posix shared memory, p325~p335

fork로 나온 parent, child와 다르게 완전 다른 프로세스간 shared memory로 데이터를 공유할 수 있다.

pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ cat make_memory.c 
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#define FILE_MODE 0666

int
main(int argc, char **argv)
{
	int		c, fd, flags;
	char	*ptr;
	off_t	length;

	flags = O_RDWR | O_CREAT;
	while ( (c = getopt(argc, argv, "e")) != -1) {
		switch (c) {
		case 'e':
			flags |= O_EXCL;
			break;
		}
	}
	if (optind != argc - 2)
		perror("usage: shmcreate [ -e ] <name> <length>");
	length = atoi(argv[optind + 1]);

	fd = shm_open(argv[optind], flags, FILE_MODE);
	ftruncate(fd, length);

	ptr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

	exit(0);
}

pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ cat unlink.c 

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#define FILE_MODE 0666



int
main(int argc, char **argv)
{
	if (argc != 2)
		perror("usage: shmunlink <name>");

	shm_unlink(argv[1]);

	exit(0);
}

pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ cat write.c 
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#define FILE_MODE 0666

int
main(int argc, char **argv)
{
	int		i, fd;
	struct stat	stat;
	unsigned char	*ptr;

	if (argc != 2)
		perror("usage: shmwrite <name>");

		/* 4open, get size, map */
	fd = shm_open(argv[1], O_RDWR, FILE_MODE);
	fstat(fd, &stat);
	ptr = mmap(NULL, stat.st_size, PROT_READ | PROT_WRITE,
			   MAP_SHARED, fd, 0);
	close(fd);

		/* 4set: ptr[0] = 0, ptr[1] = 1, etc. */
	for (i = 0; i < stat.st_size; i++)
		*ptr++ = i % 256;

	exit(0);
}

pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ cat read.c 
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#define FILE_MODE 0400

int
main(int argc, char **argv)
{
	int		i, fd;
	struct stat	stat;
	unsigned char	c, *ptr;

	if (argc != 2)
		perror("usage: shmread <name>");

		/* 4open, get size, map */
	fd = shm_open(argv[1], O_RDONLY, FILE_MODE);
	fstat(fd, &stat);
	ptr = mmap(NULL, stat.st_size, PROT_READ,
			   MAP_SHARED, fd, 0);
	close(fd);

		/* 4check that ptr[0] = 0, ptr[1] = 1, etc. */
	for (i = 0; i < stat.st_size; i++)
		if ( (c = *ptr++) == (i % 256))
			printf("ptr[%d] = %d\n", i, c);

	exit(0);
}

이를 실행하면 다음과 같다. 512 길이 메모리를 할당하고, 250~259번지를 보면 다시 0으로 돌아간다. od로 확인하면 255에서 0으로 갔음을 본다. 파일 사용하는 방법과 비슷하다.

pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ ./makemeory test 512
pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ ./write test
pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ od -A d -t u1 -t x2 /dev/shm/test 
0000000   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
           0100    0302    0504    0706    0908    0b0a    0d0c    0f0e
0000016  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31
           1110    1312    1514    1716    1918    1b1a    1d1c    1f1e
0000032  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47
           2120    2322    2524    2726    2928    2b2a    2d2c    2f2e
0000048  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63
           3130    3332    3534    3736    3938    3b3a    3d3c    3f3e
0000064  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79
           4140    4342    4544    4746    4948    4b4a    4d4c    4f4e
0000080  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95
           5150    5352    5554    5756    5958    5b5a    5d5c    5f5e
0000096  96  97  98  99 100 101 102 103 104 105 106 107 108 109 110 111
           6160    6362    6564    6766    6968    6b6a    6d6c    6f6e
0000112 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
           7170    7372    7574    7776    7978    7b7a    7d7c    7f7e
0000128 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
           8180    8382    8584    8786    8988    8b8a    8d8c    8f8e
0000144 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159
           9190    9392    9594    9796    9998    9b9a    9d9c    9f9e
0000160 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175
           a1a0    a3a2    a5a4    a7a6    a9a8    abaa    adac    afae
0000176 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191
           b1b0    b3b2    b5b4    b7b6    b9b8    bbba    bdbc    bfbe
0000192 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207
           c1c0    c3c2    c5c4    c7c6    c9c8    cbca    cdcc    cfce
0000208 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223
           d1d0    d3d2    d5d4    d7d6    d9d8    dbda    dddc    dfde
0000224 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
           e1e0    e3e2    e5e4    e7e6    e9e8    ebea    edec    efee
0000240 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
           f1f0    f3f2    f5f4    f7f6    f9f8    fbfa    fdfc    fffe
0000256   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
           0100    0302    0504    0706    0908    0b0a    0d0c    0f0e
0000272  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31
           1110    1312    1514    1716    1918    1b1a    1d1c    1f1e
0000288  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47
           2120    2322    2524    2726    2928    2b2a    2d2c    2f2e
0000304  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63
           3130    3332    3534    3736    3938    3b3a    3d3c    3f3e
0000320  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79
           4140    4342    4544    4746    4948    4b4a    4d4c    4f4e
0000336  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95
           5150    5352    5554    5756    5958    5b5a    5d5c    5f5e
0000352  96  97  98  99 100 101 102 103 104 105 106 107 108 109 110 111
           6160    6362    6564    6766    6968    6b6a    6d6c    6f6e
0000368 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
           7170    7372    7574    7776    7978    7b7a    7d7c    7f7e
0000384 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
           8180    8382    8584    8786    8988    8b8a    8d8c    8f8e
0000400 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159
           9190    9392    9594    9796    9998    9b9a    9d9c    9f9e
0000416 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175
           a1a0    a3a2    a5a4    a7a6    a9a8    abaa    adac    afae
0000432 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191
           b1b0    b3b2    b5b4    b7b6    b9b8    bbba    bdbc    bfbe
0000448 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207
           c1c0    c3c2    c5c4    c7c6    c9c8    cbca    cdcc    cfce
0000464 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223
           d1d0    d3d2    d5d4    d7d6    d9d8    dbda    dddc    dfde
0000480 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
           e1e0    e3e2    e5e4    e7e6    e9e8    ebea    edec    efee
0000496 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
           f1f0    f3f2    f5f4    f7f6    f9f8    fbfa    fdfc    fffe
0000512
pi@raspberrypi:~/Project/cCode/IPC/shared_memory $ ./read test | grep -e "ptr\[25[0-9]\]"
ptr[250] = 250
ptr[251] = 251
ptr[252] = 252
ptr[253] = 253
ptr[254] = 254
ptr[255] = 255
ptr[256] = 0
ptr[257] = 1
ptr[258] = 2
ptr[259] = 3