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