SIGPIPE

On POSIX-compliant platforms, SIGPIPE is the signal sent to a process when it attempts to write to a pipe without a process connected to the other end. The symbolic constant for SIGPIPE is defined in the header file signal.h. Symbolic signal names are used because signal numbers can vary across platforms.

Etymology

SIG is a common prefix for signal names. PIPE refers to the Unix pipe.

Description

Unix supports the principle of piping, which allows processes to send data to other processes without the need for creating temporary files. When a pipe is broken, the process writing to it is sent the SIGPIPE signal. The default reaction to this signal for a process is to terminate.

A simple example of piping is the following.

ps -e | head

This command, when run on a Unix-like machine (including Linux), returns a list of processes, limited to ten lines.

• ps -e returns a list of all processes (including those of other users).
• head selects the first ten lines.

When ps has written ten lines, head has received all it needs and exits. ps will receive a SIGPIPE when it tries to write the remaining lines, causing it to terminate as well: It is no use writing data that no one will use. Hence it is possible that the reading process terminates while writing the data. This will cause SIGPIPE to be sent to the writing process.

One can ignore SIGPIPE (using, for example, the signal system call). In this case, all system calls that would cause SIGPIPE to be sent will return -1 and set errno to EPIPE.