75 lines
2.1 KiB
C
75 lines
2.1 KiB
C
// Persistent stack example of how mmap() naturally creates a software abstraction of persistent memory.
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// Author: Terence Kelly
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// tpkelly @ { acm.org, cs.princeton.edu, eecs.umich.edu }
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// Note: Use 'truncate' to make the initial (empty) backing file,
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// whose size should be a multiple of the system page size:
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//
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// prompt> getconf PAGESIZE
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// 4096
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// prompt> truncate -s 4096 ps.img
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//
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// makes a 4096-byte empty file. The first sizeof(size_t) bytes will
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// be interprted as the number of items on the stack; the remainder
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// of the file will contain the integers contained in the stack.
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// Now you can run the program:
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//
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// prompt> ./pstack 7 13 47 pop
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// 47
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// prompt> ./pstack pop pop 99
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// 13
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// 7
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// prompt> ./pstack pop
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// 99
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//
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// Notice that items push'd in one invocation of the program persist
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// until the next invocation: The stack is persistent.
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//
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// You can use hexdump to examine the contents of the backing file:
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//
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// prompt> hexdump ps.img
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#include <assert.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include <sys/mman.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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typedef struct {
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size_t n;
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int stack[]; // zero-length per C99
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} pstack_t;
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int main(int argc, char *argv[]) {
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int fd, rc;
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struct stat s;
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size_t file_size;
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pstack_t *p;
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fd = open("ps.img", O_RDWR);
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assert(fd > -1);
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rc = fstat(fd, &s);
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assert(rc == 0);
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file_size = (size_t) s.st_size;
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assert(file_size >= sizeof(pstack_t) && file_size % sizeof(int) == 0);
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p = (pstack_t *) mmap(NULL, file_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
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assert(p != MAP_FAILED);
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for (int i = 1; i < argc; i++)
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if (strcmp(argv[i], "pop") == 0) {
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if (p->n > 0) // stack not empty
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printf("%d\n", p->stack[--p->n]);
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} else { // push
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if (sizeof(pstack_t) + (1 + p->n) * sizeof(int) <= file_size) // stack not full
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p->stack[p->n++] = atoi(argv[i]);
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}
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(void) close(fd);
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return 0;
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}
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