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dup.3p (6278B)

    

  1. '\" et
  2. .TH DUP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
  3. .\"
  4. .SH PROLOG
  5. This manual page is part of the POSIX Programmer's Manual.
  6. The Linux implementation of this interface may differ (consult
  7. the corresponding Linux manual page for details of Linux behavior),
  8. or the interface may not be implemented on Linux.
  9. .\"
  10. .SH NAME
  11. dup,
  12. dup2
  13. \(em duplicate an open file descriptor
  14. .SH SYNOPSIS
  15. .LP
  16. .nf
  17. #include <unistd.h>
  18. .P
  19. int dup(int \fIfildes\fP);
  20. int dup2(int \fIfildes\fP, int \fIfildes2\fP);
  21. .fi
  22. .SH DESCRIPTION
  23. The
  24. \fIdup\fR()
  25. function provides an alternative interface to the service provided by
  26. \fIfcntl\fR()
  27. using the F_DUPFD command. The call
  28. .IR dup ( fildes )
  29. shall be equivalent to:
  30. .sp
  31. .RS 4
  32. .nf
  34. fcntl(fildes, F_DUPFD, 0);
  35. .fi
  36. .P
  37. .RE
  38. .P
  39. The
  40. \fIdup2\fR()
  41. function shall cause the file descriptor
  42. .IR fildes2
  43. to refer to the same open file description as the file descriptor
  44. .IR fildes
  45. and to share any locks, and shall return
  46. .IR fildes2 .
  47. If
  48. .IR fildes2
  49. is already a valid open file descriptor, it shall be closed first, unless
  50. .IR fildes
  51. is equal to
  52. .IR fildes2
  53. in which case
  54. \fIdup2\fR()
  55. shall return
  56. .IR fildes2
  57. without closing it. If the close operation fails to close
  58. .IR fildes2 ,
  59. \fIdup2\fR()
  60. shall return \-1 without changing the open file description to which
  61. .IR fildes2
  62. refers. If
  63. .IR fildes
  64. is not a valid file descriptor,
  65. \fIdup2\fR()
  66. shall return \-1 and shall not close
  67. .IR fildes2 .
  68. If
  69. .IR fildes2
  70. is less than 0 or greater than or equal to
  71. {OPEN_MAX},
  72. \fIdup2\fR()
  73. shall return \-1 with
  74. .IR errno
  75. set to
  76. .BR [EBADF] .
  77. .P
  78. Upon successful completion, if
  79. .IR fildes
  80. is not equal to
  81. .IR fildes2 ,
  82. the FD_CLOEXEC flag associated with
  83. .IR fildes2
  84. shall be cleared. If
  85. .IR fildes
  86. is equal to
  87. .IR fildes2 ,
  88. the FD_CLOEXEC flag associated with
  89. .IR fildes2
  90. shall not be changed.
  91. .P
  92. If
  93. .IR fildes
  94. refers to a typed memory object, the result of the
  95. \fIdup2\fR()
  96. function is unspecified.
  97. .SH "RETURN VALUE"
  98. Upon successful completion a non-negative integer, namely the file
  99. descriptor, shall be returned; otherwise, \-1 shall be returned
  100. and
  101. .IR errno
  102. set to indicate the error.
  103. .SH ERRORS
  104. The
  105. \fIdup\fR()
  106. function shall fail if:
  107. .TP
  108. .BR EBADF
  109. The
  110. .IR fildes
  111. argument is not a valid open file descriptor.
  112. .TP
  113. .BR EMFILE
  114. All file descriptors available to the process are currently open.
  115. .P
  116. The
  117. \fIdup2\fR()
  118. function shall fail if:
  119. .TP
  120. .BR EBADF
  121. The
  122. .IR fildes
  123. argument is not a valid open file descriptor or the argument
  124. .IR fildes2
  125. is negative or greater than or equal to
  126. {OPEN_MAX}.
  127. .TP
  128. .BR EINTR
  129. The
  130. \fIdup2\fR()
  131. function was interrupted by a signal.
  132. .P
  133. The
  134. \fIdup2\fR()
  135. function may fail if:
  136. .TP
  137. .BR EIO
  138. An I/O error occurred while attempting to close
  139. .IR fildes2 .
  140. .LP
  141. .IR "The following sections are informative."
  142. .SH EXAMPLES
  143. .SS "Redirecting Standard Output to a File" S
  144. .P
  145. The following example closes standard output for the current processes,
  146. re-assigns standard output to go to the file referenced by
  147. .IR pfd ,
  148. and closes the original file descriptor to clean up.
  149. .sp
  150. .RS 4
  151. .nf
  153. #include <unistd.h>
  154. \&...
  155. int pfd;
  156. \&...
  157. close(1);
  158. dup(pfd);
  159. close(pfd);
  160. \&...
  161. .fi
  162. .P
  163. .RE
  164. .SS "Redirecting Error Messages"
  165. .P
  166. The following example redirects messages from
  167. .IR stderr
  168. to
  169. .IR stdout .
  170. .sp
  171. .RS 4
  172. .nf
  174. #include <unistd.h>
  175. \&...
  176. dup2(1, 2);
  177. \&...
  178. .fi
  179. .P
  180. .RE
  181. .SH "APPLICATION USAGE"
  182. Implementations may use file descriptors that must be inherited into
  183. child processes for the child process to remain conforming, such as for
  184. message catalog or tracing purposes. Therefore, an application that calls
  185. \fIdup2\fR()
  186. with an arbitrary integer for
  187. .IR fildes2
  188. risks non-conforming behavior, and
  189. \fIdup2\fR()
  190. can only portably be used to overwrite file descriptor values that the
  191. application has obtained through explicit actions, or for the three file
  192. descriptors corresponding to the standard file streams. In order to avoid
  193. a race condition of leaking an unintended file descriptor into a child
  194. process, an application should consider opening all file descriptors
  195. with the FD_CLOEXEC bit set unless the file descriptor is intended to
  196. be inherited across
  197. .IR exec .
  198. .SH RATIONALE
  199. The
  200. \fIdup\fR()
  201. function is redundant. Its services are also provided by the
  202. \fIfcntl\fR()
  203. function. It has been included in this volume of POSIX.1\(hy2017 primarily for historical reasons,
  204. since many existing applications use it. On the other hand, the
  205. \fIdup2\fR()
  206. function provides unique services, as no other interface is able to
  207. atomically replace an existing file descriptor.
  208. .P
  209. The
  210. \fIdup2\fR()
  211. function is not marked obsolescent because it presents a type-safe
  212. version of functionality provided in a type-unsafe version by
  213. \fIfcntl\fR().
  214. It is used in the POSIX Ada binding.
  215. .P
  216. The
  217. \fIdup2\fR()
  218. function is not intended for use in critical regions as a
  219. synchronization mechanism.
  220. .P
  221. In the description of
  222. .BR [EBADF] ,
  223. the case of
  224. .IR fildes
  225. being out of range is covered by the given case of
  226. .IR fildes
  227. not being valid. The descriptions for
  228. .IR fildes
  229. and
  230. .IR fildes2
  231. are different because the only kind of invalidity that is relevant for
  232. .IR fildes2
  233. is whether it is out of range; that is, it does not matter whether
  234. .IR fildes2
  235. refers to an open file when the
  236. \fIdup2\fR()
  237. call is made.
  238. .SH "FUTURE DIRECTIONS"
  239. None.
  240. .SH "SEE ALSO"
  241. .IR "\fIclose\fR\^(\|)",
  242. .IR "\fIfcntl\fR\^(\|)",
  243. .IR "\fIopen\fR\^(\|)"
  244. .P
  245. The Base Definitions volume of POSIX.1\(hy2017,
  246. .IR "\fB<unistd.h>\fP"
  247. .\"
  248. .SH COPYRIGHT
  249. Portions of this text are reprinted and reproduced in electronic form
  250. from IEEE Std 1003.1-2017, Standard for Information Technology
  251. -- Portable Operating System Interface (POSIX), The Open Group Base
  252. Specifications Issue 7, 2018 Edition,
  253. Copyright (C) 2018 by the Institute of
  254. Electrical and Electronics Engineers, Inc and The Open Group.
  255. In the event of any discrepancy between this version and the original IEEE and
  256. The Open Group Standard, the original IEEE and The Open Group Standard
  257. is the referee document. The original Standard can be obtained online at
  258. http://www.opengroup.org/unix/online.html .
  259. .PP
  260. Any typographical or formatting errors that appear
  261. in this page are most likely
  262. to have been introduced during the conversion of the source files to
  263. man page format. To report such errors, see
  264. https://www.kernel.org/doc/man-pages/reporting_bugs.html .