Section: Linux Programmer's Manual (3)
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popen, pclose - pipe stream to or from a process
FILE *popen(const char *command, const char *type);
int pclose(FILE *stream);
Feature Test Macro Requirements for glibc (see
_POSIX_C_SOURCE >= 2
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
function opens a process by creating a pipe, forking, and invoking the
Since a pipe is by definition unidirectional, the
argument may specify only reading or writing, not both; the resulting
stream is correspondingly read-only or write-only.
argument is a pointer to a null-terminated string containing a shell
This command is passed to
flag; interpretation, if any, is performed by the shell.
argument is a pointer to a null-terminated string which must contain
either the letter 'r' for reading or the letter 'w' for writing.
Since glibc 2.9,
this argument can additionally include the letter 'e',
which causes the close-on-exec flag
to be set on the underlying file descriptor;
see the description of the
for reasons why this may be useful.
The return value from
is a normal standard I/O stream in all respects save that it must be closed
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
unless this is altered by the command itself.
Conversely, reading from
the stream reads the command's standard output, and the command's
standard input is the same as that of the process that called
Note that output
streams are block buffered by default.
function waits for the associated process to terminate and returns the exit
status of the command as returned by
on success, returns a pointer to an open stream that
can be used to read or write to the pipe;
calls fail, or if the function cannot allocate memory,
NULL is returned.
on success, returns the exit status of the command; if
returns an error, or some other error is detected,
-1 is returned.
Both functions set
to an appropriate value in the case of an error.
function does not set
if memory allocation fails.
If the underlying
is set appropriately.
argument is invalid, and this condition is detected,
is set to
cannot obtain the child status,
is set to
For an explanation of the terms used in this section, see
The 'e' value for
is a Linux extension.
carefully read Caveats in
Since the standard input of a command opened for reading shares its seek
offset with the process that called
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
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.
This page is part of release 4.16 of the Linux
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