POPEN(3) Library Routines POPEN(3)
popen, pclose - process I/O
FILE *popen (const char *command, const char *type);
int pclose (FILE *stream);
The 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 null-terminated string contain-
ing a shell command line. This command is passed to /bin/sh using the
-c flag; interpretation, if any, is performed by the shell. The mode
argument is a pointer to a null-terminated string which must be either
r for reading or w for writing.
The return value from popen 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. Con-
versely, 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.
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 waitpid.
The popen function returns NULL if the fork(2) or pipe(2) calls fail,
or if it cannot allocate memory.
The pclose function returns -1 if stream is not associated with a
popened command, if stream already pclosed, or if waitpid returns an
The popen function does not reliably set errno.
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
failure 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). (Because of
a lack of available sh(1), the current GNO implementation always calls
This implementation makes use of waitpid(2). Because waitpid is not
currently provided by the GNO kernel, pclose is affected by the same
bug as is waitpid; other child processes' exit status may be caught
(and discarded) while waiting for the process created by popen. Conse-
quently, if the parent processes is doing any other waitpid calls
(either directly or indirectly), pclose may never return.
Under GNO, the memory region pointed to by command must be able to be
referenced up to the point that the child process does its execl(2).
This implies that if command is allocated on the stack and the function
which allocated it returns before the program calls pclose (or if the
parent exits and makes its child into a zombie), there is a chance that
the forked child of popen may try to execl an arbitrary command, the
results of which are undefined.
gsh(1) appears to hang on occasion when executing popen's arguments.
The symptom is that the pclose never returns, and the process table
(see ps(1)) shows that the forked gsh's user time is forever increas-
gsh(1), sh(1), fork(2), pipe(2), waitpid(2), fclose(3), fflush(3),
fopen(3), stdio(3), system(3)
A popen and a pclose function appeared in Version 7 AT&T UNIX.
GNO 23 February 1997 POPEN(3)
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