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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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malloc_info(3) Library Functions Manual malloc_info(3)
malloc_info - export malloc state to a stream
Standard C library (libc, -lc)
#include <malloc.h>
int malloc_info(int options, FILE *stream);
The malloc_info() function exports an XML string that describes
the current state of the memory-allocation implementation in the
caller. The string is printed on the file stream stream. The
exported string includes information about all arenas (see
malloc(3)).
As currently implemented, options must be zero.
On success, malloc_info() returns 0. On failure, it returns -1,
and errno is set to indicate the error.
EINVAL options was nonzero.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ malloc_info() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
GNU.
glibc 2.10.
The memory-allocation information is provided as an XML string
(rather than a C structure) because the information may change
over time (according to changes in the underlying implementation).
The output XML string includes a version field.
The open_memstream(3) function can be used to send the output of
malloc_info() directly into a buffer in memory, rather than to a
file.
The malloc_info() function is designed to address deficiencies in
malloc_stats(3) and mallinfo(3).
The program below takes up to four command-line arguments, of
which the first three are mandatory. The first argument specifies
the number of threads that the program should create. All of the
threads, including the main thread, allocate the number of blocks
of memory specified by the second argument. The third argument
controls the size of the blocks to be allocated. The main thread
creates blocks of this size, the second thread created by the
program allocates blocks of twice this size, the third thread
allocates blocks of three times this size, and so on.
The program calls malloc_info() twice to display the memory-
allocation state. The first call takes place before any threads
are created or memory allocated. The second call is performed
after all threads have allocated memory.
In the following example, the command-line arguments specify the
creation of one additional thread, and both the main thread and
the additional thread allocate 10000 blocks of memory. After the
blocks of memory have been allocated, malloc_info() shows the
state of two allocation arenas.
$ getconf GNU_LIBC_VERSION
glibc 2.13
$ ./a.out 1 10000 100
============ Before allocating blocks ============
<malloc version="1">
<heap nr="0">
<sizes>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="0" size="0"/>
<system type="current" size="135168"/>
<system type="max" size="135168"/>
<aspace type="total" size="135168"/>
<aspace type="mprotect" size="135168"/>
</heap>
<total type="fast" count="0" size="0"/>
<total type="rest" count="0" size="0"/>
<system type="current" size="135168"/>
<system type="max" size="135168"/>
<aspace type="total" size="135168"/>
<aspace type="mprotect" size="135168"/>
</malloc>
============ After allocating blocks ============
<malloc version="1">
<heap nr="0">
<sizes>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="0" size="0"/>
<system type="current" size="1081344"/>
<system type="max" size="1081344"/>
<aspace type="total" size="1081344"/>
<aspace type="mprotect" size="1081344"/>
</heap>
<heap nr="1">
<sizes>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="0" size="0"/>
<system type="current" size="1032192"/>
<system type="max" size="1032192"/>
<aspace type="total" size="1032192"/>
<aspace type="mprotect" size="1032192"/>
</heap>
<total type="fast" count="0" size="0"/>
<total type="rest" count="0" size="0"/>
<system type="current" size="2113536"/>
<system type="max" size="2113536"/>
<aspace type="total" size="2113536"/>
<aspace type="mprotect" size="2113536"/>
</malloc>
Program source
#include <err.h>
#include <errno.h>
#include <malloc.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
static size_t blockSize;
static size_t numThreads;
static unsigned int numBlocks;
static void *
thread_func(void *arg)
{
int tn = (int) arg;
/* The multiplier '(2 + tn)' ensures that each thread (including
the main thread) allocates a different amount of memory. */
for (unsigned int j = 0; j < numBlocks; j++)
if (malloc(blockSize * (2 + tn)) == NULL)
err(EXIT_FAILURE, "malloc-thread");
sleep(100); /* Sleep until main thread terminates. */
return NULL;
}
int
main(int argc, char *argv[])
{
int sleepTime;
pthread_t *thr;
if (argc < 4) {
fprintf(stderr,
"%s num-threads num-blocks block-size [sleep-time]\n",
argv[0]);
exit(EXIT_FAILURE);
}
numThreads = atoi(argv[1]);
numBlocks = atoi(argv[2]);
blockSize = atoi(argv[3]);
sleepTime = (argc > 4) ? atoi(argv[4]) : 0;
thr = calloc(numThreads, sizeof(*thr));
if (thr == NULL)
err(EXIT_FAILURE, "calloc");
printf("============ Before allocating blocks ============\n");
malloc_info(0, stdout);
/* Create threads that allocate different amounts of memory. */
for (size_t tn = 0; tn < numThreads; tn++) {
errno = pthread_create(&thr[tn], NULL, thread_func,
(void *) tn);
if (errno != 0)
err(EXIT_FAILURE, "pthread_create");
/* If we add a sleep interval after the start-up of each
thread, the threads likely won't contend for malloc
mutexes, and therefore additional arenas won't be
allocated (see malloc(3)). */
if (sleepTime > 0)
sleep(sleepTime);
}
/* The main thread also allocates some memory. */
for (unsigned int j = 0; j < numBlocks; j++)
if (malloc(blockSize) == NULL)
err(EXIT_FAILURE, "malloc");
sleep(2); /* Give all threads a chance to
complete allocations. */
printf("\n============ After allocating blocks ============\n");
malloc_info(0, stdout);
exit(EXIT_SUCCESS);
}
mallinfo(3), malloc(3), malloc_stats(3), mallopt(3),
open_memstream(3)
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user-space interface documentation) project. Information about
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 malloc_info(3)
Pages that refer to this page: systemd-analyze(1), mallinfo(3), malloc(3), malloc_stats(3), mallopt(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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