1MALLINFO(3) Linux Programmer's Manual MALLINFO(3)
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6 mallinfo, mallinfo2 - obtain memory allocation information
7
9 #include <malloc.h>
10 struct mallinfo mallinfo(void);
12 struct mallinfo2 mallinfo2(void);
13
15 These functions return a copy of a structure containing information
16 about memory allocations performed by malloc(3) and related functions.
17 The structure returned by each function contains the same fields. How‐
18 ever, the older function, mallinfo(), is deprecated since the type used
19 for the fields is too small (see BUGS).
20 Note that not all allocations are visible to these functions; see BUGS
22 and consider using malloc_info(3) instead.
23 The mallinfo2 structure returned by mallinfo2() is defined as follows:
25 struct mallinfo2 {
27 size_t arena; /* Non-mmapped space allocated (bytes) */
28 size_t ordblks; /* Number of free chunks */
29 size_t smblks; /* Number of free fastbin blocks */
30 size_t hblks; /* Number of mmapped regions */
31 size_t hblkhd; /* Space allocated in mmapped regions
32 (bytes) */
33 size_t usmblks; /* See below */
34 size_t fsmblks; /* Space in freed fastbin blocks (bytes) */
35 size_t uordblks; /* Total allocated space (bytes) */
36 size_t fordblks; /* Total free space (bytes) */
37 size_t keepcost; /* Top-most, releasable space (bytes) */
38 };
39 The mallinfo structure returned by the deprecated mallinfo() function
41 is exactly the same, except that the fields are typed as int.
42 The structure fields contain the following information:
44 arena The total amount of memory allocated by means other than
46 mmap(2) (i.e., memory allocated on the heap). This figure
47 includes both in-use blocks and blocks on the free list.
48 ordblks The number of ordinary (i.e., non-fastbin) free blocks.
50 smblks The number of fastbin free blocks (see mallopt(3)).
52 hblks The number of blocks currently allocated using mmap(2). (See
54 the discussion of M_MMAP_THRESHOLD in mallopt(3).)
55 hblkhd The number of bytes in blocks currently allocated using
57 mmap(2).
58 usmblks This field is unused, and is always 0. Historically, it was
60 the "highwater mark" for allocated space—that is, the maximum
61 amount of space that was ever allocated (in bytes); this
62 field was maintained only in nonthreading environments.
63 fsmblks The total number of bytes in fastbin free blocks.
65 uordblks The total number of bytes used by in-use allocations.
67 fordblks The total number of bytes in free blocks.
69 keepcost The total amount of releasable free space at the top of the
71 heap. This is the maximum number of bytes that could ideally
72 (i.e., ignoring page alignment restrictions, and so on) be
73 released by malloc_trim(3).
74
76 The mallinfo2() function was added in glibc 2.33.
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 These functions are not specified by POSIX or the C standards. A
97 mallinfo() function exists on many System V derivatives, and was speci‐
98 fied in the SVID.
99
101 Information is returned for only the main memory allocation area. Al‐
102 locations in other arenas are excluded. See malloc_stats(3) and mal‐
103 loc_info(3) for alternatives that include information about other are‐
104 nas.
105 The fields of the mallinfo structure that is returned by the older
107 mallinfo() function are typed as int. However, because some internal
108 bookkeeping values may be of type long, the reported values may wrap
109 around zero and thus be inaccurate.
110
112 The program below employs mallinfo2() to retrieve memory allocation
113 statistics before and after allocating and freeing some blocks of memo‐
114 ry. The statistics are displayed on standard output.
115 The first two command-line arguments specify the number and size of
117 blocks to be allocated with malloc(3).
118 The remaining three arguments specify which of the allocated blocks
120 should be freed with free(3). These three arguments are optional, and
121 specify (in order): the step size to be used in the loop that frees
122 blocks (the default is 1, meaning free all blocks in the range); the
123 ordinal position of the first block to be freed (default 0, meaning the
124 first allocated block); and a number one greater than the ordinal posi‐
125 tion of the last block to be freed (default is one greater than the
126 maximum block number). If these three arguments are omitted, then the
127 defaults cause all allocated blocks to be freed.
128 In the following example run of the program, 1000 allocations of 100
130 bytes are performed, and then every second allocated block is freed:
131 $ ./a.out 1000 100 2
133 ============== Before allocating blocks ==============
134 Total non-mmapped bytes (arena): 0
135 # of free chunks (ordblks): 1
136 # of free fastbin blocks (smblks): 0
137 # of mapped regions (hblks): 0
138 Bytes in mapped regions (hblkhd): 0
139 Max. total allocated space (usmblks): 0
140 Free bytes held in fastbins (fsmblks): 0
141 Total allocated space (uordblks): 0
142 Total free space (fordblks): 0
143 Topmost releasable block (keepcost): 0
144 ============== After allocating blocks ==============
146 Total non-mmapped bytes (arena): 135168
147 # of free chunks (ordblks): 1
148 # of free fastbin blocks (smblks): 0
149 # of mapped regions (hblks): 0
150 Bytes in mapped regions (hblkhd): 0
151 Max. total allocated space (usmblks): 0
152 Free bytes held in fastbins (fsmblks): 0
153 Total allocated space (uordblks): 104000
154 Total free space (fordblks): 31168
155 Topmost releasable block (keepcost): 31168
156 ============== After freeing blocks ==============
158 Total non-mmapped bytes (arena): 135168
159 # of free chunks (ordblks): 501
160 # of free fastbin blocks (smblks): 0
161 # of mapped regions (hblks): 0
162 Bytes in mapped regions (hblkhd): 0
163 Max. total allocated space (usmblks): 0
164 Free bytes held in fastbins (fsmblks): 0
165 Total allocated space (uordblks): 52000
166 Total free space (fordblks): 83168
167 Topmost releasable block (keepcost): 31168
168 Program source
170 #include <malloc.h>
172 #include <stdlib.h>
173 #include <string.h>
174 static void
176 display_mallinfo2(void)
177 {
178 struct mallinfo2 mi;
179 mi = mallinfo2();
181 printf("Total non-mmapped bytes (arena): %zu\n", mi.arena);
183 printf("# of free chunks (ordblks): %zu\n", mi.ordblks);
184 printf("# of free fastbin blocks (smblks): %zu\n", mi.smblks);
185 printf("# of mapped regions (hblks): %zu\n", mi.hblks);
186 printf("Bytes in mapped regions (hblkhd): %zu\n", mi.hblkhd);
187 printf("Max. total allocated space (usmblks): %zu\n", mi.usmblks);
188 printf("Free bytes held in fastbins (fsmblks): %zu\n", mi.fsmblks);
189 printf("Total allocated space (uordblks): %zu\n", mi.uordblks);
190 printf("Total free space (fordblks): %zu\n", mi.fordblks);
191 printf("Topmost releasable block (keepcost): %zu\n", mi.keepcost);
192 }
193 int
195 main(int argc, char *argv[])
196 {
197 #define MAX_ALLOCS 2000000
198 char *alloc[MAX_ALLOCS];
199 int numBlocks, freeBegin, freeEnd, freeStep;
200 size_t blockSize;
201 if (argc < 3 || strcmp(argv[1], "--help") == 0) {
203 fprintf(stderr, "%s num-blocks block-size [free-step "
204 "[start-free [end-free]]]\n", argv[0]);
205 exit(EXIT_FAILURE);
206 }
207 numBlocks = atoi(argv[1]);
209 blockSize = atoi(argv[2]);
210 freeStep = (argc > 3) ? atoi(argv[3]) : 1;
211 freeBegin = (argc > 4) ? atoi(argv[4]) : 0;
212 freeEnd = (argc > 5) ? atoi(argv[5]) : numBlocks;
213 printf("============== Before allocating blocks ==============\n");
215 display_mallinfo2();
216 for (int j = 0; j < numBlocks; j++) {
218 if (numBlocks >= MAX_ALLOCS) {
219 fprintf(stderr, "Too many allocations\n");
220 exit(EXIT_FAILURE);
221 }
222 alloc[j] = malloc(blockSize);
224 if (alloc[j] == NULL) {
225 perror("malloc");
226 exit(EXIT_FAILURE);
227 }
228 }
229 printf("\n============== After allocating blocks ==============\n");
231 display_mallinfo2();
232 for (int j = freeBegin; j < freeEnd; j += freeStep)
234 free(alloc[j]);
235 printf("\n============== After freeing blocks ==============\n");
237 display_mallinfo2();
238 exit(EXIT_SUCCESS);
240 }
241
243 mmap(2), malloc(3), malloc_info(3), malloc_stats(3), malloc_trim(3),
244 mallopt(3)
245
247 This page is part of release 5.13 of the Linux man-pages project. A
248 description of the project, information about reporting bugs, and the
249 latest version of this page, can be found at
250 https://www.kernel.org/doc/man-pages/.
251Linux 2021-03-22 MALLINFO(3)