11. Preprocessor
   31.1. Variadic macros
   5For variadic macros, stick with this C99-like syntax:
   7#define DPRINTF(fmt, ...)                                       \
   8    do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
  101.2. Include directives
  12Order include directives as follows:
  14#include "qemu/osdep.h"  /* Always first... */
  15#include <...>           /* then system headers... */
  16#include "..."           /* and finally QEMU headers. */
  18The "qemu/osdep.h" header contains preprocessor macros that affect the behavior
  19of core system headers like <stdint.h>.  It must be the first include so that
  20core system headers included by external libraries get the preprocessor macros
  21that QEMU depends on.
  23Do not include "qemu/osdep.h" from header files since the .c file will have
  24already included it.
  262. C types
  28It should be common sense to use the right type, but we have collected
  29a few useful guidelines here.
  312.1. Scalars
  33If you're using "int" or "long", odds are good that there's a better type.
  34If a variable is counting something, it should be declared with an
  35unsigned type.
  37If it's host memory-size related, size_t should be a good choice (use
  38ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
  39but only for RAM, it may not cover whole guest address space.
  41If it's file-size related, use off_t.
  42If it's file-offset related (i.e., signed), use off_t.
  43If it's just counting small numbers use "unsigned int";
  44(on all but oddball embedded systems, you can assume that that
  45type is at least four bytes wide).
  47In the event that you require a specific width, use a standard type
  48like int32_t, uint32_t, uint64_t, etc.  The specific types are
  49mandatory for VMState fields.
  51Don't use Linux kernel internal types like u32, __u32 or __le32.
  53Use hwaddr for guest physical addresses except pcibus_t
  54for PCI addresses.  In addition, ram_addr_t is a QEMU internal address
  55space that maps guest RAM physical addresses into an intermediate
  56address space that can map to host virtual address spaces.  Generally
  57speaking, the size of guest memory can always fit into ram_addr_t but
  58it would not be correct to store an actual guest physical address in a
  61For CPU virtual addresses there are several possible types.
  62vaddr is the best type to use to hold a CPU virtual address in
  63target-independent code. It is guaranteed to be large enough to hold a
  64virtual address for any target, and it does not change size from target
  65to target. It is always unsigned.
  66target_ulong is a type the size of a virtual address on the CPU; this means
  67it may be 32 or 64 bits depending on which target is being built. It should
  68therefore be used only in target-specific code, and in some
  69performance-critical built-per-target core code such as the TLB code.
  70There is also a signed version, target_long.
  71abi_ulong is for the *-user targets, and represents a type the size of
  72'void *' in that target's ABI. (This may not be the same as the size of a
  73full CPU virtual address in the case of target ABIs which use 32 bit pointers
  74on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
  75the target's ABI must use this type for anything that on the target is defined
  76to be an 'unsigned long' or a pointer type.
  77There is also a signed version, abi_long.
  79Of course, take all of the above with a grain of salt.  If you're about
  80to use some system interface that requires a type like size_t, pid_t or
  81off_t, use matching types for any corresponding variables.
  83Also, if you try to use e.g., "unsigned int" as a type, and that
  84conflicts with the signedness of a related variable, sometimes
  85it's best just to use the *wrong* type, if "pulling the thread"
  86and fixing all related variables would be too invasive.
  88Finally, while using descriptive types is important, be careful not to
  89go overboard.  If whatever you're doing causes warnings, or requires
  90casts, then reconsider or ask for help.
  922.2. Pointers
  94Ensure that all of your pointers are "const-correct".
  95Unless a pointer is used to modify the pointed-to storage,
  96give it the "const" attribute.  That way, the reader knows
  97up-front that this is a read-only pointer.  Perhaps more
  98importantly, if we're diligent about this, when you see a non-const
  99pointer, you're guaranteed that it is used to modify the storage
 100it points to, or it is aliased to another pointer that is.
 1022.3. Typedefs
 103Typedefs are used to eliminate the redundant 'struct' keyword.
 1052.4. Reserved namespaces in C and POSIX
 106Underscore capital, double underscore, and underscore 't' suffixes should be
 1093. Low level memory management
 111Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
 112APIs is not allowed in the QEMU codebase. Instead of these routines,
 113use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
 114g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree
 117Please note that g_malloc will exit on allocation failure, so there
 118is no need to test for failure (as you would have to with malloc).
 119Calling g_malloc with a zero size is valid and will return NULL.
 121Prefer g_new(T, n) instead of g_malloc(sizeof(T) * n) for the following
 124  a. It catches multiplication overflowing size_t;
 125  b. It returns T * instead of void *, letting compiler catch more type
 126     errors.
 128Declarations like T *v = g_malloc(sizeof(*v)) are acceptable, though.
 130Memory allocated by qemu_memalign or qemu_blockalign must be freed with
 131qemu_vfree, since breaking this will cause problems on Win32.
 1334. String manipulation
 135Do not use the strncpy function.  As mentioned in the man page, it does *not*
 136guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
 137It also zeros trailing destination bytes out to the specified length.  Instead,
 138use this similar function when possible, but note its different signature:
 139void pstrcpy(char *dest, int dest_buf_size, const char *src)
 141Don't use strcat because it can't check for buffer overflows, but:
 142char *pstrcat(char *buf, int buf_size, const char *s)
 144The same limitation exists with sprintf and vsprintf, so use snprintf and
 147QEMU provides other useful string functions:
 148int strstart(const char *str, const char *val, const char **ptr)
 149int stristart(const char *str, const char *val, const char **ptr)
 150int qemu_strnlen(const char *s, int max_len)
 152There are also replacement character processing macros for isxyz and toxyz,
 153so instead of e.g. isalnum you should use qemu_isalnum.
 155Because of the memory management rules, you must use g_strdup/g_strndup
 156instead of plain strdup/strndup.
 1585. Printf-style functions
 160Whenever you add a new printf-style function, i.e., one with a format
 161string argument and following "..." in its prototype, be sure to use
 162gcc's printf attribute directive in the prototype.
 164This makes it so gcc's -Wformat and -Wformat-security options can do
 165their jobs and cross-check format strings with the number and types
 166of arguments.
 1686. C standard, implementation defined and undefined behaviors
 170C code in QEMU should be written to the C99 language specification. A copy
 171of the final version of the C99 standard with corrigenda TC1, TC2, and TC3
 172included, formatted as a draft, can be downloaded from:
 173 http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf
 175The C language specification defines regions of undefined behavior and
 176implementation defined behavior (to give compiler authors enough leeway to
 177produce better code).  In general, code in QEMU should follow the language
 178specification and avoid both undefined and implementation defined
 179constructs. ("It works fine on the gcc I tested it with" is not a valid
 180argument...) However there are a few areas where we allow ourselves to
 181assume certain behaviors because in practice all the platforms we care about
 182behave in the same way and writing strictly conformant code would be
 183painful. These are:
 184 * you may assume that integers are 2s complement representation
 185 * you may assume that right shift of a signed integer duplicates
 186   the sign bit (ie it is an arithmetic shift, not a logical shift)
 188In addition, QEMU assumes that the compiler does not use the latitude
 189given in C99 and C11 to treat aspects of signed '<<' as undefined, as
 190documented in the GNU Compiler Collection manual starting at version 4.0.
 1927. Error handling and reporting
 1947.1 Reporting errors to the human user
 196Do not use printf(), fprintf() or monitor_printf().  Instead, use
 197error_report() or error_vreport() from error-report.h.  This ensures the
 198error is reported in the right place (current monitor or stderr), and in
 199a uniform format.
 201Use error_printf() & friends to print additional information.
 203error_report() prints the current location.  In certain common cases
 204like command line parsing, the current location is tracked
 205automatically.  To manipulate it manually, use the loc_*() from
 2087.2 Propagating errors
 210An error can't always be reported to the user right where it's detected,
 211but often needs to be propagated up the call chain to a place that can
 212handle it.  This can be done in various ways.
 214The most flexible one is Error objects.  See error.h for usage
 217Use the simplest suitable method to communicate success / failure to
 218callers.  Stick to common methods: non-negative on success / -1 on
 219error, non-negative / -errno, non-null / null, or Error objects.
 221Example: when a function returns a non-null pointer on success, and it
 222can fail only in one way (as far as the caller is concerned), returning
 223null on failure is just fine, and certainly simpler and a lot easier on
 224the eyes than propagating an Error object through an Error ** parameter.
 226Example: when a function's callers need to report details on failure
 227only the function really knows, use Error **, and set suitable errors.
 229Do not report an error to the user when you're also returning an error
 230for somebody else to handle.  Leave the reporting to the place that
 231consumes the error returned.
 2337.3 Handling errors
 235Calling exit() is fine when handling configuration errors during
 236startup.  It's problematic during normal operation.  In particular,
 237monitor commands should never exit().
 239Do not call exit() or abort() to handle an error that can be triggered
 240by the guest (e.g., some unimplemented corner case in guest code
 241translation or device emulation).  Guests should not be able to
 242terminate QEMU.
 244Note that &error_fatal is just another way to exit(1), and &error_abort
 245is just another way to abort().