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popen (3)
  • popen (3) ( Solaris man: Библиотечные вызовы )
  • popen (3) ( FreeBSD man: Библиотечные вызовы )
  • popen (3) ( Русские man: Библиотечные вызовы )
  • >> popen (3) ( Linux man: Библиотечные вызовы )
  • popen (3) ( POSIX man: Библиотечные вызовы )
  •  

    NAME

    popen, pclose - pipe stream to or from a process
     
    

    SYNOPSIS

    #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)):

    popen(), pclose(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _POSIX_SOURCE _BSD_SOURCE || _SVID_SOURCE  

    DESCRIPTION

    The popen() function opens a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.

    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 type argument 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_CLOEXEC flag 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 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(2).  

    RETURN VALUE

    The popen() function returns NULL if the fork(2) or pipe(2) calls fail, or if it cannot allocate memory.

    The pclose() function returns -1 if wait4(2) returns an error, or some other error is detected.  

    ERRORS

    The popen() function does not set errno if memory allocation fails. If the underlying fork(2) or pipe(2) fails, errno is set appropriately. If the type argument is invalid, and this condition is detected, errno is set to EINVAL.

    If pclose() cannot obtain the child status, errno is set to ECHILD.  

    CONFORMING TO

    POSIX.1-2001.

    The aqeaq value for type is a Linux extension.  

    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 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().

    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.  

    SEE ALSO

    sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)  

    COLOPHON

    This page is part of release 3.14 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


     

    Index

    NAME
    SYNOPSIS
    DESCRIPTION
    RETURN VALUE
    ERRORS
    CONFORMING TO
    BUGS
    SEE ALSO
    COLOPHON


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