.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved .TH "POPEN" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" .\" popen .SH NAME popen \- initiate pipe streams to or from a process .SH SYNOPSIS .LP \fB#include .br .sp FILE *popen(const char *\fP\fIcommand\fP\fB, const char *\fP\fImode\fP\fB); \fP \fB .br \fP .SH DESCRIPTION .LP The \fIpopen\fP() function shall execute the command specified by the string \fIcommand\fP. It shall create a pipe between the calling program and the executed command, and shall return a pointer to a stream that can be used to either read from or write to the pipe. .LP The environment of the executed command shall be as if a child process were created within the \fIpopen\fP() call using the \fIfork\fP() function, and the child invoked the \fIsh\fP utility using the call: .sp .RS .nf \fBexecl(\fP\fIshell path\fP\fB, "sh", "-c",\fP \fIcommand\fP\fB, (char *)0); \fP .fi .RE .LP where \fIshell path\fP is an unspecified pathname for the \fIsh\fP utility. .LP The \fIpopen\fP() function shall ensure that any streams from previous \fIpopen\fP() calls that remain open in the parent process are closed in the new child process. .LP The \fImode\fP argument to \fIpopen\fP() is a string that specifies I/O mode: .IP " 1." 4 If \fImode\fP is \fIr\fP, when the child process is started, its file descriptor STDOUT_FILENO shall be the writable end of the pipe, and the file descriptor \fIfileno\fP(\fIstream\fP) in the calling process, where \fIstream\fP is the stream pointer returned by \fIpopen\fP(), shall be the readable end of the pipe. .LP .IP " 2." 4 If \fImode\fP is \fIw\fP, when the child process is started its file descriptor STDIN_FILENO shall be the readable end of the pipe, and the file descriptor \fIfileno\fP(\fIstream\fP) in the calling process, where \fIstream\fP is the stream pointer returned by \fIpopen\fP(), shall be the writable end of the pipe. .LP .IP " 3." 4 If \fImode\fP is any other value, the result is undefined. .LP .LP After \fIpopen\fP(), both the parent and the child process shall be capable of executing independently before either terminates. .LP Pipe streams are byte-oriented. .SH RETURN VALUE .LP Upon successful completion, \fIpopen\fP() shall return a pointer to an open stream that can be used to read or write to the pipe. Otherwise, it shall return a null pointer and may set \fIerrno\fP to indicate the error. .SH ERRORS .LP The \fIpopen\fP() function may fail if: .TP 7 .B EMFILE {FOPEN_MAX} or {STREAM_MAX} streams are currently open in the calling process. .TP 7 .B EINVAL The \fImode\fP argument is invalid. .sp .LP The \fIpopen\fP() function may also set \fIerrno\fP values as described by \fIfork\fP() or \fIpipe\fP() . .LP \fIThe following sections are informative.\fP .SH EXAMPLES .LP None. .SH APPLICATION USAGE .LP Since open files are shared, a mode \fIr\fP command can be used as an input filter and a mode \fIw\fP command as an output filter. .LP Buffered reading before opening an input filter may leave the standard input of that filter mispositioned. Similar problems with an output filter may be prevented by careful buffer flushing; for example, with \fIfflush\fP() . .LP A stream opened by \fIpopen\fP() should be closed by \fIpclose\fP(). .LP The behavior of \fIpopen\fP() is specified for values of \fImode\fP of \fIr\fP and \fIw\fP. Other modes such as \fIrb\fP and \fIwb\fP might be supported by specific implementations, but these would not be portable features. Note that historical implementations of \fIpopen\fP() only check to see if the first character of \fImode\fP is \fIr\fP. Thus, a \fImode\fP of \fIrobert the robot\fP would be treated as \fImode\fP \fIr\fP, and a \fImode\fP of \fIanything else\fP would be treated as \fImode\fP \fIw\fP. .LP If the application calls \fIwaitpid\fP() or \fIwaitid\fP() with a \fIpid\fP argument greater than 0, and it still has a stream that was called with \fIpopen\fP() open, it must ensure that \fIpid\fP does not refer to the process started by \fIpopen\fP(). .LP To determine whether or not the environment specified in the Shell and Utilities volume of IEEE\ Std\ 1003.1-2001 is present, use the function call: .sp .RS .nf \fBsysconf(_SC_2_VERSION) \fP .fi .RE .LP (See \fIsysconf\fP() ). .SH RATIONALE .LP The \fIpopen\fP() function should not be used by programs that have set user (or group) ID privileges. The \fIfork\fP() and \fIexec\fP family of functions (except \fIexeclp\fP() and \fIexecvp\fP()), should be used instead. This prevents any unforeseen manipulation of the environment of the user that could cause execution of commands not anticipated by the calling program. .LP If the original and \fIpopen\fP()ed processes both intend to read or write or read and write a common file, and either will be using FILE-type C functions ( \fIfread\fP(), \fIfwrite\fP(), and so on), the rules for sharing file handles must be observed (see \fIInteraction of File Descriptors and Standard I/O Streams\fP ). .SH FUTURE DIRECTIONS .LP None. .SH SEE ALSO .LP \fIpclose\fP() , \fIpipe\fP() , \fIsysconf\fP() , \fIsystem\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI\fP, the Shell and Utilities volume of IEEE\ Std\ 1003.1-2001, \fIsh\fP .SH COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .