I'm working on an embedded C project and would like to initialize, at compile-time, a structure that is stored in flash (0x1200u), but I keep getting weird compile errors.
typedef struct {
float foo[2];
...other stuff...
} MY_STRUCT_DATA_TYPE
#define MY_FLASH_STRUCT ((MY_STRUCT_DATA_TYPE *)(0x1200u)) // <-- error here
MY_FLASH_STRUCT MY_InitdStruct = {
.foo[0] = 0.12345f;
.foo[1] = 0.54321f;
...other stuff...
};
The error I'm getting is "expected '{' before '(' token." Anyone know how to make this work?
Update
Added to linker file...
MEMORY
{
... other stuff ...
m_my_data_section (RW) : ORIGIN = 0x00001200, LENGTH = 0x00000400
... other stuff ...
}
SECTIONS
{
... other stuff ..
.my_data_section :
{
. = ALIGN(4);
KEEP(*(.my_data_section))
. = ALIGN(4);
} > m_my_data_section
... other stuff ...
}
C code...
static volatile const MY_STRUCT_DATA_TYPE __attribute__ ((section (".my_data_section"))) MY_InitdStruct = {
.foo[0] = 0.12345f;
.foo[1] = 0.54321f;
...other stuff...
};
I'm not sure the static or const keywords are necessary, since it's intended only for one-time use to initialize that section of flash memory at compile-time, but it doesn't hurt to restrict the label's usage.
That makes no sense at all, syntactically that is.
What you need to do is figure out how your compiler supports this, since it's not something you can do with just standard C.
With GCC, you use __attribute() to put the symbol in a particular segment, then use the linker script to put that segment in a particular piece of actual memory.
Or, just give your compiler the benefit of the doubt and try a static const structure, that should end up in flash.
In the example you gave, a ";" is missing after the declaration of MY_STRUCT_DATA_TYPE
typedef struct {
float foo[2];
...other stuff...
} MY_STRUCT_DATA_TYPE;
If it's not a copie/paste mistake, it's the kind of error which could lead to the type of error message you have
I'm working on an embedded C project and would like to initialize, at
compile-time, a structure that is stored in flash (0x1200u), but I
keep getting weird compile errors.
That's unsurprising, as C does not support what you're trying to do. You can provide initializers for objects that C allocates, including pointers, but the C language has no concept of objects that exist independently. Indeed, C makes no guarantees whatever about what happens when you do anything with (MY_STRUCT_DATA_TYPE *)(0x1200u) other than convert it back to an integer.
The error I'm getting is "expected '{' before '(' token."
The compiler is complaining because in
MY_FLASH_STRUCT MY_InitdStruct = {
, the expansion of MY_FLASH_STRUCT is not a type, therefore the construct is not a valid declaration. Nor is it a valid assignment, but even if it were, assignment statements are executable, and therefore may appear only inside functions.
Assigning an object to a specific address would be a function of the linker. Whether you can assign an object to the particular address you want is system-dependent, and the mechanism, if any, for doing so depends on your toolchain.
Related
In the linker script, I have defined
MEMORY {
sec_1 : ORIGIN = 0x1B800, LENGTH = 2048
......
}
How can I read the start address of this section in C? I would like to copy it in a variable in the C code
Basically to achieve this, you have two tasks to fulfill:
Tell the linker to save the start address of the section. This can be achieved by placing a symbol in the linker script at the beginning of your section.
Tell the compiler to save initialize a constant with an address filled in later by the linker
As for the first step: In your section sec_1 you have to place a symbol that will be placed at the start of that section:
SECTIONS
{
...
.sec_1 :
{
__SEC_1_START = ABSOLUTE(.); /* <-- add this */
...
} > sec_1
...
}
Now that the linker produces bespoke symbol, you have to make it accessible from the compiler side. In order to do so, you need somewhere some code like this:
/* Make the compiler aware of the linker symbol, by telling it, there
is something, somewhere that the linker will put together (i.e. "extern") */
extern int __SEC_1_START;
void Sec1StartPrint(void) {
void * const SEC_1_START = &__SEC_1_START;
printf("The start address for sec_1 is: %p", SEC_1_START);
}
By calling Sec1StartPrint() you should get an address output that matches your *.map file the linker created.
What is the intention to set handle to an object as pointer-to pointer but not pointer? Like following code:
FT_Library library;
FT_Error error = FT_Init_FreeType( &library );
where
typedef struct FT_LibraryRec_ *FT_Library
so &library is a FT_LIBraryRec_ handle of type FT_LIBraryRec_**
It's a way to emulate pass by reference in C, which otherwise only have pass by value.
The 'C' library function FT_Init_FreeType has two outputs, the error code and/or the library handle (which is a pointer).
In C++ we'd more naturally either:
return an object which encapsulated the success or failure of the call and the library handle, or
return one output - the library handle, and throw an exception on failure.
C APIs are generally not implemented this way.
It is not unusual for a C Library function to return a success code, and to be passed the addresses of in/out variables to be conditionally mutated, as per the case above.
The approach hides implementation. It speeds up compilation of your code. It allows to upgrade data structures used by the library without breaking existing code that uses them. Finally, it makes sure the address of that object never changes, and that you don’t copy these objects.
Here’s how the version with a single pointer might be implemented:
struct FT_Struct
{
// Some fields/properties go here, e.g.
int field1;
char* field2;
}
FT_Error Init( FT_Struct* p )
{
p->field1 = 11;
p->field2 = malloc( 100 );
if( nullptr == p->field2 )
return E_OUTOFMEMORY;
return S_OK;
}
Or C++ equivalent, without any pointers:
class FT_Struct
{
int field1;
std::vector<char> field2;
public:
FT_Struct() :
field1( 11 )
{
field2.resize( 100 );
}
};
As a user of the library, you have to include struct/class FT_Struct definition. Libraries can be very complex so this will slow down compilation of your code.
If the library is dynamic i.e. *.dll on windows, *.so on linux or *.dylib on osx, you upgrade the library and if the new version changes memory layout of the struct/class, old applications will crash.
Because of the way C++ works, objects are passed by value, i.e. you normally expect them to be movable and copiable, which is not necessarily what library author wants to support.
Now consider the following function instead:
FT_Error Init( FT_Struct** pp )
{
try
{
*pp = new FT_Struct();
return S_OK;
}
catch( std::exception& ex )
{
return E_FAIL;
}
}
As a user of the library, you no longer need to know what’s inside FT_Struct or even what size it is. You don’t need to #include the implementation details, i.e. compilation will be faster.
This plays nicely with dynamic libraries, library author can change memory layout however they please, as long as the C API is stable, old apps will continue to work.
The API guarantees you won’t copy or move the values, you can’t copy structures of unknown lengths.
I'm wondering if the following is possible:
I have a namespace-style struct setup, filled with just function pointers. These are provided in the header file like so:
typedef struct {
int32_t(*const event_construct)(struct sync_event* evt);
int32_t(*const event_destroy)(struct sync_event* evt);
int32_t(*const event_set)(struct sync_event* evt);
int32_t(*const event_wait)(struct sync_event* evt);
} namespace_sync;
extern namespace_sync const sync;
and then in the relevant source file, after all the function implementations:
...
namespace_sync const sync = {
sync_event_construct,
sync_event_destroy,
sync_event_set,
sync_event_wait
};
Say I want to add an extra function not at the end; I add it to the struct and source file, but forget to assign it. Because the function declarations match, a warning isn't generated for it, and the compiler (at least in this example, vs2013) doesn't provide a hint that there's an issue.
I've got compile-time assertion checks available, but not sure if I can verify this particular aspect, since the struct size is accurate. If vs2013 can't work with it - I'm aware it's an abysmal C compiler(!) - the newest versions of gcc will also be used, so I could limit the functionality to one compiler.
One solution going forward would be to use designated initializers:
namespace_sync const sync = {
.event_construct = sync_event_construct,
.event_destroy = sync_event_destroy,
.event_set = sync_event_set,
.event_wait = sync_event_wait
};
Any unlisted members will default to null pointers.
I would advise to not add members into the middle of a struct, because it is difficult to be sure that you have correctly updated any code that was relying on the old struct layout. However, if you really do want to do this, then one way to have the compiler indicate to you where all the uses of the struct are is to change the struct name:
typedef struct {
// ...
} namespace_sync_2;
Then the code namespace_sync const sync will cause a compilation error. This alerts you to the fact that this piece of code requires a code review to make sure that it will work correctly with the new struct layout.
I'm working in C on Linux. I've seen the usage of of the gcc __section__ attribute (especially in the Linux kernel) to collect data (usually function pointers) into custom ELF sections. How is the "stuff" that gets put in those custom sections retrieved and used?
As long as the section name results in a valid C variable name, gcc (ld, rather) generates two magic variables: __start_SECTION and __stop_SECTION. Those can be used to retrieve the start and end addresses of a section, like so:
/**
* Assuming you've tagged some stuff earlier with:
* __attribute((__section__("my_custom_section")))
*/
struct thing *iter = &__start_my_custom_section;
for ( ; iter < &__stop_my_custom_section; ++iter) {
/* do something with *iter */
}
I couldn’t find any formal documentation for this feature, only a few obscure mailing list references. If you know where the docs are, drop a comment!
If you're using your own linker script (as the Linux kernel does) you'll have to add the magic variables yourself (see vmlinux.lds.[Sh] and this SO answer).
See here for another example of using custom ELF sections.
Collecting the information together from various answers, here is a working example of how to collect information into a custom linker section and then read the information from that section using the magic variables __start_SECTION and __stop_SECTION in your C program, where SECTION is the name of the section in the link map.
The __start_SECTION and __stop_SECTION variables are made available by the linker so explicit extern references need to be created for these variables when they are used from C code.
There are also some problems if the alignment used by the compiler for calculating pointer/array offsets is different than the alignment of the objects packed in each section by the linker. One solution (used in this example) is to store only a pointer to the data in the linker section.
#include <stdio.h>
struct thing {
int val;
const char* str;
int another_val;
};
struct thing data1 = {1, "one"};
struct thing data2 = {2, "two"};
/* The following two pointers will be placed in "my_custom_section".
* Store pointers (instead of structs) in "my_custom_section" to ensure
* matching alignment when accessed using iterator in main(). */
struct thing *p_one __attribute__((section("my_custom_section"))) = &data1;
struct thing *p_two __attribute__((section("my_custom_section"))) = &data2;
/* The linker automatically creates these symbols for "my_custom_section". */
extern struct thing *__start_my_custom_section;
extern struct thing *__stop_my_custom_section;
int main(void) {
struct thing **iter = &__start_my_custom_section;
for ( ; iter < &__stop_my_custom_section; ++iter) {
printf("Have thing %d: '%s'\n", (*iter)->val, (*iter)->str);
}
return 0;
}
Linker can use the symbols defined in the code, and can assign their initial values if you use the exact name in the linker script:
_smysection = .;
*(.mysection)
*(.mysection*)
_emysection = .;
Just define a variable in C code:
const void * _smysection;
And then you can access that as a regular variable.
u32 someVar = (u32)&_smysection;
So the answer above, __start_SECTION and __stop_SECTION will work, however for the program to be able to use the information from the linker you to need to declare those variables as extern char* __start_SECTION. Enjoy!
extern char * __start_blobby;
...
printf("This section starts at %p\n", (unsigned int)&__start_blobby);
...
HI: like this.
extern const struct pseudo_ta_head __start_ta_head_section;
extern const struct pseudo_ta_head __stop_ta_head_section;
const struct pseudo_ta_head *start = &__start_ta_head_section;
const struct pseudo_ta_head *end = &__stop_ta_head_section;
I was trying to write a small debug utility and for this I need to get the function/global variable address given its name. This is built-in debug utility, which means that the debug utility will run from within the code to be debugged or in plain words I cannot parse the executable file.
Now is there a well-known way to do that ? The plan I have is to make the .debug_* sections to to be loaded into to memory [which I plan to do by a cheap trick like this in ld script]
.data {
*(.data)
__sym_start = .;
(debug_);
__sym_end = .;
}
Now I have to parse the section to get the information I need, but I am not sure this is doable or is there issues with this - this is all just theory. But it also seems like too much of work :-) is there a simple way. Or if someone can tell upfront why my scheme will not work, it ill also be helpful.
Thanks in Advance,
Alex.
If you are running under a system with dlopen(3) and dlsym(3) (like Linux) you should be able to:
char thing_string[] = "thing_you_want_to_look_up";
void * handle = dlopen(NULL, RTLD_LAZY | RTLD_NOLOAD);
// you could do RTLD_NOW as well. shouldn't matter
if (!handle) {
fprintf(stderr, "Dynamic linking on main module : %s\n", dlerror() );
exit(1);
}
void * addr = dlsym(handle, thing_string);
fprintf(stderr, "%s is at %p\n", thing_string, addr);
I don't know the best way to do this for other systems, and this probably won't work for static variables and functions. C++ symbol names will be mangled, if you are interested in working with them.
To expand this to work for shared libraries you could probably get the names of the currently loaded libraries from /proc/self/maps and then pass the library file names into dlopen, though this could fail if the library has been renamed or deleted.
There are probably several other much better ways to go about this.
edit without using dlopen
/* name_addr.h */
struct name_addr {
const char * sym_name;
const void * sym_addr;
};
typedef struct name_addr name_addr_t;
void * sym_lookup(cost char * name);
extern const name_addr_t name_addr_table;
extern const unsigned name_addr_table_size;
/* name_addr_table.c */
#include "name_addr.h"
#define PREMEMBER( X ) extern const void * X
#define REMEMBER( X ) { .sym_name = #X , .sym_addr = (void *) X }
PREMEMBER(strcmp);
PREMEMBER(printf);
PREMEMBER(main);
PREMEMBER(memcmp);
PREMEMBER(bsearch);
PREMEMBER(sym_lookup);
/* ... */
const name_addr_t name_addr_table[] =
{
/* You could do a #include here that included the list, which would allow you
* to have an empty list by default without regenerating the entire file, as
* long as your compiler only warns about missing include targets.
*/
REMEMBER(strcmp),
REMEMBER(printf),
REMEMBER(main),
REMEMBER(memcmp),
REMEMBER(bsearch),
REMEMBER(sym_lookup);
/* ... */
};
const unsigned name_addr_table_size = sizeof(name_addr_table)/sizeof(name_addr_t);
/* name_addr_code.c */
#include "name_addr.h"
#include <string.h>
void * sym_lookup(cost char * name) {
unsigned to_go = name_addr_table_size;
const name_addr_t *na = name_addr_table;
while(to_to) {
if ( !strcmp(name, na->sym_name) ) {
return na->sym_addr;
}
na++;
to_do--;
}
/* set errno here if you are using errno */
return NULL; /* Or some other illegal value */
}
If you do it this way the linker will take care of filling in the addresses for you after everything has been laid out. If you include header files for all of the symbols that you are listing in your table then you will not get warnings when you compile the table file, but it will be much easier just to have them all be extern void * and let the compiler warn you about all of them (which it probably will, but not necessarily).
You will also probably want to sort your symbols by name such that you can use a binary search of the list rather than iterate through it.
You should note that if you have members in the table which are not otherwise referenced by the program (like if you had an entry for sqrt in the table, but didn't call it) the linker will then want (need) to link those functions into your image. This can make it blow up.
Also, if you were taking advantage of global optimizations having this table will likely make those less effective since the compiler will think that all of the functions listed could be accessed via pointer from this list and that it cannot see all of the call points.
Putting static functions in this list is not straight forward. You could do this by changing the table to dynamic and doing it at run time from a function in each module, or possibly by generating a new section in your object file that the table lives in. If you are using gcc:
#define SECTION_REMEMBER(X) \
static const name_addr_t _name_addr##X = \
{.sym_name= #X , .sym_addr = (void *) X } \
__attribute__(section("sym_lookup_table" ) )
And tack a list of these onto the end of each .c file with all of the symbols that you want to remember from that file. This will require linker work so that the linker will know what to do with these members, but then you can iterate over the list by looking at the begin and end of the section that it resides in (I don't know exactly how to do this, but I know it can be done and isn't TOO difficult). This will make having a sorted list more difficult, though. Also, I'm not entirely certain initializing the .sym_name to a string literal's address would not result in cramming the string into this section, but I don't think it would. If it did then this would break things.
You can still use objdump to get a list of the symbols that the object file (probably elf) contains, and then filter this for the symbols you are interested in, and then regenerate the table file the table's members listed.