pclose(3) - NetBSD Manual Pages

POPEN(3)                  NetBSD Programmer's Manual                  POPEN(3)


NAME
popen, pclose - process I/O
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h> FILE * popen(const char *command, 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 al- low 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. 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 return value from popen() is a normal standard I/O stream in all re- spects 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, read- ing 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 VALUE
The popen() function returns NULL if the fork(2), pipe(2), or socketpair(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 wait4(2) re- turns an error.
ERRORS
The popen() function does not reliably set errno.
SEE ALSO
fork(2), sh(1), pipe(2), socketpair(2), wait4(2), fflush(3), fclose(3), fopen(3), stdio(3), system(3)
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 expect- ed. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoid- ed by calling fflush(3) before popen(). Failure to execute the shell is indistinguishable from the shell's fail- ure 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).
HISTORY
A popen() and a pclose() function appeared in Version 7 AT&T UNIX. NetBSD 1.4 May 3, 1995 2

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