libzzuf - Man Page

helper library for the zzuf multiple purpose fuzzer


libzzuf is a helper library automatically preloaded by zzuf when fuzzing applications, but it can also be used alone for debugging purposes or specific cases that cannot be covered by zzuf.


libzzuf must be preloaded using the operating system's default way of preloading libraries. For instance, on a typical Linux installation:


Environment Variables

libzzuf's initial setup is done through environment variables. After they are read, no further information can be sent to the fuzzed process.

All environment variables are optional.


This environment variable is set to the debugging level. 0 means no debugging information is sent to zzuf. 1 logs important information as well as all diverted functions. 2 logs fuzzing status and all optional file stream information.


This environment variable is set to a file descriptor where libzzuf will send debugging information. This is used to send data to the main zzuf controlling binary.


This variable is set to the initial random seed. Corresponding zzuf flag: --seed.


These variables are set to the minimal and maximal fuzzing ratios. Corresponding zzuf flag: --ratio.


If this variable is set, the random seed is incremented each time a new file is opened. Corresponding zzuf flag: --autoinc.


This variable contains byte ranges to which fuzzing should be restricted. Corresponding zzuf flag: --bytes.


This variable contains file descriptor ranges to which fuzzing should be restricted. Corresponding zzuf flag: --list.


If this variable is set, network mode is activated. Corresponding zzuf flag: --network.


This variable contains port ranges to which fuzzing should be restricted. Corresponding zzuf flag: --port.


These variables contain character ranges to protect or refuse. Corresponding zzuf flags: --protect, --refuse.


These variables contain regular expressions to indicate which files should be included or excluded from the list of fuzzed files. Corresponding zzuf flags: --include, --exclude.


If this variable is set, the fuzzed process will be prevented from installing signal handlers that usually cause coredumps. Corresponding zzuf flag: --signal.


This variable contains the maximum amount of memory that the fuzzed process is allowed to allocate. Corresponding zzuf flag: --max-memory.


If this variable is set, standard input will be fuzzed, too. Corresponding zzuf flag: --stdin.


In order to intercept file and network operations, signal handlers and memory allocations, libzzuf diverts and reimplements the following functions, which can sometimes be private C library symbols, too:

Unix file descriptor handling:

open(), dup(), dup2(), lseek(), read(), readv(), pread(), accept(), socket(), recv(), recvfrom(), recvmsg(), aio_read(), aio_return(), close()

Standard IO streams:

fopen(), freopen(), fseek(), fseeko(), rewind(), fread(), getc(), getchar(), fgetc(), fgets(), ungetc(), fclose()

Memory management:

mmap(), munmap(), malloc(), calloc(), valloc(), free(), memalign(), posix_memalign()

Required on Linux:

open64(), lseek64(), mmap64(), _IO_getc(), getline(), getdelim(), __getdelim(), getc_unlocked(), getchar_unlocked(), fgetc_unlocked(), fgets_unlocked(), fread_unlocked(), __uflow()

Required on BSD systems:

fgetln(), __srefill(), __srget()

Required on Mac OS X:


Required on HP-UX:

__open64(), __lseek64(), __filbuf()

Required on OpenSolaris:

freopen64(), fseeko64(), fsetpos64()

Signal handling:

signal(), sigaction()

If an application manipulates file descriptors (reading data, seeking around) using functions that are not in that list, libzzuf will not fuzz its input consistently and the results should not be trusted. You can use a tool such as ltrace(1) on Linux to know the missing functions.

On BSD systems, such as FreeBSD or Mac OS X, __srefill() is enough to monitor all standard IO streams functions. On other systems, such as Linux, each function is reimplemented on a case by case basis. One important unimplemented function is fscanf(), because of its complexity. Missing functions will be added upon user request.

See Also



Copyright © 2002-2015 Sam Hocevar <>.

libzzuf and this manual page are free software. They come without any warranty, to the extent permitted by applicable law. You can redistribute them and/or modify them under the terms of the Do What the Fuck You Want to Public License, Version 2, as published by the WTFPL Task Force. See for more details.

zzuf's webpage can be found at An overview of the architecture and inner works is at

Referenced By

zzat(1), zzuf(1).

2015-01-06 libzzuf 0.15