phoenix-rtos-doc / libc / functions / p / popen.part-impl.md

popen

Synopsis

#include <stdio.h>

FILE *popen(const char *command, const char *mode);

Status

Implemented

Conformance

IEEE Std 1003.1-2017

Description

The popen() function shall execute the command specified by the string command. It shall create a pipe between the calling program and the executed command, and shall return a pointer to a stream that can be used to either read from or write to the pipe.

The environment of the executed command shall be as if a child process were created within the popen() call using the fork() function, and the child invoked the sh utility using the call:

execl(shell path, "sh", "-c", command, (char *)0);

Where shell path is an unspecified path name for the sh utility.

The popen() function shall ensure that any streams from previous popen() calls that remain open in the parent process are closed in the new child process.

The mode argument to popen() is a string that specifies I/O mode:

If mode is r, when the child process is started, its file descriptor STDOUT_FILENO shall be the writable end of the pipe, and the file descriptor fileno(stream) in the calling process, where stream is the stream pointer returned by popen(), shall be the readable end of the pipe.
If mode is w, when the child process is started its file descriptor STDIN_FILENO shall be the readable end of the pipe, and the file descriptor fileno(stream) in the calling process, where stream is the stream pointer returned by popen(), shall be the writable end of the pipe.
If mode is any other value, the result is unspecified.

After popen(), both the parent and the child process shall be capable of executing independently before either terminates.

Pipe streams are byte-oriented.

Return value

Upon successful completion, popen() shall return a pointer to an open stream that can be used to read or write to the pipe. Otherwise, it shall return a null pointer and may set errno to indicate the error.

Errors

The popen() function shall fail if:

EMFILE - {STREAM_MAX} streams are currently open in the calling process.

The popen() function may fail if:

EMFILE - {FOPEN_MAX} streams are currently open in the calling process.
EINVAL - The mode argument is invalid.

The popen() function may also set errno values as described by fork or pipe.

Tests

Untested

Known bugs

None