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1.1 root 1: /*
2: * libc/gen/malloc/malloc.c
3: * Memory allocation routines.
4: * The memory arena is a circular linked list rooted at __a_first.
5: * Each arena is subdivided into two or more blocks.
6: * The first word of each block gives its length,
7: * with the low order bit set if the block is free;
8: * block lengths are always multiples of 2 to allow free marking.
9: * A length word containing 0 marks the end of an arena;
10: * in this case the following pointer points to the next arena.
11: */
12:
13: #include <stdio.h>
14: #include <sys/malloc.h>
15:
16: MBLOCK *__a_scanp = NULL; /* start search here */
17: MBLOCK *__a_first = NULL; /* first arena */
18: MBLOCK *__a_top = NULL; /* end of last sbrk */
19: unsigned __a_count = 0; /* number of blocks */
20:
21: extern char *sbrk();
22:
23: /*
24: * Get a new arena from sbrk() and hook it to the old list.
25: * Return 1 if there is any chance of malloc succeeding, else 0.
26: * Note that this assumes the argument to sbrk() is unsigned;
27: * bad news if sbrk() expects int and shrinks the break if negative.
28: */
29: static
30: int
31: newarena(size) unsigned size;
32: {
33: register MBLOCK *mp, *pmp, *linkmp;
34: register unsigned len, max;
35: static char failed = 0;
36:
37: if (failed) /* no more room */
38: return 0;
39:
40: /* Force sbrk() to alignment boundary. */
41: if ((len = ((unsigned)sbrk(0)) & (ALIGNMENT - 1)) != 0
42: && (sbrk(ALIGNMENT - len) == BADSBRK)) {
43: failed = 1;
44: return 1;
45: }
46:
47: /* Add space for end mblock, round up to 2^ARENASIZE */
48: len = roundup(size + sizeof(MBLOCK), ARENASIZE);
49: if (len < size)
50: len = size;
51:
52: __a_scanp = __a_first; /* rescan from the begining */
53:
54: /*
55: * If there isn't enough space get what we can it may be enough.
56: * This means further calls to newarena must fail.
57: */
58: while ((mp = (MBLOCK *)sbrk(len)) == BADSBRK) {
59: if (failed == 0 && (max = ulimit(3)) != -1 && len > max)
60: len = max; /* use system limit as upper bound */
61: failed = 1;
62: if (len <= DECRSIZE)
63: return 1; /* even zero may be ok */
64: len -= DECRSIZE;
65: if (sizeof(MBLOCK) > len)
66: len = sizeof(MBLOCK);
67: }
68:
69: if (__a_top == NULL) { /* first time through */
70: __a_count = 2;
71: __a_first = __a_scanp = linkmp = mp;
72: }
73: else if (__a_top == mp) { /* new arena follows old */
74: /*
75: * The following assumes that len + sizeof(MBLOCK)
76: * will not be greater than the maximum unsigned value,
77: * which will be true if 2^ARENASIZE > sizeof(MBLOCK).
78: */
79: --mp;
80: len += sizeof(MBLOCK);
81: linkmp = mp->uval.next;
82: __a_count++;
83: }
84: else { /* discontigous arenas */
85: pmp = __a_top - 1;
86: linkmp = pmp->uval.next; /* save old pointer */
87: pmp->uval.next = mp; /* old points to new */
88: __a_count += 2;
89: }
90: mp->blksize = (len - sizeof(MBLOCK)) | FREE;
91: __a_top = bumpp(mp, len);
92: pmp = __a_top - 1;
93: pmp->blksize = 0;
94: pmp->uval.next = linkmp;
95: return 1;
96: }
97:
98: /*
99: * Allocate memory.
100: * Successive free blocks are consolidated when found.
101: */
102: char *
103: malloc(size) unsigned size;
104: {
105: register MBLOCK *mp, *prevmp;
106: register unsigned len, needed, counter;
107: static char msg[] = "Bad pointer in malloc.\r\n";
108:
109: if (size == 0)
110: return NULL;
111: needed = roundup(size + sizeof(unsigned), BLOCKSIZE);
112: if (needed < size)
113: return NULL;
114:
115: do { /* until we find enough or newarena fails */
116: for (counter = __a_count, mp = __a_scanp, prevmp = NULL; counter--; ) {
117: if (!isfree(len = mp->blksize)) { /* used block or pointer */
118: mp = (len) ? bumpp(mp, len) : mp->uval.next;
119: prevmp = NULL;
120: continue;
121: }
122:
123: if (prevmp != NULL) { /* consolidate free */
124: #if 0
125: /*
126: * The following assumes adjacent free blocks can be consolidated without
127: * overflow of the size. The overflow test is conditionalized out here,
128: * but it may be required on some machines. In i8086 LARGE model, the code
129: * works without the test but only barely. When the memory arena gets larger
130: * than 64K, sbrk() returns mp pointing to the same memory location as
131: * newarena()/__a_top, but with a different segment:offset representation;
132: * thus the "if (__a_top == mp)" test in newarena fails (even though they
133: * point to the same memory location) and newarena() leaves a 0 end marker
134: * between the arenas.
135: */
136: if (prevmp->blksize + realsize(len) > len) {
137: len = ((mp = prevmp)->blksize += realsize(len));
138: __a_count--;
139: }
140: #else
141: len = ((mp = prevmp)->blksize += realsize(len));
142: __a_count--;
143: #endif
144: }
145:
146: if (len < needed) { /* free but too small */
147: mp = bumpp(prevmp = mp, realsize(len));
148: continue;
149: }
150:
151: if ((len -= needed) < LEASTFREE)
152: /* grab the entire block */
153: __a_scanp = bumpp(mp, mp->blksize = needed = realsize(mp->blksize));
154: else {
155: /* split into used and free portions */
156: mp->blksize = needed;
157: __a_scanp = bumpp(mp, needed);
158: __a_scanp->blksize = len;
159: __a_count++;
160: }
161: return mp->uval.usera;
162: }
163:
164: /*
165: * There should have been __a_count blocks bringing us full circle.
166: */
167: if (mp != __a_scanp) {
168: write(2, msg, sizeof(msg) - 1);
169: abort();
170: }
171:
172: /* Not enough room in the current arena, allocate a new one. */
173: } while (newarena(needed));
174: return NULL;
175: }
176:
177: /*
178: * Free a block.
179: * Some sanity checking.
180: * Adjacent free block consolidation happens in malloc(), not here.
181: */
182: void
183: free(cp) char *cp;
184: {
185: register MBLOCK *mp;
186: register unsigned len;
187:
188: if (NULL == cp) /* ansi 4.10.3.2: free(NULL) has no effect */
189: return;
190:
191: mp = mblockp(cp);
192: len = mp->blksize;
193: if (len < LEASTFREE) { /* This can't exist */
194: static char msg[] = "Bad pointer in free.\r\n";
195:
196: write(2, msg, sizeof(msg) - 1);
197: abort();
198: }
199: mp->blksize |= FREE; /* mark free */
200:
201: /*
202: * If freed block precedes scan pointer reset the scan pointer.
203: */
204: if (bumpp(mp, realsize(len)) == __a_scanp)
205: __a_scanp = mp;
206: }
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