POPEN

Section: C Library Functions (3)
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BSD mandoc

NAME

popen pclose - process I/O

SYNOPSIS

Fd #include <stdio.h> Ft FILE * Fn popen const char *command const char *type Ft int Fn pclose FILE *stream

DESCRIPTION

The Fn popen function ``opens'' a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the Fa type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.

The Fa 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 Fa 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 Fn popen is a normal standard I/O stream in all respects save that it must be closed with Fn pclose rather than Fn 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 Fn 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 Fn popen .

Note that output Fn popen streams are fully buffered by default.

The Fn pclose function waits for the associated process to terminate and returns the exit status of the command as returned by Fn wait4 .

RETURN VALUE

The Fn popen function returns NULL if the fork(2) or pipe(2) calls fail, or if it cannot allocate memory.

The Fn pclose function returns -1 if Fa stream is not associated with a ``popened'' command, if Fa stream already ``pclosed'' or if wait4 returns an error.

ERRORS

The Fn popen function does not reliably set errno

BUGS

Since the standard input of a command opened for reading shares its seek offset with the process that called Fn 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 Fn 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 Fn popen argument always calls sh, never calls csh.

HISTORY

A Fn popen and a Fn pclose function appeared in AT&T System v7 .

This document was created by man2html, using the manual pages.
Time: 16:28:57 GMT, April 18, 2022