mcheck / mprobe - glibc

Split from the chapter glibc: Virtual Memory Allocation And Paging into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <mcheck.h>

int mcheck(void (*abortfunc)(enum mcheck_status mstatus));
int mcheck_pedantic(void (*abortfunc)(enum mcheck_status mstatus));
void mcheck_check_all(void);
enum mcheck_status mprobe(void *ptr);

/* mcheck.h lines 27-34 */
enum mcheck_status { MCHECK_DISABLED = -1, MCHECK_OK, MCHECK_FREE,
                     MCHECK_HEAD, MCHECK_TAIL };

No __USE_GNU / __USE_MISC wrapper: mcheck.h declares everything unconditionally (/usr/include/mcheck.h lines 41-53).

Semantics

mcheck installs debugging hooks for the malloc family that run heap consistency checks: double frees and clobbered bookkeeping data in front of a block are detected (mcheck.3). It must be called before the first allocation; it returns 0 then, -1 if called too late (in which case it does nothing). Linking with -lmcheck inserts an implicit mcheck(NULL) early enough. On inconsistency, abortfunc is invoked with the status; a NULL abortfunc means print to stderr and abort. mcheck_pedantic re-checks every block on every allocation call (slow); mcheck_check_all forces a sweep; mprobe checks one block on demand and returns MCHECK_DISABLED if mcheck was never active, MCHECK_OK when clean, else MCHECK_HEAD (prefix clobbered), MCHECK_TAIL (suffix clobbered) or MCHECK_FREE (freed twice).

Gotchas and portability

• Since glibc 2.34 the hooks are disabled in the main libc: mcheck needs LD_PRELOAD=libc_malloc_debug.so at runtime, or it silently does nothing (glibc manual, Heap Consistency Checking; NEWS 2.34).
• MALLOC_CHECK_ (a mallopt(3) topic) detects the same error classes without relinking (mcheck.3 NOTES).
• Modern practice is ASan/valgrind; mcheck is a legacy tool.
• musl 1.2.5: absent – no mcheck.h, no symbols anywhere in the tree.

References

• man3: mcheck.3 (covers mprobe, mcheck_pedantic, mcheck_check_all)
• glibc manual: memory.texi, node Heap Consistency Checking
• headers: /usr/include/mcheck.h (lines 27-53)
• symbols: nm -D --with-symbol-versions libc.so.6