version 2, including all changes.
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POPEN |
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!!!POPEN |
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NAME |
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SYNOPSIS |
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DESCRIPTION |
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RETURN VALUE |
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ERRORS |
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CONFORMING TO |
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BUGS |
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HISTORY |
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SEE ALSO |
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---- |
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!!NAME |
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popen, pclose - process I/O |
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!!SYNOPSIS |
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__#include __ |
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__FILE *popen(const char *__''command''__, const char |
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*__''type''__);__ |
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__int pclose(FILE *__''stream''__);__ |
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!!DESCRIPTION |
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The __popen()__ function opens a process by creating a |
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pipe, forking, and invoking the shell. Since a pipe is by |
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definition unidirectional, the ''type'' argument may |
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specify only reading or writing, not both; the resulting |
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stream is correspondingly read-only or |
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write-only. |
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The ''command'' argument is a pointer to a |
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null-terminated string containing a shell command line. This |
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command is passed to ''/bin/sh'' using the __-c__ |
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flag; interpretation, if any, is performed by the shell. The |
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''mode'' argument is a pointer to a null-terminated |
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string which must be either `r' for reading or `w' for |
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writing. |
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The return value from __popen()__ is a normal standard |
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I/O stream in all respects save that it must be closed with |
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__pclose()__ rather than __fclose()__. Writing to such |
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a stream writes to the standard input of the command; the |
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command's standard output is the same as that of the process |
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that called __popen()__, unless this is altered by the |
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command itself. Conversely, reading from a ``popened'' |
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stream reads the command's standard output, and the |
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command's standard input is the same as that of the process |
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that called __popen__. |
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Note that output __popen__ streams are fully buffered by |
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default. |
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The __pclose__ function waits for the associated process |
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to terminate and returns the exit status of the command as |
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returned by __wait4__. |
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!!RETURN VALUE |
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The __popen__ function returns __NULL__ if the |
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fork(2) or pipe(2) calls fail, or if it cannot |
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allocate memory. |
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The __pclose__ function returns -1 if __wait4__ |
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returns an error, or some other error is |
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detected. |
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!!ERRORS |
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The __popen__ function does not set ''errno'' if |
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memory allocation fails. If the underlying __fork()__ or |
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__pipe()__ fails, ''errno'' is set appropriately. If |
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the ''mode'' argument is invalid, and this condition is |
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detected, ''errno'' is set to __EINVAL__. |
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If __pclose()__ cannot obtain the child status, |
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''errno'' is set to __ECHILD__. |
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!!CONFORMING TO |
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POSIX.2 |
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!!BUGS |
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Since the standard input of a command opened for reading |
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shares its seek offset with the process that called |
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__popen()__, if the original process has done a buffered |
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read, the command's input position may not be as expected. |
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Similarly, the output from a command opened for writing may |
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become intermingled with that of the original process. The |
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latter can be avoided by calling fflush(3) before |
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__popen__. |
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Failure to execute the shell is indistinguishable from the |
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shell's failure to execute command, or an immediate exit of |
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the command. The only hint is an exit status of |
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127. |
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!!HISTORY |
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A __popen()__ and a __pclose()__ function appeared in |
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Version 7 AT__ |
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!!SEE ALSO |
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fork(2), sh(1), pipe(2), |
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perry |
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wait4(2), fflush(3), fclose(3), |
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perry |
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fopen(3), stdio(3), |
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system(3) |
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---- |