gcov uses three files for doing profiling. The names of these files are derived from the original source file by substituting the file suffix with either .bb, .bbg, or .da. All of these files are placed in the same directory as the source file, and contain data stored in a platform-independent method.
The .bb and .bbg files are generated when the source file is compiled with the GCC -ftest-coverage option. The .bb file contains a list of source files (including headers), functions within those files, and line numbers corresponding to each basic block in the source file.
The .bb file format consists of several lists of 4-byte integers which correspond to the line numbers of each basic block in the file. Each list is terminated by a line number of 0. A line number of −1 is used to designate that the source file name (padded to a 4-byte boundary and followed by another −1) follows. In addition, a line number of −2 is used to designate that the name of a function (also padded to a 4-byte boundary and followed by a −2) follows.
The .bbg file is used to reconstruct the program flow graph for the source file. It contains a list of the program flow arcs (possible branches taken from one basic block to another) for each function which, in combination with the .bb file, enables gcov to reconstruct the program flow.
In the .bbg file, the format is:
name of function #0
checksum of function #0
number of basic blocks for function #0 (4-byte number)
total number of arcs for function #0 (4-byte number)
count of arcs in basic block #0 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
destination basic block of arc #1 (4-byte number)
flag bits (4-byte number)
...
destination basic block of arc #N (4-byte number)
flag bits (4-byte number)
count of arcs in basic block #1 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
...
A −1 (stored as a 4-byte number) is used to separate each function's list of basic blocks, and to verify that the file has been read correctly.
The function name is stored as a −1 (4 bytes), the length (4 bytes), the name itself (padded to 4-byte boundary) followed by a −1 (4 bytes).
The flags are defined as follows:
The .da file is generated when a program containing object files built with the GCC -fprofile-arcs option is executed. A separate .da file is created for each source file compiled with this option, and the name of the .da file is stored as an absolute pathname in the resulting object file. This path name is derived from the object file name by substituting a .da suffix.
The .da consists of one or more blocks with the following structure:
"magic" number −123 (4-byte number)
number of functions (4-byte number)
length of the "extension block" in bytes
extension block (variable length)
name of function #0 (the same format as in .bbg file)
checksum of function #0
number of instrumented arcs (4-byte number)
count of arc #0 (8-byte number)
count of arc #1 (8-byte number)
...
count of arc #M_0 (8-byte number)
name of function #1 (the same format as in .bbg file)
checksum of function #1
...
Multiple program runs might merge data into a single block, or might append a new block. The current structure of the extension block is as follows:
number of instrumented arcs in whole program (4-byte number)
sum all of instrumented arcs in whole program (8-byte number)
maximal value of counter in whole program (8-byte number)
number of instrumented arcs in the object file (4-byte number)
sum all of instrumented arcs in the object file (8-byte number)
maximal value of counter in the object file (8-byte number)
All three of these files use the functions in gcov-io.h to store integers; the functions in this header provide a machine-independent mechanism for storing and retrieving data from a stream.