1FFI::Platypus::Buffer(3U)ser Contributed Perl DocumentatiFoFnI::Platypus::Buffer(3)
2
6 FFI::Platypus::Buffer - Convert scalars to C buffers
7
9 version 1.58
10
12 use FFI::Platypus::Buffer;
13 my($pointer, $size) = scalar_to_buffer $scalar;
14 my $scalar2 = buffer_to_scalar $pointer, $size;
15
17 A common pattern in C is to pass a "buffer" or region of memory into a
18 function with a pair of arguments, an opaque pointer and the size of
19 the memory region. In Perl the equivalent structure is a scalar
20 containing a string of bytes. This module provides portable functions
21 for converting a Perl string or scalar into a buffer and back.
22 These functions are implemented using pack and unpack and so they
24 should be relatively fast.
25 Both functions are exported by default, but you can explicitly export
27 one or neither if you so choose.
28 A better way to do this might be with custom types see
30 FFI::Platypus::API and FFI::Platypus::Type. These functions were taken
31 from the now obsolete FFI::Util module, as they may be useful in some
32 cases.
33 Caution: This module provides great power in the way that you interact
35 with C code, but with that power comes great responsibility. Since you
36 are dealing with blocks of memory you need to take care to understand
37 the underlying ownership model of these pointers.
38
40 scalar_to_buffer
41 my($pointer, $size) = scalar_to_buffer $scalar;
42 Convert a string scalar into a buffer. Returned in order are a pointer
44 to the start of the string scalar's memory region and the size of the
45 region.
46 You should NEVER try to free $pointer.
48 When you pass this pointer and size into a C function, it has direct
50 access to the data stored in your scalar, so it is important that you
51 not resize or free the scalar while it is in use by the C code.
52 Typically if you are passing a buffer into a C function which reads or
53 writes to the buffer, but does not keep the pointer for later use you
54 are okay. If the buffer is in use long term by the C code, then you
55 should consider copying the buffer instead. For example:
56 use FFI::Platypus::Buffer qw( scalar_to_buffer );
58 use FFI::Platypus::Memory qw( malloc memcpy free )
59 my($ptr, $size) = scalar_to_buffer $string;
61 c_function_that_does_not_keep_ptr( $ptr, $size); # okay
62 my($ptr, $size) = scalar_to_buffer $string;
64 my $ptr_copy = malloc($size);
65 memcpy($ptr_copy, $ptr, $size);
66 c_function_that_DOES_keep_ptr( $ptr_copy, $size); # also okay
67 ...
69 # later when you know that the c code is no longer using the pointer
71 # Since you allocated the copy, you are responsible for free'ing it.
72 free($ptr_copy);
73 scalar_to_pointer
75 my $pointer = scalar_to_pointer $scalar;
76 Get the pointer to the scalar. (Similar to "scalar_to_buffer" above,
78 but the size of the scalar is not computed or returned).
79 Not exported by default, but may be exported on request.
81 buffer_to_scalar
83 my $scalar = buffer_to_scalar $pointer, $size;
84 Convert the buffer region defined by the pointer and size into a string
86 scalar.
87 Because of the way memory management works in Perl, the buffer is
89 copied from the buffer into the scalar. If this pointer was returned
90 from C land, then you should only free it if you allocated it.
91 grow
93 grow $scalar, $size, \%options;
94 Ensure that the scalar can contain at least $size bytes. The following
96 are recognized:
97 clear => boolean
99 If true, $scalar is cleared prior to being enlarged. This avoids
100 copying the existing contents to the reallocated memory if they are
101 not needed.
102 For example, after
104 $scalar = "my string";
106 grow $scalar, 100, { clear => 0 };
107 "$scalar == "my string"", while after
109 $scalar = "my string";
111 grow $scalar, 100;
112 "length($scalar) == 0"
114 It defaults to "true".
116 set_length => boolean
118 If true, the length of the string in the $scalar is set to $size.
119 (See the discussion in "set_used_length".) This is useful if a
120 foreign function writes exactly $size bytes to $scalar, as it
121 avoids a subsequent call to "set_used_length". Contrast this
122 grow my $scalar, 100;
124 read_exactly_100_bytes_into_scalar( scalar_to_pointer($scalar) );
125 @chars = unpack( 'c*', $scalar );
126 with this:
128 grow my $scalar, 100, { set_length => 0 };
130 read_exactly_100_bytes_into_scalar( scalar_to_pointer($scalar) );
131 set_used_length( $scalar, 100 );
132 @chars = unpack( 'c*', $scalar );
133 It defaults to "true".
135 Any pointers obtained with "scalar_to_pointer" or "scalar_to_buffer"
137 are no longer valid after growing the scalar.
138 Not exported by default, but may be exported on request.
140 set_used_length
142 set_used_length $scalar, $length;
143 Update Perl's notion of the length of the string in the scalar. A
145 string scalar keeps track of two lengths: the number of available bytes
146 and the number of used bytes. When a string scalar is used as a buffer
147 by a foreign function, it is necessary to indicate to Perl how many
148 bytes were actually written to it so that Perl's string functions (such
149 as "substr" or "unpack") will work correctly.
150 If $length is larger than what the scalar can hold, it is set to the
152 maximum possible size.
153 In the following example, the foreign routine "read_doubles" may fill
155 the buffer with up to a set number of doubles, returning the number
156 actually written.
157 my $sizeof_double = $ffi->sizeof( 'double' );
159 my $max_doubles = 100;
160 my $max_length = $max_doubles * $sizeof_double;
161 my $buffer; # length($buffer) == 0
163 grow $buffer, $max_length; # length($buffer) is still 0
164 my $pointer = scalar_to_pointer($buffer);
165 my $num_read = read_doubles( $pointer, $max_doubles );
167 # length($buffer) is still == 0
168 set_used_length $buffer, $num_read * $sizeof_double;
170 # length($buffer) is finally != 0
171 # unpack the native doubles into a Perl array
173 my @doubles = unpack( 'd*', $buffer ); # @doubles == $num_read
174 Not exported by default, but may be exported on request.
176 window
178 window $scalar, $pointer;
179 window $scalar, $pointer, $size;
180 window $scalar, $pointer, $size, $utf8;
181 This makes the scalar a read-only window into the arbitrary region of
183 memory defined by $pointer, pointing to the start of the region and
184 $size, the size of the region. If $size is omitted then it will assume
185 a C style string and use the C "strlen" function to determine the size
186 (the terminating '\0' will not be included).
187 This can be useful if you have a C function that returns a buffer pair
189 (pointer, size), and want to access it from Perl without having to copy
190 the data. This can also be useful when interfacing with programming
191 languages that store strings as a address/length pair instead of a
192 pointer to null-terminated sequence of bytes.
193 You can specify $utf8 to set the UTF-8 flag on the scalar. Note that
195 the behavior of setting the UTF-8 flag on a buffer that does not
196 contain UTF-8 as understood by the version of Perl that you are running
197 is undefined.
198 Hint: If you have a buffer that needs to be free'd by C once the scalar
200 falls out of scope you can use Variable::Magic to apply magic to the
201 scalar and free the pointer once it falls out of scope.
202 use FFI::Platypus::Buffer qw( scalar_to_pointer );
204 use FFI::Platypus::Memory qw( strdup free );
205 use Variable::Magic qw( wizard cast );
206 my $free_when_out_of_scope = wizard(
208 free => sub {
209 my $ptr = scalar_to_pointer ${$_[0]};
210 free $ptr;
211 }
212 );
213 my $ptr = strdup "Hello Perl";
215 my $scalar;
216 window $scalar, $ptr, 10;
217 cast $scalar, $free_when_out_of_scope;
218 undef $ptr; # don't need to track the pointer anymore.
219 # we can now use scalar as a regular read-only Perl variable
221 print $scalar, "\n"; # prints "Hello Perl" without the \0
222 # this will free the C pointer
224 undef $scalar;
225 Hint: Returning a scalar string from a Perl function actually copies
227 the value. If you want to return a string without copying then you
228 need to return a reference.
229 sub c_string
231 {
232 my $ptr = strdup "Hello Perl";
233 my $scalar;
234 window $scalar, $ptr, 10;
235 cast $scalar, $free_when_out_of_scope;
236 \$scalar;
237 }
238 my $ref = c_string();
240 print $$ref, "\n"; # prints "Hello Perl" without the \0
241 Not exported by default, but may be exported on request.
243
245 FFI::Platypus
246 Main Platypus documentation.
247
249 Author: Graham Ollis <plicease@cpan.org>
250 Contributors:
252 Bakkiaraj Murugesan (bakkiaraj)
254 Dylan Cali (calid)
256 pipcet
258 Zaki Mughal (zmughal)
260 Fitz Elliott (felliott)
262 Vickenty Fesunov (vyf)
264 Gregor Herrmann (gregoa)
266 Shlomi Fish (shlomif)
268 Damyan Ivanov
270 Ilya Pavlov (Ilya33)
272 Petr Písař (ppisar)
274 Mohammad S Anwar (MANWAR)
276 Håkon Hægland (hakonhagland, HAKONH)
278 Meredith (merrilymeredith, MHOWARD)
280 Diab Jerius (DJERIUS)
282 Eric Brine (IKEGAMI)
284 szTheory
286 José Joaquín Atria (JJATRIA)
288 Pete Houston (openstrike, HOUSTON)
290
292 This software is copyright (c) 2015-2022 by Graham Ollis.
293 This is free software; you can redistribute it and/or modify it under
295 the same terms as the Perl 5 programming language system itself.
296perl v5.34.1 2022-06-20 FFI::Platypus::Buffer(3)