The File I/O remote protocol extension (short: File-I/O) allows the
target to use the hosts file system and console I/O when calling various
system calls. System calls on the target system are translated into a
remote protocol packet to the host system which then performs the needed
actions and returns with an adequate response packet to the target system.
This simulates file system operations even on targets that lack file systems.
The protocol is defined host- and target-system independent. It uses
it's own independent representation of datatypes and values. Both,
gdb and the target's gdb stub are responsible for
translating the system dependent values into the unified protocol values
when data is transmitted.
The communication is synchronous. A system call is possible only
when GDB is waiting for the C, c, S or s
packets. While gdb handles the request for a system call,
the target is stopped to allow deterministic access to the target's
memory. Therefore File-I/O is not interuptible by target signals. It
is possible to interrupt File-I/O by a user interrupt (Ctrl-C), though.
The target's request to perform a host system call does not finish
the latest C, c, S or s action. That means,
after finishing the system call, the target returns to continuing the
previous activity (continue, step). No additional continue or step
request from gdb is required.
(gdb) continue
<- target requests 'system call X'
target is stopped, gdb executes system call
-> GDB returns result
... target continues, GDB returns to wait for the target
<- target hits breakpoint and sends a Txx packet
The protocol is only used for files on the host file system and
for I/O on the console. Character or block special devices, pipes,
named pipes or sockets or any other communication method on the host
system are not supported by this protocol.
D.7.2. Protocol basics
The File-I/O protocol uses the F packet, as request as well
as as reply packet. Since a File-I/O system call can only occur when
gdb is waiting for the continuing or stepping target, the
File-I/O request is a reply that gdb has to expect as a result
of a former C, c, S or s packet.
This F packet contains all information needed to allow gdb
to call the appropriate host system call:
A unique identifier for the requested system call.
All parameters to the system call. Pointers are given as addresses
in the target memory address space. Pointers to strings are given as
pointer/length pair. Numerical values are given as they are.
Numerical control values are given in a protocol specific representation.
At that point gdb has to perform the following actions.
If parameter pointer values are given, which point to data needed as input
to a system call, gdb requests this data from the target with a
standard m packet request. This additional communication has to be
expected by the target implementation and is handled as any other m
packet.
gdb translates all value from protocol representation to host
representation as needed. Datatypes are coerced into the host types.
gdb calls the system call
It then coerces datatypes back to protocol representation.
If pointer parameters in the request packet point to buffer space in which
a system call is expected to copy data to, the data is transmitted to the
target using a M or X packet. This packet has to be expected
by the target implementation and is handled as any other M or X
packet.
Eventually gdb replies with another F packet which contains all
necessary information for the target to continue. This at least contains
Return value.
errno, if has been changed by the system call.
"Ctrl-C" flag.
After having done the needed type and value coercion, the target continues
the latest continue or step action.
D.7.3. The Frequest packet
The F request packet has the following format:
Fcall-id,parameter…
call-id is the identifier to indicate the host system call to be called.
This is just the name of the function.
parameter… are the parameters to the system call.
Parameters are hexadecimal integer values, either the real values in caseof scalar datatypes, as pointers to target buffer space in case of compounddatatypes and unspecified memory areas or as pointer/length pairs in caseof string parameters. These are appended to the call-id, each separatedfrom its predecessor by a comma. All values are transmitted in ASCIIstring representation, pointer/length pairs separated by a slash.
D.7.4. The Freply packet
The F reply packet has the following format:
Fretcode,errno,Ctrl-C flag;call specific attachment
retcode is the return code of the system call as hexadecimal value.
errno is the errno set by the call, in protocol specific representation.
This parameter can be omitted if the call was successful.
Ctrl-C flag is only send if the user requested a break. In this
case, errno must be send as well, even if the call was successful.
The Ctrl-C flag itself consists of the character 'C':
F0,0,C
or, if the call was interupted before the host call has been performed:
F-1,4,C
assuming 4 is the protocol specific representation of EINTR.
D.7.5. Memory transfer
Structured data which is transferred using a memory read or write as e.g.
a struct stat is expected to be in a protocol specific format with
all scalar multibyte datatypes being big endian. This should be done by
the target before the F packet is sent resp. by gdb before
it transfers memory to the target. Transferred pointers to structured
data should point to the already coerced data at any time.
D.7.6. The Ctrl-C message
A special case is, if the Ctrl-C flag is set in the gdb
reply packet. In this case the target should behave, as if it had
gotten a break message. The meaning for the target is "system call
interupted by SIGINT". Consequentially, the target should actually stop
(as with a break message) and return to gdb with a T02
packet. In this case, it's important for the target to know, in which
state the system call was interrupted. Since this action is by design
not an atomic operation, we have to differ between two cases:
The system call hasn't been performed on the host yet.
The system call on the host has been finished.
These two states can be distinguished by the target by the value of the
returned errno. If it's the protocol representation of EINTR, the system
call hasn't been performed. This is equivalent to the EINTR handling
on POSIX systems. In any other case, the target may presume that the
system call has been finished -- successful or not -- and should behave
as if the break message arrived right after the system call.
gdb must behave reliable. If the system call has not been called
yet, gdb may send the F reply immediately, setting EINTR as
errno in the packet. If the system call on the host has been finished
before the user requests a break, the full action must be finshed by
gdb. This requires sending M or X packets as they fit.
The F packet may only be send when either nothing has happened
or the full action has been completed.
D.7.7. Console I/O
By default and if not explicitely closed by the target system, the file
descriptors 0, 1 and 2 are connected to the gdb console. Output
on the gdb console is handled as any other file output operation
(write(1, …) or write(2, …)). Console input is handled
by gdb so that after the target read request from file descriptor
0 all following typing is buffered until either one of the following
conditions is met:
The user presses Ctrl-C. The behaviour is as explained above, the
read
system call is treated as finished.
The user presses Enter. This is treated as end of input with a trailing
line feed.
The user presses Ctrl-D. This is treated as end of input. No trailing
character, especially no Ctrl-D is appended to the input.
If the user has typed more characters as fit in the buffer given to
the read call, the trailing characters are buffered in gdb until
either another read(0, …) is requested by the target or debugging
is stopped on users request.
D.7.8. The isatty(3) call
A special case in this protocol is the library call isatty which
is implemented as it's own call inside of this protocol. It returns
1 to the target if the file descriptor given as parameter is attached
to the gdb console, 0 otherwise. Implementing through system calls
would require implementing ioctl and would be more complex than
needed.
D.7.9. The system(3) call
The other special case in this protocol is the system call which
is implemented as it's own call, too. gdb is taking over the full
task of calling the necessary host calls to perform the system
call. The return value of system is simplified before it's returned
to the target. Basically, the only signal transmitted back is EINTR
in case the user pressed Ctrl-C. Otherwise the return value consists
entirely of the exit status of the called command.
Due to security concerns, the system call is refused to be called
by gdb by default. The user has to allow this call explicitly by
entering
set remote system-call-allowed 1
Set the call.
Disabling the system call is done by
set remote system-call-allowed 0
Set the call.
The current setting is shown by typing
show remote system-call-allowed
Set the call.
D.7.10. List of supported calls
D.7.10.1. open
Synopsis: int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
Request:
Fopen,pathptr/len,flags,mode
flags is the bitwise or of the following values:
O_CREAT
If the file does not exist it will be created. The host
rules apply as far as file ownership and time stamps
are concerned.
O_EXCL
When used with O_CREAT, if the file already exists it is
an error and open() fails.
O_TRUNC
If the file already exists and the open mode allows
writing (O_RDWR or O_WRONLY is given) it will be
truncated to length 0.
O_APPEND
The file is opened in append mode.
O_RDONLY
The file is opened for reading only.
O_WRONLY
The file is opened for writing only.
O_RDWR
The file is opened for reading and writing.
Each other bit is silently ignored.
mode is the bitwise or of the following values:
S_IRUSR
User has read permission.
S_IWUSR
User has write permission.
S_IRGRP
Group has read permission.
S_IWGRP
Group has write permission.
S_IROTH
Others have read permission.
S_IWOTH
Others have write permission.
Each other bit is silently ignored.
Return value: open returns the new file descriptor or -1 if an error
occured.
Errors:
EEXIST
pathname already exists and O_CREAT and O_EXCL were used.
EISDIR
pathname refers to a directory.
EACCES
The requested access is not allowed.
ENAMETOOLONG
pathname was too long.
ENOENT
A directory component in pathname does not exist.
ENODEV
pathname refers to a device, pipe, named pipe or socket.
EROFS
pathname refers to a file on a read-only filesystem and
write access was requested.
EFAULT
pathname is an invalid pointer value.
ENOSPC
No space on device to create the file.
EMFILE
The process already has the maximum number of files open.
ENFILE
The limit on the total number of files open on the system
has been reached.
EINTR
The call was interrupted by the user.
D.7.10.2. close
Synopsis: int close(int fd);
Request:
Fclose,fd
Return value:
close returns zero on success, or -1 if an error occurred.
Errors:
EBADF
fd isn't a valid open file descriptor.
EINTR
The call was interrupted by the user.
D.7.10.3. read
Synopsis: int read(int fd, void *buf, unsigned int count);
Request:
Fread,fd,bufptr,count
Return value:
On success, the number of bytes read is returned.
Zero indicates end of file. If count is zero, read
returns zero as well. On error, -1 is returned.
Errors:
EBADF
fd is not a valid file descriptor or is not open for
reading.
EFAULT
buf is an invalid pointer value.
EINTR
The call was interrupted by the user.
D.7.10.4. write
Synopsis: int write(int fd, const void *buf, unsigned int count);
Request:
Fwrite,fd,bufptr,count
Return value:
On success, the number of bytes written are returned.
Zero indicates nothing was written. On error, -1
is returned.
Errors:
EBADF
fd is not a valid file descriptor or is not open for
writing.
EFAULT
buf is an invalid pointer value.
EFBIG
An attempt was made to write a file that exceeds the
host specific maximum file size allowed.
ENOSPC
No space on device to write the data.
EINTR
The call was interrupted by the user.
D.7.10.5. lseek
Synopsis: long lseek (int fd, long offset, int flag);
Request:
Flseek,fd,offset,flag
flag is one of:
SEEK_SET
The offset is set to offset bytes.
SEEK_CUR
The offset is set to its current location plus offset
bytes.
SEEK_END
The offset is set to the size of the file plus offset
bytes.
Return value: On success, the resulting unsigned offset in bytes from
the beginning of the file is returned. Otherwise, a
value of -1 is returned.
Errors:
EBADF
fd is not a valid open file descriptor.
ESPIPE
fd is associated with the gdb console.
EINVAL
flag is not a proper value.
EINTR
The call was interrupted by the user.
D.7.10.6. rename
Synopsis: int rename(const char *oldpath, const char *newpath);
Request:
Frename,oldpathptr/len,newpathptr/len
Return value:
On success, zero is returned. On error, -1 is returned.
Errors:
EISDIR
newpath is an existing directory, but oldpath is not a
directory.
EEXIST
newpath is a non-empty directory.
EBUSY
oldpath or newpath is a directory that is in use by some
process.
EINVAL
An attempt was made to make a directory a subdirectory
of itself.
ENOTDIR
A component used as a directory in oldpath or new
path is not a directory. Or oldpath is a directory
and newpath exists but is not a directory.
EFAULT
oldpathptr or newpathptr are invalid pointer values.
EACCES
No access to the file or the path of the file.
ENAMETOOLONG
oldpath or newpath was too long.
ENOENT
A directory component in oldpath or newpath does not exist.
EROFS
The file is on a read-only filesystem.
ENOSPC
The device containing the file has no room for the new
directory entry.
EINTR
The call was interrupted by the user.
D.7.10.7. unlink
Synopsis: int unlink(const char *pathname);
Request:
Funlink,pathnameptr/len
Return value:
On success, zero is returned. On error, -1 is returned.
Errors:
EACCES
No access to the file or the path of the file.
EPERM
The system does not allow unlinking of directories.
EBUSY
The file pathname cannot be unlinked because it's
being used by another process.
EFAULT
pathnameptr is an invalid pointer value.
ENAMETOOLONG
pathname was too long.
ENOENT
A directory component in pathname does not exist.
ENOTDIR
A component of the path is not a directory.
EROFS
The file is on a read-only filesystem.
EINTR
The call was interrupted by the user.
D.7.10.8. stat/fstat
Synopsis: int stat(const char *pathname, struct stat *buf);
int fstat(int fd, struct stat *buf);
Request:
Fstat,pathnameptr/len,bufptr
Ffstat,fd,bufptr
Return value:
On success, zero is returned. On error, -1 is returned.
Errors:
EBADF
fd is not a valid open file.
ENOENT
A directory component in pathname does not exist or the
path is an empty string.
ENOTDIR
A component of the path is not a directory.
EFAULT
pathnameptr is an invalid pointer value.
EACCES
No access to the file or the path of the file.
ENAMETOOLONG
pathname was too long.
EINTR
The call was interrupted by the user.
D.7.10.9. gettimeofday
Synopsis: int gettimeofday(struct timeval *tv, void *tz);
Request:
Fgettimeofday,tvptr,tzptr
Return value:
On success, 0 is returned, -1 otherwise.
Errors:
EINVAL
tz is a non-NULL pointer.
EFAULT
tvptr and/or tzptr is an invalid pointer value.
D.7.10.10. isatty
Synopsis: int isatty(int fd);
Request:
Fisatty,fd
Return value:
Returns 1 if fd refers to the gdb console, 0 otherwise.
Errors:
EINTR
The call was interrupted by the user.
D.7.10.11. system
Synopsis: int system(const char *command);
Request:
Fsystem,commandptr/len
Return value:
The value returned is -1 on error and the return status
of the command otherwise. Only the exit status of the
command is returned, which is extracted from the hosts
system return value by calling WEXITSTATUS(retval).
In case /bin/sh could not be executed, 127 is returned.
Errors:
EINTR
The call was interrupted by the user.
D.7.11. Protocol specific representation of datatypes
D.7.11.1. Integral datatypes
The integral datatypes used in the system calls are
int, unsigned int, long, unsigned long, mode_t and time_t
Int, unsigned int, mode_t and time_t are
implemented as 32 bit values in this protocol.
Long and unsigned long are implemented as 64 bit types.
Refer to Section D.7.12.5 Limits, for corresponding MIN and MAX values (similar to those
in limits.h) to allow range checking on host and target.
time_t datatypes are defined as seconds since the Epoch.
All integral datatypes transferred as part of a memory read or write of a
structured datatype e.g. a struct stat have to be given in big endian
byte order.
D.7.11.2. Pointer values
Pointers to target data are transmitted as they are. An exception
is made for pointers to buffers for which the length isn't
transmitted as part of the function call, namely strings. Strings
are transmitted as a pointer/length pair, both as hex values, e.g.
1aaf/12
which is a pointer to data of length 18 bytes at position 0x1aaf.
The length is defined as the full string length in bytes, including
the trailing null byte. Example:
``hello, world'' at address 0x123456
is transmitted as
123456/d
D.7.11.3. struct stat
The buffer of type struct stat used by the target and gdb is defined
as follows:
struct stat {
unsigned int st_dev; /* device */
unsigned int st_ino; /* inode */
mode_t st_mode; /* protection */
unsigned int st_nlink; /* number of hard links */
unsigned int st_uid; /* user ID of owner */
unsigned int st_gid; /* group ID of owner */
unsigned int st_rdev; /* device type (if inode device) */
unsigned long st_size; /* total size, in bytes */
unsigned long st_blksize; /* blocksize for filesystem I/O */
unsigned long st_blocks; /* number of blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last change */
};
The integral datatypes are conforming to the definitions given in the
approriate section ((refer to Section D.7.11.1 Integral datatypes, for details) so this
structure is of size 64 bytes.
The values of several fields have a restricted meaning and/or
range of values.
st_dev: 0 file
1 console
st_ino: No valid meaning for the target. Transmitted unchanged.
st_mode: Valid mode bits are described in Appendix C. Any other
bits have currently no meaning for the target.
st_uid: No valid meaning for the target. Transmitted unchanged.
st_gid: No valid meaning for the target. Transmitted unchanged.
st_rdev: No valid meaning for the target. Transmitted unchanged.
st_atime, st_mtime, st_ctime:
These values have a host and file system dependent
accuracy. Especially on Windows hosts the file systems
don't support exact timing values.
The target gets a struct stat of the above representation and is
responsible to coerce it to the target representation before
continuing.
Note that due to size differences between the host and target
representation of stat members, these members could eventually
get truncated on the target.
D.7.11.4. struct timeval
The buffer of type struct timeval used by the target and gdb
is defined as follows:
struct timeval {
time_t tv_sec; /* second */
long tv_usec; /* microsecond */
};
The integral datatypes are conforming to the definitions given in the
approriate section ((refer to Section D.7.11.1 Integral datatypes, for details) so this
structure is of size 8 bytes.
D.7.12. Constants
The following values are used for the constants inside of the
protocol. gdb and target are resposible to translate these
values before and after the call as needed.
D.7.12.1. Open flags
All values are given in hexadecimal representation.
