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HP-UX 11i Version 3: February 2007

Technical documentation

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zero — /dev/zero special file


/dev/zero is a zero special file. Reads from a zero special file always return characters whose value is 0 (\0 characters).

Data written on a zero special file is discarded or ignored.

Seeks on a zero special file always succeed.

When /dev/zero is memory mapped by calling mmap(), the associated memory object behaves as a MAP_ANONYMOUS object. It is initialized to all zeros. Writes to the object modify the contents of the object which are observed by subsequent reads to this object.

Both MAP_SHARED and MAP_PRIVATE mmap() are allowed.

When it is mapped shared, the memory object can be shared only with the descendants of the current process. Modifications made to the MAP_SHARED object are visible only to the process and its descendants.

When it is mapped private, any modifications done after fork() are visible only to the process.


In the following example, the buffer buf is filled with len \0 characters.

fildes = open("/dev/zero",...) read(fildes, buf, len)

In the following example, the process now has a range of len \0 characters at memory location address:

fildes = open("/dev/zero",...) address = mmap(0, len, PROT_READ | PROT_WRITE, MAP_PRIVATE, fildes, any_offset)




mmap(2), null(7).

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