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Rev Author # Line
1 perry 1 !!NAME
2 popen, pclose - process I/O
3
4 CraigBox 4 !!SYNOPSIS
1 perry 5
3 SamJansen 6 __#include <stdio.h>__
1 perry 7
3 SamJansen 8 __FILE *popen(const char *__''command''__, const char*__''type''__);__
9 __int pclose(FILE *__''stream''__);__
1 perry 10
11 !!DESCRIPTION
12
13 The __popen()__ function opens a process by creating a
14 pipe, forking, and invoking the shell. Since a pipe is by
15 definition unidirectional, the ''type'' argument may
16 specify only reading or writing, not both; the resulting
17 stream is correspondingly read-only or
18 write-only.
19
20
21 The ''command'' argument is a pointer to a
22 null-terminated string containing a shell command line. This
23 command is passed to ''/bin/sh'' using the __-c__
24 flag; interpretation, if any, is performed by the shell. The
25 ''mode'' argument is a pointer to a null-terminated
26 string which must be either `r' for reading or `w' for
27 writing.
28
29
30 The return value from __popen()__ is a normal standard
31 I/O stream in all respects save that it must be closed with
32 __pclose()__ rather than __fclose()__. Writing to such
33 a stream writes to the standard input of the command; the
34 command's standard output is the same as that of the process
35 that called __popen()__, unless this is altered by the
36 command itself. Conversely, reading from a ``popened''
37 stream reads the command's standard output, and the
38 command's standard input is the same as that of the process
39 that called __popen__.
40
41
42 Note that output __popen__ streams are fully buffered by
43 default.
44
45
46 The __pclose__ function waits for the associated process
47 to terminate and returns the exit status of the command as
48 returned by __wait4__.
4 CraigBox 49
1 perry 50 !!RETURN VALUE
51
52 The __popen__ function returns __NULL__ if the
53 fork(2) or pipe(2) calls fail, or if it cannot
54 allocate memory.
55
56 The __pclose__ function returns -1 if __wait4__
57 returns an error, or some other error is
58 detected.
4 CraigBox 59
1 perry 60 !!ERRORS
61
62 The __popen__ function does not set ''errno'' if
63 memory allocation fails. If the underlying __fork()__ or
64 __pipe()__ fails, ''errno'' is set appropriately. If
65 the ''mode'' argument is invalid, and this condition is
66 detected, ''errno'' is set to [EINVAL].
67
4 CraigBox 68 If __pclose()__ cannot obtain the child status, ''errno'' is set to [ECHILD].
1 perry 69
70 !!CONFORMING TO
71
4 CraigBox 72 POSIX.2
1 perry 73
74 !!BUGS
75
4 CraigBox 76 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__.
1 perry 77
4 CraigBox 78 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.
1 perry 79
80 !!HISTORY
81
4 CraigBox 82 A __popen()__ and a __pclose()__ function appeared in Version 7 AT&T Unix.
1 perry 83
84 !!SEE ALSO
85
4 CraigBox 86 fork(2), sh(1), pipe(2), wait4(2), fflush(3), fclose(3), fopen(3), stdio(3), system(3)
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