SIGPIPE
Encyclopedia
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
.
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.
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.
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
-compliant platforms, SIGPIPE is the signal
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
sent to a process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
when it attempts to write to a pipe
Pipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
without a process connected to the other end. The symbolic constant
C preprocessor
The C preprocessor is the preprocessor for the C and C++ computer programming languages. The preprocessor handles directives for source file inclusion , macro definitions , and conditional inclusion ....
for SIGPIPE is defined in the header file
Header file
Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...
signal.hSignal.hsignal.h is a header file defined in the C Standard Library to specify how a program handles signals while it executes. A signal can report some exceptional behavior within the program , or a signal can report some asynchronous event outside the program .A signal can be generated...
. 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 pipePipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
.
Description
UnixUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
supports the principle of piping
Pipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
, 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
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
machine (including Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
), returns a list of processes, limited to ten lines.
- psPs (Unix)In most Unix-like operating systems, the ps program displays the currently-running processes. A related Unix utility named top provides a real-time view of the running processes....
-e returns a list of all processes (including those of other users). - headHead (Unix)head is a program on Unix and Unix-like systems used to display the first few lines of a text file or piped data. The command syntax is: head [options] <file_name>...
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
Sigaction (Unix)
In computing, sigaction is a function API defined by POSIX to give the programmer access to what should be a program's behavior when receiving specific OS signals.- General :...
system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
). In this case, all system calls that would cause SIGPIPE to be sent will return -1 and set errno to EPIPE.