|POPEN(3)||Library Functions Manual||POPEN(3)|
— process I/O
char *command, const char
popen() function “opens”
a process by creating a pipe, forking, and invoking the shell. Since a pipe
is by definition unidirectional, the type argument may
specify only reading or writing, not both; the resulting stream is
correspondingly read-only or write-only.
The command argument is a pointer to a
NUL-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 type argument is a pointer to a
NUL-terminated string which must be either “r” or
“re” for reading or “w” or “we”
for writing. If the letter “e” is present in the string then
the close-on-exec flag shall be set on the file descriptor underlying the
FILE that is returned.
The return value from
popen() is a normal
standard I/O stream in all respects except that it must be closed with
pclose() rather than
fclose(3). 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
command's standard output, and the command's standard input is the same as
that of the process that called
popen() output streams are fully
buffered by default. In addition, fork handlers established using
pthread_atfork(3) are not called
when a multithreaded program calls
pclose() function waits for the
associated process to terminate and returns the exit status of the command
as returned by wait4(2).
pclose() function returns -1 if
stream is not associated with a
“popened” command, if stream already
“pclosed”, or if wait4(2)
returns an error.
popen() function does not reliably set
popen() and a
pclose() function appeared in
Version 7 AT&T UNIX.
Never supply the
popen() function with a
command containing any part of an unsanitized user-supplied string. Shell
meta-characters present will be honored by the
sh(1) command interpreter.
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
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.
|February 5, 2016||OpenBSD-current|