gen/FreeBSD/popen.3

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  32 .\"     @(#)popen.3     8.2 (Berkeley) 5/3/95
  33 .\" $FreeBSD: src/lib/libc/gen/popen.3,v 1.16 2003/06/08 10:01:51 charnier Exp $
  34 .\"
  35 .Dd May 3, 1995
  36 .Dt POPEN 3
  37 .Os
  38 .Sh NAME
  39 .Nm popen ,
  40 .Nm pclose
  41 .Nd process
  42 .Tn I/O
  43 .Sh LIBRARY
  44 .Lb libc
  45 .Sh SYNOPSIS
  46 .In stdio.h
  47 .Ft FILE *
  48 .Fn popen "const char *command" "const char *type"
  49 .Ft int
  50 .Fn pclose "FILE *stream"
  51 .Sh DESCRIPTION
  52 The
  53 .Fn popen
  54 function
  55 .Dq opens
  56 a process by creating a bidirectional pipe
  57 forking,
  58 and invoking the shell.
  59 Any streams opened by previous
  60 .Fn popen
  61 calls in the parent process are closed in the new child process.
  62 Historically,
  63 .Fn popen
  64 was implemented with a unidirectional pipe;
  65 hence many implementations of
  66 .Fn popen
  67 only allow the
  68 .Fa type
  69 argument to specify reading or writing, not both.
  70 Since
  71 .Fn popen
  72 is now implemented using a bidirectional pipe, the
  73 .Fa type
  74 argument may request a bidirectional data flow.
  75 The
  76 .Fa type
  77 argument is a pointer to a null-terminated string
  78 which must be
  79 .Ql r
  80 for reading,
  81 .Ql w
  82 for writing, or
  83 .Ql r+
  84 for reading and writing.
  85 .Pp
  86 The
  87 .Fa command
  88 argument is a pointer to a null-terminated string
  89 containing a shell command line.
  90 This command is passed to
  91 .Pa /bin/sh
  92 using the
  93 .Fl c
  94 flag; interpretation, if any, is performed by the shell.
  95 .Pp
  96 The return value from
  97 .Fn popen
  98 is a normal standard
  99 .Tn I/O
 100 stream in all respects
 101 save that it must be closed with
 102 .Fn pclose
 103 rather than
 104 .Fn fclose .
 105 Writing to such a stream
 106 writes to the standard input of the command;
 107 the command's standard output is the same as that of the process that called
 108 .Fn popen ,
 109 unless this is altered by the command itself.
 110 Conversely, reading from a
 111 .Dq popened
 112 stream reads the command's standard output, and
 113 the command's standard input is the same as that of the process that called
 114 .Fn popen .
 115 .Pp
 116 Note that output
 117 .Fn popen
 118 streams are fully buffered by default.
 119 .Pp
 120 The
 121 .Fn pclose
 122 function waits for the associated process to terminate
 123 and returns the exit status of the command
 124 as returned by
 125 .Xr wait4 2 .
 126 .Sh RETURN VALUES
 127 The
 128 .Fn popen
 129 function returns
 130 .Dv NULL
 131 if the
 132 .Xr fork 2
 133 or
 134 .Xr pipe 2
 135 calls fail,
 136 or if it cannot allocate memory.
 137 .Pp
 138 The
 139 .Fn pclose
 140 function
 141 returns \-1 if
 142 .Fa stream
 143 is not associated with a
 144 .Dq popened
 145 command, if
 146 .Fa stream
 147 already
 148 .Dq pclosed ,
 149 or if
 150 .Xr wait4 2
 151 returns an error.
 152 .Sh ERRORS
 153 The
 154 .Fn popen
 155 function does not reliably set
 156 .Va errno .
 157 .Sh SEE ALSO
 158 .Xr sh 1 ,
 159 .Xr fork 2 ,
 160 .Xr pipe 2 ,
 161 .Xr wait4 2 ,
 162 .Xr fclose 3 ,
 163 .Xr fflush 3 ,
 164 .Xr fopen 3 ,
 165 .Xr stdio 3 ,
 166 .Xr system 3
 167 .Sh BUGS
 168 Since the standard input of a command opened for reading
 169 shares its seek offset with the process that called
 170 .Fn popen ,
 171 if the original process has done a buffered read,
 172 the command's input position may not be as expected.
 173 Similarly, the output from a command opened for writing
 174 may become intermingled with that of the original process.
 175 The latter can be avoided by calling
 176 .Xr fflush 3
 177 before
 178 .Fn popen .
 179 .Pp
 180 Failure to execute the shell
 181 is indistinguishable from the shell's failure to execute command,
 182 or an immediate exit of the command.
 183 The only hint is an exit status of 127.
 184 .Pp
 185 The
 186 .Fn popen
 187 function
 188 always calls
 189 .Xr sh 1 ,
 190 never calls
 191 .Xr csh 1 .
 192 .Sh HISTORY
 193 A
 194 .Fn popen
 195 and a
 196 .Fn pclose
 197 function appeared in
 198 .At v7 .
 199 .Pp
 200 Bidirectional functionality was added in
 201 .Fx 2.2.6 .