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Diffstat (limited to 'gdb/doc/gdb.texinfo')
| -rw-r--r-- | gdb/doc/gdb.texinfo | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo index 33bac26..2ec680a 100644 --- a/gdb/doc/gdb.texinfo +++ b/gdb/doc/gdb.texinfo @@ -12494,6 +12494,53 @@ returned. In contrast, the @code{finish} command (@pxref{Continuing and Stepping, ,Continuing and Stepping}) resumes execution until the selected stack frame returns naturally. +@value{GDBN} needs to know how the @var{expression} argument should be set for +the inferior. The concrete registers assignment depends on the OS ABI and the +type being returned by the selected stack frame. For example it is common for +OS ABI to return floating point values in FPU registers while integer values in +CPU registers. Still some ABIs return even floating point values in CPU +registers. Larger integer widths (such as @code{long long int}) also have +specific placement rules. @value{GDBN} already knows the OS ABI from its +current target so it needs to find out also the type being returned to make the +assignment into the right register(s). + +Normally, the selected stack frame has debug info. @value{GDBN} will always +use the debug info instead of the implicit type of @var{expression} when the +debug info is available. For example, if you type @kbd{return -1}, and the +function in the current stack frame is declared to return a @code{long long +int}, @value{GDBN} transparently converts the implicit @code{int} value of -1 +into a @code{long long int}: + +@smallexample +Breakpoint 1, func () at gdb.base/return-nodebug.c:29 +29 return 31; +(@value{GDBP}) return -1 +Make func return now? (y or n) y +#0 0x004004f6 in main () at gdb.base/return-nodebug.c:43 +43 printf ("result=%lld\n", func ()); +(@value{GDBP}) +@end smallexample + +However, if the selected stack frame does not have a debug info, e.g., if the +function was compiled without debug info, @value{GDBN} has to find out the type +to return from user. Specifying a different type by mistake may set the value +in different inferior registers than the caller code expects. For example, +typing @kbd{return -1} with its implicit type @code{int} would set only a part +of a @code{long long int} result for a debug info less function (on 32-bit +architectures). Therefore the user is required to specify the return type by +an appropriate cast explicitly: + +@smallexample +Breakpoint 2, 0x0040050b in func () +(@value{GDBP}) return -1 +Return value type not available for selected stack frame. +Please use an explicit cast of the value to return. +(@value{GDBP}) return (long long int) -1 +Make selected stack frame return now? (y or n) y +#0 0x00400526 in main () +(@value{GDBP}) +@end smallexample + @node Calling @section Calling Program Functions |