In order for an operation to be considered ``atomic'', it must not be interrupted for any reason at all. The entire operation occurs at once. The POSIX standard dictates in /usr/include/posix1_lim.h that the maximum buffer size for an atomic operation on a pipe is:
#define _POSIX_PIPE_BUF 512
Up to 512 bytes can be written or retrieved from a pipe atomically. Anything that crosses this threshold will be split, and not atomic. Under Linux, however, the atomic operational limit is defined in ``linux/limits.h'' as:
#define PIPE_BUF 4096
As you can see, Linux accommodates the minimum number of bytes required by POSIX, quite considerably I might add. The atomicity of a pipe operation becomes important when more than one process is involved (FIFOS). For example, if the number of bytes written to a pipe exceeds the atomic limit for a single operation, and multiple processes are writing to the pipe, the data will be ``interleaved'' or ``chunked''. In other words, one process may insert data into the pipeline between the writes of another.