1mallinfo(3) Library Functions Manual mallinfo(3)
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6 mallinfo, mallinfo2 - obtain memory allocation information
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9 Standard C library (libc, -lc)
10
12 #include <malloc.h>
13 struct mallinfo mallinfo(void);
15 struct mallinfo2 mallinfo2(void);
16
18 These functions return a copy of a structure containing information
19 about memory allocations performed by malloc(3) and related functions.
20 The structure returned by each function contains the same fields. How‐
21 ever, the older function, mallinfo(), is deprecated since the type used
22 for the fields is too small (see BUGS).
23 Note that not all allocations are visible to these functions; see BUGS
25 and consider using malloc_info(3) instead.
26 The mallinfo2 structure returned by mallinfo2() is defined as follows:
28 struct mallinfo2 {
30 size_t arena; /* Non-mmapped space allocated (bytes) */
31 size_t ordblks; /* Number of free chunks */
32 size_t smblks; /* Number of free fastbin blocks */
33 size_t hblks; /* Number of mmapped regions */
34 size_t hblkhd; /* Space allocated in mmapped regions
35 (bytes) */
36 size_t usmblks; /* See below */
37 size_t fsmblks; /* Space in freed fastbin blocks (bytes) */
38 size_t uordblks; /* Total allocated space (bytes) */
39 size_t fordblks; /* Total free space (bytes) */
40 size_t keepcost; /* Top-most, releasable space (bytes) */
41 };
42 The mallinfo structure returned by the deprecated mallinfo() function
44 is exactly the same, except that the fields are typed as int.
45 The structure fields contain the following information:
47 arena The total amount of memory allocated by means other than
49 mmap(2) (i.e., memory allocated on the heap). This figure
50 includes both in-use blocks and blocks on the free list.
51 ordblks The number of ordinary (i.e., non-fastbin) free blocks.
53 smblks The number of fastbin free blocks (see mallopt(3)).
55 hblks The number of blocks currently allocated using mmap(2). (See
57 the discussion of M_MMAP_THRESHOLD in mallopt(3).)
58 hblkhd The number of bytes in blocks currently allocated using
60 mmap(2).
61 usmblks This field is unused, and is always 0. Historically, it was
63 the "highwater mark" for allocated space—that is, the maximum
64 amount of space that was ever allocated (in bytes); this
65 field was maintained only in nonthreading environments.
66 fsmblks The total number of bytes in fastbin free blocks.
68 uordblks The total number of bytes used by in-use allocations.
70 fordblks The total number of bytes in free blocks.
72 keepcost The total amount of releasable free space at the top of the
74 heap. This is the maximum number of bytes that could ideally
75 (i.e., ignoring page alignment restrictions, and so on) be
76 released by malloc_trim(3).
77
79 For an explanation of the terms used in this section, see at‐
80 tributes(7).
81 ┌────────────┬───────────────┬─────────────────────────────────────────┐
83 │Interface │ Attribute │ Value │
84 ├────────────┼───────────────┼─────────────────────────────────────────┤
85 │mallinfo(), │ Thread safety │ MT-Unsafe init const:mallopt │
86 │mallinfo2() │ │ │
87 └────────────┴───────────────┴─────────────────────────────────────────┘
88 mallinfo()/ mallinfo2() would access some global internal objects. If
89 modify them with non-atomically, may get inconsistent results. The
90 identifier mallopt in const:mallopt mean that mallopt() would modify
91 the global internal objects with atomics, that make sure mallinfo()/
92 mallinfo2() is safe enough, others modify with non-atomically maybe
93 not.
94
96 None.
97
99 mallinfo()
100 glibc 2.0. SVID.
101 mallinfo2()
103 glibc 2.33.
104
106 Information is returned for only the main memory allocation area.
107 Allocations in other arenas are excluded. See malloc_stats(3) and
108 malloc_info(3) for alternatives that include information about other
109 arenas.
110 The fields of the mallinfo structure that is returned by the older
112 mallinfo() function are typed as int. However, because some internal
113 bookkeeping values may be of type long, the reported values may wrap
114 around zero and thus be inaccurate.
115
117 The program below employs mallinfo2() to retrieve memory allocation
118 statistics before and after allocating and freeing some blocks of
119 memory. The statistics are displayed on standard output.
120 The first two command-line arguments specify the number and size of
122 blocks to be allocated with malloc(3).
123 The remaining three arguments specify which of the allocated blocks
125 should be freed with free(3). These three arguments are optional, and
126 specify (in order): the step size to be used in the loop that frees
127 blocks (the default is 1, meaning free all blocks in the range); the
128 ordinal position of the first block to be freed (default 0, meaning the
129 first allocated block); and a number one greater than the ordinal
130 position of the last block to be freed (default is one greater than the
131 maximum block number). If these three arguments are omitted, then the
132 defaults cause all allocated blocks to be freed.
133 In the following example run of the program, 1000 allocations of 100
135 bytes are performed, and then every second allocated block is freed:
136 $ ./a.out 1000 100 2
138 ============== Before allocating blocks ==============
139 Total non-mmapped bytes (arena): 0
140 # of free chunks (ordblks): 1
141 # of free fastbin blocks (smblks): 0
142 # of mapped regions (hblks): 0
143 Bytes in mapped regions (hblkhd): 0
144 Max. total allocated space (usmblks): 0
145 Free bytes held in fastbins (fsmblks): 0
146 Total allocated space (uordblks): 0
147 Total free space (fordblks): 0
148 Topmost releasable block (keepcost): 0
149 ============== After allocating blocks ==============
151 Total non-mmapped bytes (arena): 135168
152 # of free chunks (ordblks): 1
153 # of free fastbin blocks (smblks): 0
154 # of mapped regions (hblks): 0
155 Bytes in mapped regions (hblkhd): 0
156 Max. total allocated space (usmblks): 0
157 Free bytes held in fastbins (fsmblks): 0
158 Total allocated space (uordblks): 104000
159 Total free space (fordblks): 31168
160 Topmost releasable block (keepcost): 31168
161 ============== After freeing blocks ==============
163 Total non-mmapped bytes (arena): 135168
164 # of free chunks (ordblks): 501
165 # of free fastbin blocks (smblks): 0
166 # of mapped regions (hblks): 0
167 Bytes in mapped regions (hblkhd): 0
168 Max. total allocated space (usmblks): 0
169 Free bytes held in fastbins (fsmblks): 0
170 Total allocated space (uordblks): 52000
171 Total free space (fordblks): 83168
172 Topmost releasable block (keepcost): 31168
173 Program source
175 #include <malloc.h>
177 #include <stdlib.h>
178 #include <string.h>
179 static void
181 display_mallinfo2(void)
182 {
183 struct mallinfo2 mi;
184 mi = mallinfo2();
186 printf("Total non-mmapped bytes (arena): %zu\n", mi.arena);
188 printf("# of free chunks (ordblks): %zu\n", mi.ordblks);
189 printf("# of free fastbin blocks (smblks): %zu\n", mi.smblks);
190 printf("# of mapped regions (hblks): %zu\n", mi.hblks);
191 printf("Bytes in mapped regions (hblkhd): %zu\n", mi.hblkhd);
192 printf("Max. total allocated space (usmblks): %zu\n", mi.usmblks);
193 printf("Free bytes held in fastbins (fsmblks): %zu\n", mi.fsmblks);
194 printf("Total allocated space (uordblks): %zu\n", mi.uordblks);
195 printf("Total free space (fordblks): %zu\n", mi.fordblks);
196 printf("Topmost releasable block (keepcost): %zu\n", mi.keepcost);
197 }
198 int
200 main(int argc, char *argv[])
201 {
202 #define MAX_ALLOCS 2000000
203 char *alloc[MAX_ALLOCS];
204 size_t blockSize, numBlocks, freeBegin, freeEnd, freeStep;
205 if (argc < 3 || strcmp(argv[1], "--help") == 0) {
207 fprintf(stderr, "%s num-blocks block-size [free-step "
208 "[start-free [end-free]]]\n", argv[0]);
209 exit(EXIT_FAILURE);
210 }
211 numBlocks = atoi(argv[1]);
213 blockSize = atoi(argv[2]);
214 freeStep = (argc > 3) ? atoi(argv[3]) : 1;
215 freeBegin = (argc > 4) ? atoi(argv[4]) : 0;
216 freeEnd = (argc > 5) ? atoi(argv[5]) : numBlocks;
217 printf("============== Before allocating blocks ==============\n");
219 display_mallinfo2();
220 for (size_t j = 0; j < numBlocks; j++) {
222 if (numBlocks >= MAX_ALLOCS) {
223 fprintf(stderr, "Too many allocations\n");
224 exit(EXIT_FAILURE);
225 }
226 alloc[j] = malloc(blockSize);
228 if (alloc[j] == NULL) {
229 perror("malloc");
230 exit(EXIT_FAILURE);
231 }
232 }
233 printf("\n============== After allocating blocks ==============\n");
235 display_mallinfo2();
236 for (size_t j = freeBegin; j < freeEnd; j += freeStep)
238 free(alloc[j]);
239 printf("\n============== After freeing blocks ==============\n");
241 display_mallinfo2();
242 exit(EXIT_SUCCESS);
244 }
245
247 mmap(2), malloc(3), malloc_info(3), malloc_stats(3), malloc_trim(3),
248 mallopt(3)
249Linux man-pages 6.05 2023-07-20 mallinfo(3)