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Test Your Knowledge in Computers #555
Difficulty: Easy
In C, if double b = 3.14159265359; printf("b=%10f ", b); then the output with be b=3.141593
True or False?
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pipe(2) [freebsd man page]

PIPE(2) 						      BSD System Calls Manual							   PIPE(2)

NAME
pipe, pipe2 -- create descriptor pair for interprocess communication LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> int pipe(int fildes[2]); int pipe2(int fildes[2], int flags); DESCRIPTION
The pipe() system call creates a pipe, which is an object allowing bidirectional data flow, and allocates a pair of file descriptors. The pipe2() system call allows control over the attributes of the file descriptors via the flags argument. Values for flags are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>: O_CLOEXEC Set the close-on-exec flag for the new file descriptors. O_NONBLOCK Set the non-blocking flag for the ends of the pipe. If the flags argument is 0, the behavior is identical to a call to pipe(). By convention, the first descriptor is normally used as the read end of the pipe, and the second is normally the write end, so that data written to fildes[1] appears on (i.e., can be read from) fildes[0]. This allows the output of one program to be sent to another program: the source's standard output is set up to be the write end of the pipe, and the sink's standard input is set up to be the read end of the pipe. The pipe itself persists until all its associated descriptors are closed. A pipe that has had an end closed is considered widowed. Writing on such a pipe causes the writing process to receive a SIGPIPE signal. Widowing a pipe is the only way to deliver end-of-file to a reader: after the reader consumes any buffered data, reading a widowed pipe returns a zero count. The bidirectional nature of this implementation of pipes is not portable to older systems, so it is recommended to use the convention for using the endpoints in the traditional manner when using a pipe in one direction. RETURN VALUES
The pipe() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The pipe() and pipe2() system calls will fail if: [EMFILE] Too many descriptors are active. [ENFILE] The system file table is full. [ENOMEM] Not enough kernel memory to establish a pipe. The pipe2() system call will also fail if: [EINVAL] The flags argument is invalid. SEE ALSO
sh(1), fork(2), read(2), socketpair(2), write(2) HISTORY
The pipe() function appeared in Version 3 AT&T UNIX. Bidirectional pipes were first used on AT&T System V Release 4 UNIX. The pipe2() function appeared in FreeBSD 10.0. BSD
May 1, 2013 BSD

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PIPE(2) 						     Linux Programmer's Manual							   PIPE(2)

NAME
pipe, pipe2 - create pipe SYNOPSIS
#include <unistd.h> int pipe(int pipefd[2]); #define _GNU_SOURCE #include <unistd.h> int pipe2(int pipefd[2], int flags); DESCRIPTION
pipe() creates a pipe, a unidirectional data channel that can be used for interprocess communication. The array pipefd is used to return two file descriptors referring to the ends of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe. For fur- ther details, see pipe(7). If flags is 0, then pipe2() is the same as pipe(). The following values can be bitwise ORed in flags to obtain different behavior: O_NONBLOCK Set the O_NONBLOCK file status flag on the two new open file descriptions. Using this flag saves extra calls to fcntl(2) to achieve the same result. O_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the two new file descriptors. See the description of the same flag in open(2) for reasons why this may be useful. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EFAULT pipefd is not valid. EINVAL (pipe2()) Invalid value in flags. EMFILE Too many file descriptors are in use by the process. ENFILE The system limit on the total number of open files has been reached. VERSIONS
pipe2() was added to Linux in version 2.6.27; glibc support is available starting with version 2.9. CONFORMING TO
pipe(): POSIX.1-2001. pipe2() is Linux-specific. EXAMPLE
The following program creates a pipe, and then fork(2)s to create a child process; the child inherits a duplicate set of file descriptors that refer to the same pipe. After the fork(2), each process closes the descriptors that it doesn't need for the pipe (see pipe(7)). The parent then writes the string contained in the program's command-line argument to the pipe, and the child reads this string a byte at a time from the pipe and echoes it on standard output. #include <sys/wait.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> int main(int argc, char *argv[]) { int pipefd[2]; pid_t cpid; char buf; if (argc != 2) { fprintf(stderr, "Usage: %s <string> ", argv[0]); exit(EXIT_FAILURE); } if (pipe(pipefd) == -1) { perror("pipe"); exit(EXIT_FAILURE); } cpid = fork(); if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); } if (cpid == 0) { /* Child reads from pipe */ close(pipefd[1]); /* Close unused write end */ while (read(pipefd[0], &buf, 1) > 0) write(STDOUT_FILENO, &buf, 1); write(STDOUT_FILENO, " ", 1); close(pipefd[0]); _exit(EXIT_SUCCESS); } else { /* Parent writes argv[1] to pipe */ close(pipefd[0]); /* Close unused read end */ write(pipefd[1], argv[1], strlen(argv[1])); close(pipefd[1]); /* Reader will see EOF */ wait(NULL); /* Wait for child */ exit(EXIT_SUCCESS); } } SEE ALSO
fork(2), read(2), socketpair(2), write(2), popen(3), pipe(7) COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2009-09-15 PIPE(2)

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