How does processes intract with unix kernel to perform read operation in C?

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Hello, I am working on a project and my project needs to utilize memory as much as possible. I got a confusion at this point. As everyone knows system memory is divided into user space and kernel space. I want to know when I call a read() system call or fread() then whether the kernel creates a buffer in kernel space and fetches the data to it and then transfers data to the buffer in user space or it directly provides a link to copy data to the buffer in user space. In another words, I can say whether a single copy is done or a double copy happens?

Thanks in advance.

Vinay Tiwary

Posted 27-Jan-14 18:21pm

Vinay Kumar Tiwary

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Richard MacCutchan · Answer 1 · 2014-01-27T22:59:00

Solution 1

Most drivers read blocks of fixed size from the device and then transfer the specific parts of the requested data to the user. In some cases the driver will need to do multiple reads to get the correct portions of the file. The driver uses kernel memory for the read from (or write to) the device.

Posted 27-Jan-14 22:59pm

Richard MacCutchan

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Vinay Kumar Tiwary 28-Jan-14 5:57am

Hi Richard thanks for reply. You mean to say, whenever we perform a read operation, first data is transferred to kernel memory and then the kernel transfers it to user space. Is it the same case always or it may differ in different OS?

Richard MacCutchan 28-Jan-14 7:29am

As far as I know that is the way it works in most operating systems. However I am sure you could ask Google for further information.

Vinay Kumar Tiwary 28-Jan-14 7:42am

hahaha.. I already asked Google before posting question over here but couldn't find a satisfactory answer.