diff options
Diffstat (limited to 'gdb/doc')
| -rw-r--r-- | gdb/doc/ChangeLog | 5 | ||||
| -rw-r--r-- | gdb/doc/gdb.texinfo | 124 |
2 files changed, 129 insertions, 0 deletions
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog index 74db55e..319e0c3 100644 --- a/gdb/doc/ChangeLog +++ b/gdb/doc/ChangeLog @@ -1,3 +1,8 @@ +2017-12-15 Sergio Durigan Junior <sergiodj@redhat.com> + + PR cli/16224 + * gdb.texinfo (ptype): Add documentation for new flag '/o'. + 2017-12-13 Simon Marchi <simon.marchi@ericsson.com> * python.texi (Manipulating breakpoints using Python): Document diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo index da3ed28..60ed80c 100644 --- a/gdb/doc/gdb.texinfo +++ b/gdb/doc/gdb.texinfo @@ -17216,6 +17216,130 @@ names are substituted when printing other types. @item T Print typedefs defined in the class. This is the default, but the flag exists in case you change the default with @command{set print type typedefs}. + +@item o +Print the offsets and sizes of fields in a struct, similar to what the +@command{pahole} tool does. This option implies the @code{/tm} flags. + +For example, given the following declarations: + +@smallexample +struct tuv +@{ + int a1; + char *a2; + int a3; +@}; + +struct xyz +@{ + int f1; + char f2; + void *f3; + struct tuv f4; +@}; + +union qwe +@{ + struct tuv fff1; + struct xyz fff2; +@}; + +struct tyu +@{ + int a1 : 1; + int a2 : 3; + int a3 : 23; + char a4 : 2; + int64_t a5; + int a6 : 5; + int64_t a7 : 3; +@}; +@end smallexample + +Issuing a @kbd{ptype /o struct tuv} command would print: + +@smallexample +(@value{GDBP}) ptype /o struct tuv +/* offset | size */ type = struct tuv @{ +/* 0 | 4 */ int a1; +/* XXX 4-byte hole */ +/* 8 | 8 */ char *a2; +/* 16 | 4 */ int a3; + + /* total size (bytes): 24 */ + @} +@end smallexample + +Notice the format of the first column of comments. There, you can +find two parts separated by the @samp{|} character: the @emph{offset}, +which indicates where the field is located inside the struct, in +bytes, and the @emph{size} of the field. Another interesting line is +the marker of a @emph{hole} in the struct, indicating that it may be +possible to pack the struct and make it use less space by reorganizing +its fields. + +It is also possible to print offsets inside an union: + +@smallexample +(@value{GDBP}) ptype /o union qwe +/* offset | size */ type = union qwe @{ +/* 24 */ struct tuv @{ +/* 0 | 4 */ int a1; +/* XXX 4-byte hole */ +/* 8 | 8 */ char *a2; +/* 16 | 4 */ int a3; + + /* total size (bytes): 24 */ + @} fff1; +/* 40 */ struct xyz @{ +/* 0 | 4 */ int f1; +/* 4 | 1 */ char f2; +/* XXX 3-byte hole */ +/* 8 | 8 */ void *f3; +/* 16 | 24 */ struct tuv @{ +/* 16 | 4 */ int a1; +/* XXX 4-byte hole */ +/* 24 | 8 */ char *a2; +/* 32 | 4 */ int a3; + + /* total size (bytes): 24 */ + @} f4; + + /* total size (bytes): 40 */ + @} fff2; + + /* total size (bytes): 40 */ + @} +@end smallexample + +In this case, since @code{struct tuv} and @code{struct xyz} occupy the +same space (because we are dealing with an union), the offset is not +printed for them. However, you can still examine the offset of each +of these structures' fields. + +Another useful scenario is printing the offsets of a struct containing +bitfields: + +@smallexample +(@value{GDBP}) ptype /o struct tyu +/* offset | size */ type = struct tyu @{ +/* 0:31 | 4 */ int a1 : 1; +/* 0:28 | 4 */ int a2 : 3; +/* 0: 5 | 4 */ int a3 : 23; +/* 3: 3 | 1 */ signed char a4 : 2; +/* XXX 3-bit hole */ +/* XXX 4-byte hole */ +/* 8 | 8 */ int64_t a5; +/* 16:27 | 4 */ int a6 : 5; +/* 16:56 | 8 */ int64_t a7 : 3; + + /* total size (bytes): 24 */ + @} +@end smallexample + +Note how the offset information is now extended to also include how +many bits are left to be used in each bitfield. @end table @kindex ptype |