= babeltrace2-filter.lttng-utils.debug-info(7) :manpagetype: component class :revdate: 14 September 2019 :compcls: compcls:filter.lttng-utils.debug-info :defdebuginfoname: `debug_info` == NAME babeltrace2-filter.lttng-utils.debug-info - Babeltrace 2's debugging information filter component class for LTTng traces == DESCRIPTION A Babeltrace~2 {compcls} message iterator creates and emits copies of upstream messages, augmenting LTTng event messages with debugging information when it's available and possible. ---- Messages without debugging information | | +----------------------------+ | | flt.lttng-utils.debug-info | | | | '->@ in out @--> Messages with +----------------------------+ debugging information ---- include::common-see-babeltrace2-intro.txt[] A {compcls} message iterator uses the LTTng state dump events as well as the event common context's `ip` (instruction pointer) and `vpid` (process ID) fields to locate and read the corresponding debugging information. The message iterator can find the extra debugging information in an executable file or in a directory containing debugging information which the compiler creates. The new LTTng events (copies of the original ones with added debugging information) contain, when possible, a new event common context's structure field (besides the `ip` field) named {defdebuginfoname} by default (you can use the param:debug-info-field-name parameter to choose another name). This structure field contains the following fields: `bin` vtype:[string]:: Executable path or name followed with `@ADDR` or `+ADDR`, where `ADDR` is the address (hexadecimal) where it was loaded while being traced. + `@ADDR` means `ADDR` is an absolute address, and `+ADDR` means `ADDR` is a relative address. + Examples: `my-program@0x4b7fdd23`, `my-program+0x18d7c`. [[field-func]]`func` vtype:[string]:: Function name followed with `+OFFSET`, where `OFFSET` is the offset (hexadecimal) from the beginning of the function symbol in the executable file. + Example: `load_user_config+0x194`. [[field-src]]`src` vtype:[string]:: Source file path or name followed with `:LINE`, where `LINE` is the line number in this source file at which the event occurred. + Example: `user-config.c:1025`. Any of the previous fields can be an empty string if the debugging information was not available for the analyzed original LTTng event. A {compcls} message iterator systematically copies the upstream messages, but it only augments compatible LTTng event classes. This means that the message iterator copies messages of non-LTTng trace (see <>) without alteration. === Compile an executable for debugging information analysis With GCC or Clang, you need to compile the program or library source files in debug mode with the nlopt:-g option. This option makes the compiler generate debugging information in the operating system's native format. This format is recognized by a {compcls} component: it can translate the instruction pointer field of an event's common context to a source file and line number, along with the name of the surrounding function. IMPORTANT: This component class only supports the debugging information in DWARF format, version~2 or later. Use the nlopt:-gdwarf or nlopt:-gdwarf-VERSION (where `VERSION` is the DWARF version) compiler options to explicitly generate DWARF debugging information. If you don't compile the executable's source files with the nlopt:-g option or with an equivalent option, no DWARF information is available: the message iterator uses ELF symbols from the executable file instead. In this case, the events that the message iterator creates do not contain the source file and line number (see the <>), but only the name of the nearest function symbol with an offset in bytes to the location in the executable from which the LTTng event occurred (see the <>). If the executable file has neither ELF symbols nor DWARF information, the {compcls} message iterator cannot map the event to its source location: the message iterator still copies the upstream messages but without altering them. [[lttng-prereq]] === LTTng prerequisites A {compcls} message iterator can only analyze user space events which https://lttng.org[LTTng]~2.8.0 or later generates. To get debugging information for LTTng-UST events which occur in executables and libraries which the system's loader loads (what you can see with man:ldd(1)): . Add the `ip` and `vpid` context fields to user space event records: + -- [role="term"] ---- $ lttng add-context --userspace --type=ip --type=vpid ---- -- + See man:lttng-add-context(1) for more details. . Enable the LTTng-UST state dump events: + -- [role="term"] ---- $ lttng enable-event --userspace 'lttng_ust_statedump:*' ---- -- + See man:lttng-enable-event(1) and man:lttng-ust(3) for more details. To get debugging information for LTTng-UST events which occur in dynamically loaded objects, for example plugins: . Do the previous steps (add context fields and enable the LTTng-UST state dump events). . Enable the LTTng-UST dynamic linker tracing helper events: + -- [role="term"] ---- $ lttng enable-event --userspace 'lttng_ust_dl:*' ---- -- + See man:lttng-ust-dl(3) for more details. . When you are ready to trace, start your application with the `LD_PRELOAD` environment variable set to `liblttng-ust-dl.so`: + -- [role="term"] ---- $ LD_PRELOAD=liblttng-ust-dl.so my-app ---- -- === Separate debugging information You can store DWARF debugging information outside the executable itself, whether it is to reduce the executable's file size or simply to facilitate sharing the debugging information. This is usually achieved via one of two mechanisms, namely _build ID_ and _debug link_. Their use and operation is described in the https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html[Debugging Information in Separate Files] section of GDB's documentation. A {compcls} message iterator can find separate debugging information files automatically, as long as they meet the requirements stated in this manual page. The debugging information lookup order is the same as GDB's, namely: . Within the executable file itself. . Through the build ID method in the `/usr/lib/debug/.build-id` directory. . In the various possible debug link locations. The message iterator uses the first debugging information file that it finds. You can use the param:debug-info-dir initialization parameter to override the default `/usr/lib/debug` directory used in the build ID and debug link methods. NOTE: It is currently not possible to make this component search for debugging information in multiple directories. === Target prefix The debugging information analysis that a {compcls} message iterator performs uses the paths to the executables as collected during tracing as the default mechanism to resolve DWARF and ELF information. If the trace was recorded on a separate machine, however, you can use the param:target-prefix parameter to specify a prefix directory, that is, the root of the target file system. For example, if an instrumented executable's path is `/usr/bin/foo` on the target system, you can place this file at `/home/user/target/usr/bin/foo` on the system on which you use a {compcls} component. In this case, the target prefix to use is `/home/user/target`. == INITIALIZATION PARAMETERS param:debug-info-dir='DIR' vtype:[optional string]:: Use 'DIR' as the directory from which to load debugging information with the build ID and debug link methods instead of `/usr/lib/debug`. param:debug-info-field-name='NAME' vtype:[optional string]:: Name the debugging information structure field in the common context of the created events 'NAME' instead of the default {defdebuginfoname}. param:full-path=`yes` vtype:[optional boolean]:: Use the full path when writing the executable name (`bin`) and source file name (`src`) fields in the {defdebuginfoname} context field of the created events. param:target-prefix='DIR' vtype:[optional string]:: Use 'DIR' as the root directory of the target file system instead of `/`. == PORTS ---- +----------------------------+ | flt.lttng-utils.debug-info | | | @ in out @ +----------------------------+ ---- === Input `in`:: Single input port. === Output `out`:: Single output port. include::common-footer.txt[] == SEE ALSO man:babeltrace2-intro(7), man:babeltrace2-plugin-lttng-utils(7), man:lttng(1), man:lttng-add-context(1)