promise of America, " says the Hollywood A-lister Will Smith, "is such a great idea. Because this is the only country in the world in which Chris Gardner could exist."

Is that true? Gardner's story - a rags-toriches tale of a black homeless man who fights his way to a stockbroking fortune - is certainly a most American one. But, for much of The Pursuit of Happyness, Smith's Oscar-tipped interpretation of Gardner's life, Reagan's America of the early 1980s seems like a broken promise - by turns greedy and unforgiving. Gardner's story is a moving parable of that time.

Chris Gardner was born in Milwaukee in 1954. Enduring a childhood in and out of foster care, he suffered abuse at the hands of his stepfather, saw his mother imprisoned for welfare fraud, and was raped by a neighbour. But Gardner was bright, and after graduating at the top of his (albeit small) class, he joined the navy, where he started to train to become a medic. But he never made it to medical school. Instead, he decided to sell medical equipment in San Francisco - in particular, a bone density scanner that few doctors wanted to buy.

Just after the birth of his first son, and with the bone density scanner breaking him financially, Gardner had his Damascene moment. He saw an impeccably dressed man parking his red Ferrari. Gardner asked the man two questions: "What do you do?" and "How do you do that?" The man was a stockbroker, earning $80,000 a month.

Gardner applied for internships with brokers all over the city, and was accepted as a trainee at a minor firm, only to be sacked after a few days when the man who hired him was himself sacked. Gardner's money problems were escalating. He was evicted from his apartment. The Internal Revenue Service was demanding tax. He was jailed for 10 days for non-payment of parking fines totalling $1,200, at which point his girlfriend left with his son, Chris junior. But Gardner was adamant that his son should stay with him, and he fought to gain custody.

He was tenacious, too, in his goal of becoming a stockbroker. He continued to apply for internships, and arrived at his last interview for Dean Witter stockbrokers fresh out of prison. Despite his grubby appearance, and confessing the full scale of his miseries, he won a place. One of the senior brokers, it turned out, had recently been through a messy divorce, and admired Gardner's honesty.

He had his break, but he still had no money and no home. To pay for food he gave blood. At night, he and his son slept in parks, boarding houses, and even the bathrooms of a train station, where he'd bathe Chris junior in a sink. Eventually, they found refuge at a Methodist church shelter.

All the while, Gardner worked hard to impress at Dean Witter, becoming the only trainee in his intake to be taken on fulltime. In 1983, he left to work at Bear Stearns and, in 1987, set up his own institutional broker's firm, Gardner Rich & Co Inc in Chicago. He also bought Michael Jordan's Ferrari.

Until two years ago, when Gardner was invited to talk on a US television show called 20/20, he had rarely discussed his struggles. But as soon as his story became public, his phone rang non-stop. A book deal was negotiated. Public appearances were arranged. And, naturally, Hollywood was interested.

"Immediately I said, 'My God, this is a fantastic story', " recalls Todd Black, one of the film's producers. "It's like Rocky - a guy who is completely beaten down and goes on to succeed."

The Pursuit of Happyness differs somewhat from Gardner's experiences. Gardner's infant son becomes a talking, emotive five-year-old (played by Smith's own son, Jaden). Other characters were created who were amalgams of various people, including the mother of his young son.

Gardner is relaxed about this. "I told [Steve Conrad, the scriptwriter] my story, " he recalls, "and he decided which elements could be used. Steve was very clear with me that he was a dramatist, not a biographer. . . I was very happy with the story. It captured the essence of the struggle."

He could surely have never anticipated the response. Gardner's "unique black story" has been championed by the Rev Al Sharpton on prime-time television. His story has produced tears and laughter on Oprah. He has been given standing ovations on aeroplanes. "I've never seen anything like this in my life, " he says. "It reminds me of nothing other than the final days of a presidential campaign."

America really is the only country in which Chris Gardner could exist. Gardner fought his way out of trouble by proving that he could make a lot of rich men even richer at Dean Witter. And he has since proved, through his life story, that he can make another bunch of moneyed Californians more greenbacks still. He has also, through his wealth, evolved from grateful recipient into benevolent provider, and has invested heavily in homeless charities and other philanthropic projects.

Gardner, it seems, has had his eyes open the entire time. As an example of what makes America tick, he recalls one client who he had solicited by phone in his early days at Dean Witter. The businessman had assumed that Gardner was white, and had made racist jokes. Gardner said nothing. At the end of the call, the client said: "Buy me 50,000 shares of whatever you called me about." When they eventually met, the client didn't bat an eyelid. In fact, he gave Gardner his whole business.

"That's when I learned, " said Gardner, "that in this business, it's not a black thing.

It's not a white thing. It's a green thing."

The Pursuit of Happyness is on general release