| 1 | .\" -*-nroff-*- |
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| 4 | .RS |
| 5 | .nf |
| 6 | .ft B |
| 7 | .. |
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| 13 | .. |
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| 17 | \h'-\w'\\$1\ 'u'\\$1\ \c |
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| 19 | .. |
| 20 | .ie t .ds o \(bu |
| 21 | .el .ds o o |
| 22 | .TH mdup 3 "4 January" "Straylight/Edgeware" "mLib utilities library" |
| 23 | .SH NAME |
| 24 | mdup \- renumber file descriptors |
| 25 | .SH SYNOPSIS |
| 26 | .nf |
| 27 | .B "#include <mLib/mdup.h>" |
| 28 | |
| 29 | .B "typedef struct {" |
| 30 | .B "\h'4'int cur;" |
| 31 | .B "\h'4n'int want;" |
| 32 | .B "} mdup_fd;" |
| 33 | |
| 34 | .BI "int mdup(mdup_fd *" v ", size_t " n ");" |
| 35 | .fi |
| 36 | .SH DESCRIPTION |
| 37 | The |
| 38 | .B mdup |
| 39 | function renumbers file descriptors, using the |
| 40 | .BR dup (2) |
| 41 | and |
| 42 | .BR dup2 (2) |
| 43 | system calls. Its arguments are a pointer |
| 44 | .I v |
| 45 | to a vector of |
| 46 | .B mdup_fd |
| 47 | structures, and the length |
| 48 | .I n |
| 49 | of this vector, in elements. Each `slot' (element) in the vector |
| 50 | .I v |
| 51 | represents a file. The slot's |
| 52 | .B cur |
| 53 | member names the current file descriptor for this file; the |
| 54 | .B want |
| 55 | member is the file descriptor to move it to. In order to keep a file |
| 56 | alive when you don't care which descriptor it ends up with, set |
| 57 | .I want |
| 58 | = \-1. Several slots may specify the same |
| 59 | .B cur |
| 60 | descriptor; but they all have to declare different |
| 61 | .BR want s |
| 62 | (except that several slots may have |
| 63 | .I want |
| 64 | = \-1. |
| 65 | .PP |
| 66 | On successful exit, the function will have rearranged the file |
| 67 | descriptors as requested. To reflect this, the |
| 68 | .B cur |
| 69 | members will all be set to match the |
| 70 | .B want |
| 71 | members (except where the latter are \-1). |
| 72 | .PP |
| 73 | If there is a failure, then some rearrangement may have been performed |
| 74 | and some not; the |
| 75 | .B cur |
| 76 | members are set to reflect which file descriptors are to be used. |
| 77 | .PP |
| 78 | The old file descriptors are |
| 79 | .IR closed . |
| 80 | This is different from usual |
| 81 | .BR dup (2) |
| 82 | behaviour, of course, but essential for reliable error handling. If you |
| 83 | want to keep a particular source file descriptor open as well as make a |
| 84 | new copy then specify two slots with the same |
| 85 | .BR cur , |
| 86 | one with |
| 87 | .B want " = " cur |
| 88 | and one with the desired output descriptor. |
| 89 | .PP |
| 90 | The |
| 91 | .B mdup |
| 92 | function is capable of arbitrary file descriptor remappings. In |
| 93 | particular, it works correctly even if the desired remappings contain |
| 94 | cycles. |
| 95 | .SS "Background: the problem that mdup solves" |
| 96 | The |
| 97 | .B mdup |
| 98 | function is intended to be used to adjust file descriptors prior to |
| 99 | invoking one of the |
| 100 | .B exec |
| 101 | system calls. The standard use of |
| 102 | .BR dup (2) |
| 103 | to establish the child process's standard input/output/error files is |
| 104 | prone to errors in the case where the newly opened file in fact already |
| 105 | has one of the relevant file descriptors. |
| 106 | .PP |
| 107 | Consider the case where we want to run a process with separate pipes |
| 108 | attached to each of the standard descriptors. Typical code looks like |
| 109 | this. |
| 110 | .VS |
| 111 | #define P_INIT { \-1, \-1 } |
| 112 | int p_in[2] = P_INIT, p_out[2] = P_INIT, p_err[2] = P_INIT; |
| 113 | pid_t kid = -1; |
| 114 | int i; |
| 115 | |
| 116 | if (pipe(p_in) || pipe(p_out) || pipe(p_err)) goto error; |
| 117 | if ((kid = fork()) < 0) goto error; |
| 118 | if (!kid) { |
| 119 | if (dup2(p_in[0], STDIN_FILENO) < 0 || |
| 120 | dup2(p_out[1], STDOUT_FILENO) < 0 || |
| 121 | dup2(p_err[2], STDERR_FILENO) < 0 || |
| 122 | close(p_in[0]) || close(p_out[0]) || close(p_err[0]) || |
| 123 | close(p_in[1]) || close(p_out[1]) || close(p_err[1])) |
| 124 | _exit(127); |
| 125 | execvp("/bin/sh", "sh", "-c", "...", (char *)0); |
| 126 | } |
| 127 | \&... |
| 128 | .VE |
| 129 | Now suppose that, in the parent process, the standard input, output and |
| 130 | error descriptors are all initially closed. After the calls to |
| 131 | .BR pipe (2), |
| 132 | descriptors 0, 1, and 2 refer to |
| 133 | .BR p_in[0] , |
| 134 | .BR p_in[1] , |
| 135 | and |
| 136 | .B p_out[0] |
| 137 | respectively. In the child process, the calls to |
| 138 | .BR dup2 (2) |
| 139 | rearrange these. But then the |
| 140 | .BR close (2) |
| 141 | calls will immediately close all three descriptors, before |
| 142 | .BR exec ing |
| 143 | the child. |
| 144 | .PP |
| 145 | Here's how to rewrite the above function using |
| 146 | .BR mdup . |
| 147 | .VS |
| 148 | #define P_INIT { \-1, \-1 } |
| 149 | int p_in[2] = P_INIT, p_out[2] = P_INIT, p_err[2] = P_INIT; |
| 150 | pid_t kid = -1; |
| 151 | mdup_fd md[3]; |
| 152 | int i; |
| 153 | |
| 154 | if (pipe(p_in) || pipe(p_out) || pipe(p_err)) goto error; |
| 155 | if ((kid = fork()) < 0) goto error; |
| 156 | if (!kid) { |
| 157 | if (close(p_in[1] || close(p_out[0]) || close(p_err[0])) |
| 158 | goto _exit(127); |
| 159 | md[0].cur = p_in[0]; md[0].want = STDIN_FILENO; |
| 160 | md[1].cur = p_out[1]; md[1].want = STDOUT_FILENO; |
| 161 | md[2].cur = p_err[1]; md[2].want = STDERR_FILENO; |
| 162 | if (mdup(md, 3)) _exit(127); |
| 163 | execvp("/bin/sh", "sh", "-c", "...", (char *)0); |
| 164 | } |
| 165 | \&... |
| 166 | .VE |
| 167 | One can see that, not only is the resulting program more correct, it's |
| 168 | also simpler. Note that we close the unwanted ends of the pipes |
| 169 | .I before |
| 170 | invoking |
| 171 | .BR mdup . |
| 172 | Closing them afterwards risks interfering with the newly assigned |
| 173 | descriptors which are meant to be passed to the child process. Note |
| 174 | also that |
| 175 | .B mdup |
| 176 | has taken responsibility for closing the other descriptors for the |
| 177 | wanted ends of the pipes. |
| 178 | .SH "SEE ALSO" |
| 179 | .BR dup (2), |
| 180 | .BR dup2 (2), |
| 181 | .BR mLib (3). |
| 182 | .SH AUTHOR |
| 183 | Mark Wooding, <mdw@distorted.org.uk> |
| 184 | |