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POPEN(3) NetBSD Library Functions Manual POPEN(3)
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popen, popenve, pclose -- process I/O
Standard C Library (libc, -lc)
popen(const char *command, const char *type);
popenve(const char *path, char * const *argv, char * const *envp,
const char *type);
The popen() function ``opens'' a process by creating an IPC connection,
forking, and invoking the shell. Historically, popen() was implemented
with a unidirectional pipe; hence many implementations of popen() only
allow the type argument to specify reading or writing, not both. Since
popen() is now implemented using sockets, the type may request a bidirec-
tional data flow. The type argument is a pointer to a null-terminated
string which must be `r' for reading, `w' for writing, or `r+' for read-
ing and writing. In addition if the character `e' is present in the type
string, the file descriptor used internally is set to be closed on
The command argument is a pointer to a null-terminated string containing
a shell command line. This command is passed to /bin/sh using the -c
flag; interpretation, if any, is performed by the shell.
The popenve() function is similar to popen() but the first three argu-
ments are passed to execve(2) and there is no shell involved in the com-
The return value from popen() and popenve() is a normal standard I/O
stream in all respects save that it must be closed with pclose() rather
than 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 popen(), unless this is altered by the command
itself. Conversely, reading from a ``popened'' stream reads the com-
mand's standard output, and the command's standard input is the same as
that of the process that called popen().
Note that output popen() streams are fully buffered by default.
The pclose() function waits for the associated process to terminate and
returns the exit status of the command as returned by wait4().
The popen() function returns NULL if the vfork(2), pipe(2), or
socketpair(2) calls fail, or if it cannot allocate memory, preserving the
errno from those functions.
The pclose() function returns -1 if stream is not associated with a
``popened'' command, if stream has already been ``pclosed'', setting
errno to ESRCH or if wait4(2) returns an error, preserving the errno
returned by wait4(2).
sh(1), execve(2), fork(2), pipe(2), socketpair(2), wait4(2), fclose(3),
fflush(3), fopen(3), shquote(3), stdio(3), system(3)
The popen() and pclose() functions conform to IEEE Std 1003.2-1992
A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
Since the standard input of a command opened for reading shares its seek
offset with the process that called 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 fflush(3) before popen().
Failure to execute the shell is indistinguishable from the shell's fail-
ure to execute command, or an immediate exit of the command. The only
hint is an exit status of 127.
The popen() argument always calls sh(1), never calls csh(1).
The popenve() function first appeared in NetBSD 8.
NetBSD 9.99 January 19, 2015 NetBSD 9.99