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MALLOC(9) FreeBSD Kernel Developer's Manual MALLOC(9) NAME malloc, mallocarray, free - kernel memory allocator SYNOPSIS #include <sys/types.h> #include <sys/malloc.h> void * malloc(size_t size, int type, int flags); void * mallocarray(size_t nmemb, size_t size, int type, int flags); void free(void *addr, int type, size_t size); DESCRIPTION The malloc() function allocates uninitialized memory in kernel address space for an object whose size is specified by size. The mallocarray() function is the same as malloc(), but allocates space for an array of nmemb objects and checks for arithmetic overflow. The free() function releases memory at address addr that was previously allocated by malloc() or mallocarray() for re-use. The same object size originally provided to malloc() should be specified by size, because free() will operate faster knowing this. If tracking the size is difficult, specify size as 0. If addr is a null pointer, no action occurs. The flags argument affects the operational characteristics of malloc() and mallocarray() as follows: M_WAITOK If memory is currently unavailable, malloc() may call sleep to wait for resources to be released by other processes. M_NOWAIT Causes malloc() to return NULL if the request cannot be immediately fulfilled due to resource shortage. M_CANFAIL In the M_WAITOK case, if not enough memory is available, return NULL instead of calling panic(9). If mallocarray() detects an overflow or malloc() detects an excessive allocation, return NULL instead of calling panic(9). M_ZERO Causes allocated memory to be zeroed. One of M_NOWAIT or M_WAITOK must be specified via the flags argument. The type argument broadly identifies the kernel subsystem for which the allocated memory was needed, and is commonly used to maintain statistics about kernel memory usage. These statistics can be examined using vmstat(8) or systat(1) if either of the kernel options(4) KMEMSTATS or DEBUG are enabled. The following types are currently defined: M_DEVBUF Device driver memory. M_PCB Protocol control blocks. M_RTABLE Routing tables. M_PF Packet filter structures. M_IFADDR Interface addresses. M_IFGROUP Interface groups. M_SYSCTL Sysctl persistent buffers. M_COUNTERS Per-CPU counters via counters_alloc(9). M_IOCTLOPS Ioctl data buffers. M_IOV Large IOVs. M_MOUNT VFS mount structs. M_NFSREQ NFS request headers. M_NFSMNT NFS mount structures. M_LOG Messages in kernel log stash. M_VNODE Dynamically allocated vnodes. M_DQUOT UFS quota entries. M_UFSMNT UFS mount structures. M_SHM SVID compatible shared memory segments. M_VMMAP VM map structures. M_SEM SVID compatible semaphores. M_DIRHASH UFS directory hash structures. M_ACPI ACPI structures. M_VMPMAP VM pmap data. M_FILEDESC Open file descriptor tables. M_SIGIO Sigio structures. M_PROC Proc structures. M_SUBPROC Proc sub-structures. M_MFSNODE MFS vnode private part. M_NETADDR Export host address structures. M_NFSSVC NFS server structures. M_NFSD NFS server daemon structures. M_IPMOPTS Internet multicast options. M_IPMADDR Internet multicast addresses. M_IFMADDR Link-level multicast addresses. M_MRTABLE Multicast routing tables. M_ISOFSMNT ISOFS mount structures. M_ISOFSNODE ISOFS vnode private part. M_MSDOSFSMNT MSDOS FS mount structures. M_MSDOSFSFAT MSDOS FS FAT tables. M_MSDOSFSNODE MSDOS FS vnode private part. M_TTYS Allocated tty structures. M_EXEC Argument lists & other mem used by exec. M_MISCFSMNT Miscellaneous FS mount structures. M_FUSEFS FUSE FS mount structures. M_PINSYSCALL pinsyscalls(2) related data. M_PFKEY Pfkey data. M_TDB Transforms database. M_XDATA IPsec data. M_PAGEDEP File page dependencies. M_INODEDEP Inode dependencies. M_NEWBLK New block allocation. M_INDIRDEP Indirect block dependencies. M_VMSWAP VM swap structures. M_UVMAMAP UVM amap and related. M_UVMAOBJ UVM aobj and related. M_USB USB general. M_USBDEV USB device driver. M_USBHC USB host controller. M_WITNESS witness(4) memory. M_MEMDESC Memory range. M_CRYPTO_DATA crypto(9) data buffers. M_CREDENTIALS ipsec(4) related credentials. M_IP6OPT IPv6 options. M_IP6NDP IPv6 Neighbor Discovery structures. M_TEMP Miscellaneous temporary data buffers. M_NTFSMNT NTFS mount structures. M_NTFSNTNODE NTFS ntnode information. M_NTFSFNODE NTFS fnode information. M_NTFSDIR NTFS directory buffers. M_NTFSNTHASH NTFS ntnode hash tables. M_NTFSNTVATTR NTFS file attribute information. M_NTFSRDATA NTFS resident data. M_NTFSDECOMP NTFS decompression temporary storage. M_NTFSRUN NTFS vrun storage. M_KEVENT kqueue(2) data structures. M_SYNCACHE SYN cache hash array. M_UDFMOUNT UDF mount structures. M_UDFFENTRY UDF file entries. M_UDFFID UDF file IDs. M_AGP AGP memory. M_DRM Direct Rendering Manager. CONTEXT malloc() and mallocarray() can be called during autoconf, from process context, or from interrupt context if M_NOWAIT is passed via flags. They can't be called from interrupt context if M_WAITOK is passed via flags. free() can be called during autoconf, from process context, or from interrupt context. RETURN VALUES malloc() and mallocarray() return a kernel virtual address that is suitably aligned for storage of any type of object. DIAGNOSTICS A kernel compiled with the DIAGNOSTIC configuration option attempts to detect memory corruption caused by such things as writing outside the allocated area and unbalanced calls to malloc() or mallocarray(), and free(). Failing consistency checks will cause a panic or a system console message: o panic: "malloc: bogus type" o panic: "malloc: out of space in kmem_map" o panic: "malloc: allocation too large" o panic: "malloc: wrong bucket" o panic: "malloc: lost data" o panic: "mallocarray: overflow" o panic: "free: unaligned addr" o panic: "free: duplicated free" o panic: "free: multiple frees" o panic: "free: non-malloced addr" o panic: "free: size too large" o panic: "free: size too small" o panic: "kmeminit: minbucket too small/struct freelist too big" o "multiply freed item <addr>" o "Data modified on freelist: <data object description>" SEE ALSO systat(1), vmstat(8) FreeBSD 14.1-RELEASE-p8 January 19, 2024 FreeBSD 14.1-RELEASE-p8