POPEN(3)

POPEN(3) | 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 bidirectional 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 reading 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 arguments are passed to execve(2) and there is no shell involved in the command 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 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 `wait4`().

# 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), 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.

# 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 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).

The `popenve`() function first appeared in NetBSD 8.

January 19, 2015 | NetBSD 9.3