<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Copyright (C) 1988-2020 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." --> <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Protocol Basics (Debugging with GDB)</title> <meta name="description" content="Protocol Basics (Debugging with GDB)"> <meta name="keywords" content="Protocol Basics (Debugging with GDB)"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <link href="index.html#Top" rel="start" title="Top"> <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" rel="up" title="File-I/O Remote Protocol Extension"> <link href="The-F-Request-Packet.html#The-F-Request-Packet" rel="next" title="The F Request Packet"> <link href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" rel="prev" title="File-I/O Overview"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> </head> <body lang="en"> <a name="Protocol-Basics"></a> <div class="header"> <p> Next: <a href="The-F-Request-Packet.html#The-F-Request-Packet" accesskey="n" rel="next">The F Request Packet</a>, Previous: <a href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" accesskey="p" rel="prev">File-I/O Overview</a>, Up: <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" accesskey="u" rel="up">File-I/O Remote Protocol Extension</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> </div> <hr> <a name="Protocol-Basics-1"></a> <h4 class="subsection">E.13.2 Protocol Basics</h4> <a name="index-protocol-basics_002c-file_002di_002fo"></a> <p>The File-I/O protocol uses the <code>F</code> packet as the request as well as reply packet. Since a File-I/O system call can only occur when <small>GDB</small> is waiting for a response from the continuing or stepping target, the File-I/O request is a reply that <small>GDB</small> has to expect as a result of a previous ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’ or ‘<samp>s</samp>’ packet. This <code>F</code> packet contains all information needed to allow <small>GDB</small> to call the appropriate host system call: </p> <ul> <li> A unique identifier for the requested system call. </li><li> 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 flags are given in a protocol-specific representation. </li></ul> <p>At this point, <small>GDB</small> has to perform the following actions. </p> <ul> <li> If the parameters include pointer values to data needed as input to a system call, <small>GDB</small> requests this data from the target with a standard <code>m</code> packet request. This additional communication has to be expected by the target implementation and is handled as any other <code>m</code> packet. </li><li> <small>GDB</small> translates all value from protocol representation to host representation as needed. Datatypes are coerced into the host types. </li><li> <small>GDB</small> calls the system call. </li><li> It then coerces datatypes back to protocol representation. </li><li> If the system call is expected to return data in buffer space specified by pointer parameters to the call, the data is transmitted to the target using a <code>M</code> or <code>X</code> packet. This packet has to be expected by the target implementation and is handled as any other <code>M</code> or <code>X</code> packet. </li></ul> <p>Eventually <small>GDB</small> replies with another <code>F</code> packet which contains all necessary information for the target to continue. This at least contains </p> <ul> <li> Return value. </li><li> <code>errno</code>, if has been changed by the system call. </li><li> “Ctrl-C” flag. </li></ul> <p>After having done the needed type and value coercion, the target continues the latest continue or step action. </p> <hr> <div class="header"> <p> Next: <a href="The-F-Request-Packet.html#The-F-Request-Packet" accesskey="n" rel="next">The F Request Packet</a>, Previous: <a href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" accesskey="p" rel="prev">File-I/O Overview</a>, Up: <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" accesskey="u" rel="up">File-I/O Remote Protocol Extension</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> </div> </body> </html>