Manpage of POPEN
POPENSection: Linux Programmer's Manual (3)
NAMEpopen, pclose - pipe stream to or from a process
#include <stdio.h>FILE *popen(const char *command, const char *type);int pclose(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
- _POSIX_C_SOURCE >= 2
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTIONThe popen() function opens a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the typeargument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.
The commandargument is a pointer to a null-terminated string containing a shell command line. This command is passed to /bin/shusing the -cflag; interpretation, if any, is performed by the shell.
The typeargument is a pointer to a null-terminated string which must contain either the letter aqraq for reading or the letter aqwaq for writing. Since glibc 2.9, this argument can additionally include the letter aqeaq, which causes the close-on-exec flag (FD_CLOEXEC) to be set on the underlying file descriptor; see the description of the O_CLOEXECflag in open(2) for reasons why this may be useful.
The return value from popen() is a normal standard I/O stream in all respects save that it must be closed with pclose() rather than fclose(3). 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, 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 popen().
Note that output popen() streams are block 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(2).
RETURN VALUEpopen(): on success, returns a pointer to an open stream that can be used to read or write to the pipe; if the fork(2) or pipe(2) calls fail, or if the function cannot allocate memory, NULL is returned.
pclose(): on success, returns the exit status of the command; if wait4(2) returns an error, or some other error is detected, -1 is returned.
ERRORSThe popen() function does not set errnoif memory allocation fails. If the underlying fork(2) or pipe(2) fails, errnois set appropriately. If the typeargument is invalid, and this condition is detected, errnois set to EINVAL.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
|popen(), pclose()||Thread safety||MT-Safe|
CONFORMING TOPOSIX.1-2001, POSIX.1-2008.
BUGSSince 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 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 popen().
SEE ALSOsh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)
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