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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
POPEN(3P) POSIX Programmer's ManualPOPEN(3P)This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
popen — initiate pipe streams to or from a process
#include <stdio.h> FILE *popen(const char *command, const char *mode);
Thepopen() function shall execute the command specified by the stringcommand. 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 thepopen() call using thefork() function, and the child invoked thesh utility using the call: execl(shell path, "sh", "-c",command, (char *)0); whereshell path is an unspecified pathname for thesh utility. Thepopen() function shall ensure that any streams from previouspopen() calls that remain open in the parent process are closed in the new child process. Themode argument topopen() is a string that specifies I/O mode: 1. Ifmode isr, when the child process is started, its file descriptor STDOUT_FILENO shall be the writable end of the pipe, and the file descriptorfileno(stream) in the calling process, wherestream is the stream pointer returned bypopen(), shall be the readable end of the pipe. 2. Ifmode isw, when the child process is started its file descriptor STDIN_FILENO shall be the readable end of the pipe, and the file descriptorfileno(stream) in the calling process, wherestream is the stream pointer returned bypopen(), shall be the writable end of the pipe. 3. Ifmode is any other value, the result is unspecified. Afterpopen(), both the parent and the child process shall be capable of executing independently before either terminates. Pipe streams are byte-oriented.
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 seterrno to indicate the error.
Thepopen() function shall fail if:EMFILE{STREAM_MAX} streams are currently open in the calling process. Thepopen() function may fail if:EMFILE{FOPEN_MAX} streams are currently open in the calling process.EINVALThemode argument is invalid. Thepopen() function may also seterrno values as described byfork(3p) orpipe(3p).The following sections are informative.Using popen() to Obtain a List of Files from the ls Utility The following example demonstrates the use ofpopen() andpclose() to execute the commandls* in order to obtain a list of files in the current directory: #include <stdio.h> ... FILE *fp; int status; char path[PATH_MAX]; fp = popen("ls *", "r"); if (fp == NULL) /* Handle error */; while (fgets(path, PATH_MAX, fp) != NULL) printf("%s", path); status = pclose(fp); if (status == -1) { /* Error reported by pclose() */ ... } else { /* Use macros described under wait() to inspect `status' in order to determine success/failure of command executed by popen() */ ... }Since open files are shared, a moder command can be used as an input filter and a modew command as an output filter. Buffered reading before opening an input filter may leave the standard input of that filter mispositioned. Similar problems with an output filter may be prevented by careful buffer flushing; for example, withfflush(3p). A stream opened bypopen() should be closed bypclose(). The behavior ofpopen() is specified for values ofmode ofr andw. Other modes such asrb andwb might be supported by specific implementations, but these would not be portable features. Note that historical implementations ofpopen() only check to see if the first character ofmode isr. Thus, amode ofrobert therobot would be treated asmode r, and amode ofanything else would be treated asmode w. If the application callswaitpid() orwaitid() with apid argument greater than 0, and it still has a stream that was called withpopen() open, it must ensure thatpid does not refer to the process started bypopen(). To determine whether or not the environment specified in the Shell and Utilities volume of POSIX.1‐2017 is present, use the function call: sysconf(_SC_2_VERSION) (Seesysconf(3p)).
Thepopen() function should not be used by programs that have set user (or group) ID privileges. Thefork() andexec family of functions (exceptexeclp() andexecvp()), should be used instead. This prevents any unforeseen manipulation of the environment of the user that could cause execution of commands not anticipated by the calling program. If the original andpopen()ed processes both intend to read or write or read and write a common file, and either will be using FILE-type C functions (fread(),fwrite(), and so on), the rules for sharing file handles must be observed (seeSection 2.5.1,Interaction of File Descriptors and Standard I/O Streams).
None.
Section 2.5,Standard I/O Streams,fork(3p),pclose(3p),pipe(3p),sysconf(3p),system(3p),wait(3p),waitid(3p) The Base Definitions volume of POSIX.1‐2017,stdio.h(0p) The Shell and Utilities volume of POSIX.1‐2017,sh(1p)
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online athttp://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017POPEN(3P)Pages that refer to this page:stdio.h(0p), awk(1p), pclose(3p), stdin(3p)
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