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@(#)popen.3 8.2 (Berkeley) 5/3/95
$FreeBSD: head/lib/libc/gen/popen.3 84306 2001-10-01 16:09:29Z ru $
.Dd May 3, 1995 .Dt POPEN 3 .Os .Sh NAME .Nm popen , .Nm pclose .Nd process .Tn I/O .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdio.h .Ft FILE * .Fn popen "const char *command" "const char *type" .Ft int .Fn pclose "FILE *stream" .Sh DESCRIPTION The .Fn popen function .Dq opens a process by creating a bidirectional pipe forking, and invoking the shell. Any streams opened by previous .Fn popen calls in the parent process are closed in the new child process. Historically, .Fn popen was implemented with a unidirectional pipe; hence many implementations of .Fn popen only allow the .Fa type argument to specify reading or writing, not both. Since .Fn popen is now implemented using a bidirectional pipe, the .Fa type argument may request a bidirectional data flow. The .Fa type argument is a pointer to a null-terminated string which must be .Ql r for reading, .Ql w for writing, or .Ql r+ for reading and writing.
p The .Fa command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to
a /bin/sh using the .Fl c flag; interpretation, if any, is performed by the shell.
p The return value from .Fn popen is a normal standard .Tn I/O stream in all respects save that it must be closed with .Fn pclose rather than .Fn fclose . Writing to such a stream writes to the standard input of the command; the command's standard output is the same as that of the process that called .Fn popen , unless this is altered by the command itself. Conversely, reading from a .Dq popened stream reads the command's standard output, and the command's standard input is the same as that of the process that called .Fn popen .
p Note that output .Fn popen streams are fully buffered by default.
p The .Fn pclose function waits for the associated process to terminate and returns the exit status of the command as returned by .Fn wait4 . .Sh RETURN VALUES The .Fn popen function returns .Dv NULL if the .Xr fork 2 or .Xr pipe 2 calls fail, or if it cannot allocate memory.
p The .Fn pclose function returns -1 if .Fa stream is not associated with a .Dq popened command, if .Fa stream already .Dq pclosed , or if .Xr wait4 returns an error. .Sh ERRORS The .Fn popen function does not reliably set .Va errno . .Sh SEE ALSO .Xr sh 1 , .Xr fork 2 , .Xr pipe 2 , .Xr wait4 2 , .Xr fclose 3 , .Xr fflush 3 , .Xr fopen 3 , .Xr stdio 3 , .Xr system 3 .Sh BUGS Since the standard input of a command opened for reading shares its seek offset with the process that called .Fn popen , if the original process has done a buffered read, the command's input position may not be as expected. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoided by calling .Xr fflush 3 before .Fn popen .
p Failure to execute the shell is indistinguishable from the shell's failure to execute command, or an immediate exit of the command. The only hint is an exit status of 127.
p The .Fn popen argument always calls .Xr sh 1 , never calls .Xr csh 1 . .Sh HISTORY A .Fn popen and a .Fn pclose function appeared in .At v7 .
p Bidirectional functionality was added in .Fx 2.2.6 .