popen(3) - NetBSD Manual Pages

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POPEN(3)                NetBSD Library Functions Manual               POPEN(3)


NAME

     popen, popenve, pclose -- process I/O


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdio.h>

     FILE *
     popen(const char *command, const char *type);

     FILE *
     popenve(const char *path, char * const *argv, char * const *envp,
         const char *type);

     int
     pclose(FILE *stream);


DESCRIPTION

     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
     exec(3).

     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-
     mand invocation.

     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(2).


RETURN VALUES

     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).


SEE ALSO

     sh(1), execve(2), fork(2), pipe(2), socketpair(2), vfork(2), wait4(2),
     fclose(3), fflush(3), fopen(3), shquote(3), stdio(3), system(3)


STANDARDS

     The popen() and pclose() functions conform to IEEE Std 1003.2-1992
     (``POSIX.2'').


HISTORY

     A popen() and a pclose() function appeared in Version 7 AT&T UNIX.

     The popenve() function first appeared in NetBSD 8.


BUGS

     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 the 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).

NetBSD 10.1                   September 11, 2021                   NetBSD 10.1