malloc_info(3)

💖 malloc_info(3) 💖

okay so real talk bestie, this page is from the Library Functions Manual, and it's about a little function that just… tells you what your allocator is up to. and honestly? it's giving investigative journalist. 🕵️♀️✨

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🌸 NAME 🌸

malloc_info — export malloc state to a stream 🫶

📚 LIBRARY 📚

Standard C library (libc, -lc)

✨ SYNOPSIS ✨

#include <malloc.h>

int malloc_info(int options, FILE *stream);

👑 DESCRIPTION 👑

The malloc_info() function exports an XML string that describes the current state of the memory-allocation implementation in the caller. That string gets printed on the file stream stream. and like, not to be dramatic, but it includes info about all the arenas (peep malloc(3) for the lore on those 💅).

As currently implemented, options must be zero. no other value, no negotiating, zero. periodt.

🎀 RETURN VALUE 🎀

On success, malloc_info() returns 0. On failure, it returns -1, and errno is set to indicate the error. slay when it works, tears when it doesn't 😭

💀 ERRORS 💀

• EINVAL — options was nonzero. that's the whole list. you had one job options.

🦋 ATTRIBUTES 🦋

For an explanation of the terms used in this section, see attributes(7).

• malloc_info() — Thread safety — MT-Safe

highkey a flex that it's MT-Safe, we love a thread-safe icon 👑

📜 STANDARDS 📜

GNU.

🕰️ HISTORY 🕰️

glibc 2.10.

🧚 NOTES 🧚

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. so like, the version field is your little safety net, iykyk 💜

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. no disk, no drama, just vibes into a buffer 🌈

The malloc_info() function is designed to address deficiencies in malloc_stats(3) and mallinfo(3). it's the glow-up, fr fr.

💅 EXAMPLES 💅

The program below takes up to four command-line arguments, of which the first three are mandatory. Let's break it down bestie:

1. The first argument specifies the number of threads that the program should create.
2. All of the threads, including the main thread, allocate the number of blocks of memory specified by the second argument.
3. 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. it's a whole escalating situation and we're obsessed 📈

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);
       }

🔥 SEE ALSO 🔥

mallinfo(3), malloc(3), malloc_stats(3), mallopt(3), open_memstream(3)

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okay that's the whole page bestie, no notes, no crumbs left behind 🍓 this was brought to you by Linux man-pages 6.20, dated 2026-02-08 — go forth and inspect your arenas, you main character 👑✨💖