Trace File Format (Debugging with GDB)

Appendix I Trace File Format

The trace file comes in three parts: a header, a textual description section, and a trace frame section with binary data.

The header has the form \x7fTRACE0\n. The first byte is 0x7f so as to indicate that the file contains binary data, while the 0 is a version number that may have different values in the future.

The description section consists of multiple lines of ASCII text separated by newline characters (0xa). The lines may include a variety of optional descriptive or context-setting information, such as tracepoint definitions or register set size. GDB will ignore any line that it does not recognize. An empty line marks the end of this section.

R size: Specifies the size of a register block in bytes. This is equal to the size of a g packet payload in the remote protocol. size is an ascii decimal number. There should be only one such line in a single trace file.
status status: Trace status. status has the same format as a qTStatus remote packet reply. There should be only one such line in a single trace file.
tp payload: Tracepoint definition. The payload has the same format as qTfP/qTsP remote packet reply payload. A single tracepoint may take multiple lines of definition, corresponding to the multiple reply packets.
tsv payload: Trace state variable definition. The payload has the same format as qTfV/qTsV remote packet reply payload. A single variable may take multiple lines of definition, corresponding to the multiple reply packets.
tdesc payload: Target description in XML format. The payload is a single line of the XML file. All such lines should be concatenated together to get the original XML file. This file is in the same format as qXfer features payload, and corresponds to the main target.xml file. Includes are not allowed.

The trace frame section consists of a number of consecutive frames. Each frame begins with a two-byte tracepoint number, followed by a four-byte size giving the amount of data in the frame. The data in the frame consists of a number of blocks, each introduced by a character indicating its type (at least register, memory, and trace state variable). The data in this section is raw binary, not a hexadecimal or other encoding; its endianness matches the target’s endianness.

R bytes: Register block. The number and ordering of bytes matches that of a g packet in the remote protocol. Note that these are the actual bytes, in target order, not a hexadecimal encoding.
M address length bytes...: Memory block. This is a contiguous block of memory, at the 8-byte address address, with a 2-byte length length, followed by length bytes.
V number value: Trace state variable block. This records the 8-byte signed value value of trace state variable numbered number.

Future enhancements of the trace file format may include additional types of blocks.