What environment variables control dyld?

Question

There are a bunch of environment variables that control dyld launch, several of them very useful for debugging performance problems. Not all of them are documented.

Answer 1

These ones are explained in the dyld man page (at least on macOS 10.13)

DYLD_FRAMEWORK_PATH
DYLD_FALLBACK_FRAMEWORK_PATH
DYLD_VERSIONED_FRAMEWORK_PATH
DYLD_LIBRARY_PATH
DYLD_FALLBACK_LIBRARY_PATH
DYLD_VERSIONED_LIBRARY_PATH
DYLD_PRINT_TO_FILE
DYLD_SHARED_REGION
DYLD_INSERT_LIBRARIES
DYLD_FORCE_FLAT_NAMESPACE
DYLD_IMAGE_SUFFIX
DYLD_PRINT_OPTS
DYLD_PRINT_ENV
DYLD_PRINT_LIBRARIES
DYLD_BIND_AT_LAUNCH
DYLD_DISABLE_DOFS
DYLD_PRINT_APIS
DYLD_PRINT_BINDINGS
DYLD_PRINT_INITIALIZERS
DYLD_PRINT_REBASINGS
DYLD_PRINT_SEGMENTS
DYLD_PRINT_STATISTICS
DYLD_PRINT_DOFS
DYLD_PRINT_RPATHS
DYLD_SHARED_CACHE_DIR
DYLD_SHARED_CACHE_DONT_VALIDATE

This one is documented in man dyld, but isn't listed in the list at the top:

DYLD_PRINT_STATISTICS_DETAILS

These are undocumented:

DYLD_ROOT_PATH
DYLD_PATHS_ROOT
DYLD_DISABLE_PREFETCH
DYLD_PRINT_LIBRARIES_POST_LAUNCH
DYLD_NEW_LOCAL_SHARED_REGIONS
DYLD_NO_FIX_PREBINDING
DYLD_PREBIND_DEBUG
DYLD_PRINT_TO_STDERR
DYLD_PRINT_WEAK_BINDINGS
DYLD_PRINT_WARNINGS
DYLD_PRINT_CS_NOTIFICATIONS
DYLD_PRINT_INTERPOSING
DYLD_PRINT_CODE_SIGNATURES
DYLD_USE_CLOSURES
DYLD_IGNORE_PREBINDING
DYLD_SKIP_MAIN

DYLD_ROOT_PATH and DYLD_PATHS_ROOT appear to be synonyms and allow you to reset the "root" for searching for libraries/frameworks/etc. This is available on macOS/iPhoneSimulator but not iOS.

DYLD_DISABLE_PREFETCH disables the pre-fetching of the content of __DATA and __LINKEDIT segments.

DYLD_PRINT_LIBRARIES_POST_LAUNCH is the same as DYLD_PRINT_LIBRARIES but prints them right after launch has finished.

DYLD_NEW_LOCAL_SHARED_REGIONS and DYLD_NO_FIX_PREBINDING are ignored and don't do anything anymore.

DYLD_PREBIND_DEBUG prints out debug information on why prebinding was not used.

DYLD_PRINT_TO_STDERR only applies to iOS and forces output to stderr (instead of stdout) to help it show up on console logs.

DYLD_PRINT_WEAK_BINDINGS prints debug information on weak bindings.

DYLD_PRINT_WARNINGS prints a bunch of warnings (mostly regards to closures and how they are being used).

DYLD_PRINT_CS_NOTIFICATIONS prints information about the core symbolicator.

DYLD_PRINT_INTERPOSING prints details about interposes that occur.

DYLD_PRINT_CODE_SIGNATURES prints details about code signatures (specifically successes and failures).

DYLD_USE_CLOSURES is a dyld3 feature, but doesn't appear to work for anybody non-internal (need CSR_ALLOW_APPLE_INTERNAL set).

DYLD_IGNORE_PREBINDING has three values ("all", "app", "nonsplit") with nonsplit being the default if a value is not supplied.

DYLD_SKIP_MAIN is an apple only feature used for testing dyld (need CSR_ALLOW_APPLE_INTERNAL set).