I am getting the following error
rudimentary_calc.c: In function ‘main’:
rudimentary_calc.c:9:6: error: conflicting types for ‘getline’
9 | int getline(char line[], int max) ;
| ^~~~~~~
In file included from rudimentary_calc.c:1:
/usr/include/stdio.h:616:18: note: previous declaration of ‘getline’ was here
616 | extern __ssize_t getline (char **__restrict __lineptr,
| ^~~~~~~
when I ran the following code
#include <stdio.h>
#define maxline 100
int main()
{
double sum, atof(char[]);
char line[maxline];
int getline(char line[], int max) ;
sum = 0;
while (getline(line, maxline) > 0)
printf("\t %g \n", sum += atof(line));
return 0;
}
What am I doing wrong? I am very new to C, so I don't know what went wrong.
Generally, you should not have to declare "built-in" functions as long as you #include the appropriate header files (in this case stdio.h). The compiler is complaining that your declaration is not exactly the same as the one in stdio.h.
The venerable K&R book defines a function named getline. The GNU C library also defines a non-standard function named getline. It is not compatible with the function defined in K&R. It is declared in the standard <stdio.h> header. So there is a name conflict (something that every C programmer has do deal with).
You can instruct GCC to ignore non-standard names found in standard headers. You need to supply a compilation flag such as -std=c99 or -std=c11 or any other std=c<year> flag that yout compiler supports.
Live demo
Always use one of these flags, plus at least -Wall, to compile any C code, including code from K&R. You may encounter some compiler warnings or even errors. This is good. Thy will tell you that there are some code constructs that were good in the days of K&R, but are considered problematic now. You want to know about those. The book is rather old and the best practices and the C language itself have evolved since.
Related
I'm new to kernel development, and I need to write a Linux kernel module that performs several matrix multiplications (I'm working on an x64_64 platform). I'm trying to use fixed-point values for these operations, however during compilation, the compiler encounters this error:
error: SSE register return with SSE disabled
I don't know that much about SSE or this issue in particular, but from what i've found and according to most answers to questions about this problem, it is related to the usage of Floating-Point (FP) arithmetic in kernel space, which seems to be rarely a good idea (hence the utilization of Fixed-Point arithmetics). This error seems weird to me because I'm pretty sure I'm not using any FP values or operations, however it keeps popping up and in some ways that seem weird to me. For instance, I have this block of code:
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
const int scale = 16;
#define DOUBLE_TO_FIXED(x) ((x) * (1 << scale))
#define FIXED_TO_DOUBLE(x) ((x) / (1 << scale))
#define MULT(x, y) ((((x) >> 8) * ((y) >> 8)) >> 0)
#define DIV(x, y) (((x) << 8) / (y) << 8)
#define OUTPUT_ROWS 6
#define OUTPUT_COLUMNS 2
struct matrix {
int rows;
int cols;
double *data;
};
double outputlayer_weights[OUTPUT_ROWS * OUTPUT_COLUMNS] =
{
0.7977986, -0.77172316,
-0.43078753, 0.67738613,
-1.04312621, 1.0552227 ,
-0.32619684, 0.14119884,
-0.72325027, 0.64673559,
0.58467862, -0.06229197
};
...
void matmul (struct matrix *A, struct matrix *B, struct matrix *C) {
int i, j, k, a, b, sum, fixed_prod;
if (A->cols != B->rows) {
return;
}
for (i = 0; i < A->rows; i++) {
for (j = 0; j < B->cols; j++) {
sum = 0;
for (k = 0; k < A->cols; k++) {
a = DOUBLE_TO_FIXED(A->data[i * A->rows + k]);
b = DOUBLE_TO_FIXED(B->data[k * B->rows + j]);
fixed_prod = MULT(a, b);
sum += fixed_prod;
}
/* Commented the following line, causes error */
//C->data[i * C->rows + j] = sum;
}
}
}
...
static int __init insert_matmul_init (void)
{
printk(KERN_INFO "INSERTING MATMUL");
return 0;
}
static void __exit insert_matmul_exit (void)
{
printk(KERN_INFO "REMOVING MATMUL");
}
module_init (insert_matmul_init);
module_exit (insert_matmul_exit);
which compiles with no errors (I left out code that I found irrelevant to the problem). I have made sure to comment any error-prone lines to get to a point where the program can be compiled with no errors, and I am trying to solve each of them one by one. However, when uncommenting this line:
C->data[i * C->rows + j] = sum;
I get this error message in a previous (unmodified) line of code:
error: SSE register return with SSE disabled
sum += fixed_prod;
~~~~^~~~~~~~~~~~~
From what I understand, there are no FP operations taking place, at least in this section, so I need help figuring out what might be causing this error. Maybe my fixed-point implementation is flawed (I'm no expert in that matter either), or maybe I'm missing something obvious. Just in case, I have tested the same logic in a user-space program (using Floating-Point values) and it seems to work fine. In either case, any help in solving this issue would be appreciated. Thanks in advance!
Edit: I have included the definition of matrix and an example matrix. I have been using the default kbuild command for building external modules, here is what my Makefile looks like:
obj-m = matrix_mult.o
KVERSION = $(shell uname -r)
all:
make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules
Linux compiles kernel code with -mgeneral-regs-only on x86, which produces this error in functions that do anything with FP or SIMD. (Except via inline asm, because then the compiler doesn't see the FP instructions, only the assembler does.)
From what I understand, there are no FP operations taking place, at least in this section, so I need help figuring out what might be causing this error.
GCC optimizes whole functions when optimization is enabled, and you are using FP inside that function. You're doing FP multiply and truncating conversion to integer with your macro and assigning the result to an int, since the MCVE you eventually provided shows struct matrix containing double *data.
If you stop the compiler from using FP instructions (like Linux does by building with -mgeneral-regs-only), it refuses to compile your file instead of doing software floating-point.
The only odd thing is that it pins down the error to an integer += instead of one of the statements that compiles to a mulsd and cvttsd2si
If you disable optimization (-O0 -mgeneral-regs-only) you get a more obvious location for the same error (https://godbolt.org/z/Tv5nG6nd4):
<source>: In function 'void matmul(matrix*, matrix*, matrix*)':
<source>:9:33: error: SSE register return with SSE disabled
9 | #define DOUBLE_TO_FIXED(x) ((x) * (1 << scale))
| ~~~~~^~~~~~~~~~~~~~~
<source>:46:21: note: in expansion of macro 'DOUBLE_TO_FIXED'
46 | a = DOUBLE_TO_FIXED(A->data[i * A->rows + k]);
| ^~~~~~~~~~~~~~~
If you really want to know what's going on with the GCC internals, you could dig into it with -fdump-tree-... options, e.g. on the Godbolt compiler explorer there's a dropdown for GCC Tree / RTL output that would let you look at the GIMPLE or RTL internal representation of your function's logic after various analyzer passes.
But if you just want to know whether there's a way to make this function work, no obviously not, unless you compile a file without -mgeneral-registers-only. All functions in a file compiled that way must only be called by callers that have used kernel_fpu_begin() before the call. (and kernel_fpu_end after).
You can't safely use kernel_fpu_begin inside a function compiled to allow it to use SSE / x87 registers; it might already have corrupted user-space FPU state before calling the function, after optimization. The symptom of getting this wrong is not a fault, it's corrupting user-space state, so don't assume that happens to work = correct. Also, depending on how GCC optimizes, the code-gen might be fine with your version, but might be broken with earlier or later GCC or clang versions. I somewhat expect that kernel_fpu_begin() at the top of this function would get called before the compiler did anything with FP instructions, but that doesn't mean it would be safe and correct.
See also Generate and optimize FP / SIMD code in the Linux Kernel on files which contains kernel_fpu_begin()?
Apparently -msse2 overrides -mgeneral-regs-only, so that's probably just an alias for -mno-mmx -mno-sse and whatever options disables x87. So you might be able to use __attribute__((target("sse2"))) on a function without changing build options for it, but that would be x86-specific. Of course, so is -mgeneral-regs-only. And there isn't a -mno-general-regs-only option to override the kernel's normal CFLAGS.
I don't have a specific suggestion for the best way to set up a build option if you really do think it's worth using kernel_fpu_begin at all, here (rather than using fixed-point the whole way through).
Obviously if you do save/restore the FPU state, you might as well use it for the loop instead of using FP to convert to fixed-point and back.
I'm playing around with gcc and g++ compiler and trying to compile some C code within those, my purpose is to see how the compiler / linker enforces that when linking a model with some function declaration to a model with that implementation of that function, the correct function are linked ( in terms of parameters passed and values returned )
for example let's take a look at this code
#include <stdio.h>
extern int foo(int b, int c);
int main()
{
int f = foo(5, 8);
printf("%d",f);
}
after compilation within my symbol table I'd have a symbol for foo, but within the elf file format there is not place that describes the arguments taken and the function signature, ( int(int,int) ), so basically if I write some other code such as this:
char foo(int a, int b, int c)
{
return (char) ( a + b + c );
}
compile that model it'll also have some symbol called foo, what if I link these models together, what's gonna happen? I have never thought of this, and how would a compiler overcome this weakness... I know that within g++ the compiler generates some prefix for every symbol regarding to it's namespace, but does it also take in mind the signature? If anyone has ever encountered this it would be great if he could shed some light upon this problem
The problem is solved with name mangling.
In compiler construction, name mangling (also called name decoration)
is a technique used to solve various problems caused by the need to
resolve unique names for programming entities in many modern
programming languages.
It provides a way of encoding additional information in the name of a
function, structure, class or another datatype in order to pass more
semantic information from the compilers to linkers.
The need arises where the language allows different entities to be
named with the same identifier as long as they occupy a different
namespace (where a namespace is typically defined by a module, class,
or explicit namespace directive) or have different signatures (such as
function overloading).
Note the simple example:
Consider the following two definitions of f() in a C++ program:
int f (void) { return 1; }
int f (int) { return 0; }
void g (void) { int i = f(), j = f(0); }
These are distinct functions, with no relation to each other apart
from the name. If they were natively translated into C with no
changes, the result would be an error — C does not permit two
functions with the same name. The C++ compiler therefore will encode
the type information in the symbol name, the result being something
resembling:
int __f_v (void) { return 1; }
int __f_i (int) { return 0; }
void __g_v (void) { int i = __f_v(), j = __f_i(0); }
Notice that g() is mangled even though there is no conflict; name
mangling applies to all symbols.
Wow, I've kept exploring and testing it on my own and I came up with a solution which quietly amazed my mind,
so I wrote the following code and compiled it on a gcc compiler
main.c
#include <stdio.h>
extern int foo(int a, char b);
int main()
{
int g = foo(5, 6);
printf("%d", g);
return 0;
}
foo.c
typedef struct{
int a;
int b;
char c;
char d;
} mystruct;
mystruct foo(int a, int b)
{
mystruct myl;
my.a = a;
my.b = a + 1;
my.c = (char) b;
my.d = (char b + 1;
return my1;
}
now I compiled foo.c to foo.o with gcc firstly and checked the symbol table using
readelf and I had some entry called foo
also after that I compiled main.c to main.o checked the symbol table and it also had some entry called foo, I linked those two together and surprisingly it worked, I ran main.o and obviously encountered some segmentation fault, which makes sense as the actual implementation of foo as implemented in foo.o probably expects three parameters (first one should be struct adders), a parameter which isn't passed in main.o under it's definition to foo then the actual implementation accesses some memory that doesn't belong to it from the stack frame of main, then tries accessing addresses that it thought it got, and ends up with segmentation fault, that's fine,
now I compiled both models again with g++ and not gcc and what came up was amazing.. I found out that the symbol entry under foo.o was _Z3fooii and under main.o it was _Z3fooic, now my guess is that the ii suffix means int int and ic suffix means int char which probably refers to the parameters that should be passed to function hence allowing the compiler to know some function deceleration gets the actual implementation. so I changed my foo declaration in main.c to
extern int foo(int a, int b);
re-compiled and this time got the symbol _Z3fooii, I linked both models again and amazingly this time it worked, I tried running it and again encountered segmentation fault, which again also makes sense as the compiler wont always even authorize correct return values.. anyways what was my original thought - that g++ includes function signature within symbol name and thus enforces the linker to give function implementation get correct parameters to correct function declaration
I have mainly two kinds of compile warning:
1. implicit declaration of function
in a.c, it has char *foo(char *ptr1, char *ptr2), in b.c, some functions use this foo function without any declaration, and I found seems compiler will treat the function foo return value as integer, and even I can pass some variables less or more than foo function declaration
2. enumerated type mixed with another type
My target chip is ARM11, it seems that even I don't solve these two kinds of compile warning, my program can run without any issues, but I believe it must have some risk behind these. Can anyone give me some good example that these two kinds of compile warning can cause some unexpected issues?
Meanwhile, if these two warnings have potential risk, why c compiler allow these kinds warning happen but not set them to error directly? any story behind?
Implicit declaration. E.g. you have function: float foo(float a), which isn't declared when you call it. Implicit rules will create auto-declaration with following signature: int foo(double) (if passed argument is float). So value you pass will be converted to double, but foo expects float. The same with return - calling code expects int, but returned float. Values would be a complete mess.
enum mixed with other type. Enumerated type have list of values it could take. If you trying to assign numeric value to it, there is a chance that it isn't one of listed values; if later your code expects only specified range and presumes nothing else could be there - it could misbehave.
Simple example:
File: warn.c
#include <stdio.h>
double foo(double x)
{
return myid(x);
}
int
main (void)
{
double x = 1.0;
fprintf (stderr, "%lg == %lg\n", x, foo (x));
return 0;
}
File: foo.c
double
myid (double x)
{
return x;
}
Compile and run:
$ gcc warn.c foo.c -Wall
warn.c: In function ‘foo’:
warn.c:5: warning: implicit declaration of function ‘myfabs’
$ ./a.out
1 == 0
Old C standard (C90) had this strange "default int" rule and for compatibility it is supported even in latest compilers.
I want to use gcc to do some compile-time checking on function inputs if the compiler knows that they are constants.
I have a solution that very almost works, and as far as I can see, it should work.
Note: __builtin_constant_p(expression) is supposed to returns whether an expression is known to be a constant at compile time.
Assuming we want to check whether port<2 when calling uart(port), the following code should work:
#include <stdio.h>
void _uart(int port) {
printf("port is %d", port);
}
#define uart(port) \
static_assert(__builtin_constant_p(port)? port<2: 1, "parameter port must be < 2"); \
_uart(port)
int main(void) {
int x=1;
uart(x);
}
This works when calling uart(). Unfortunately, it doesn't quite work for non-constant x. For some reason static_assert can't handle the case where x is not a constant, even though in theory __builtin_constant_p() won't even pass it a constant. The error message I get is:
c:\>gcc a.cpp -std=c++0x -Os
a.cpp: In function 'int main()':
a.cpp:13: error: 'x' cannot appear in a constant-expression
Any ideas?
Your code works with g++ (GCC) 4.8.2.
- but not with optimization, as you correctly noted.
If only we could use
static_assert(__builtin_choose_expr(__builtin_constant_p(port), \
port<2, 1), "parameter port must be < 2")
- but unfortunately the __builtin_choose_expr construct is currently only available for C.
However, there is a C++ patch which sadly didn't make it into the release yet.
You can try the trick used in the Linux kernel:
What is ":-!!" in C code?
The (somewhat horrible) Linux kernel macro is less strict about what kinds of expressions are allowed in the parameter.
As this is my first post to stackoverflow I want to thank you all for your valuable posts that helped me a lot in the past.
I use MinGW (gcc 4.4.0) on Windows-7(64) - more specifically I use Nokia Qt + MinGW but Qt is not involved in my Question.
I need to find the address and -more important- the length of specific functions of my application at runtime, in order to encode/decode these functions and implement a software protection system.
I already found a solution on how to compute the length of a function, by assuming that static functions placed one after each other in a source-file, it is logical to be also sequentially placed in the compiled object file and subsequently in memory.
Unfortunately this is true only if the whole CPP file is compiled with option: "g++ -O0" (optimization level = 0).
If I compile it with "g++ -O2" (which is the default for my project) the compiler seems to relocate some of the functions and as a result the computed function length seems to be both incorrect and negative(!).
This is happening even if I put a "#pragma GCC optimize 0" line in the source file,
which is supposed to be the equivalent of a "g++ -O0" command line option.
I suppose that "g++ -O2" instructs the compiler to perform some global file-level optimization (some function relocation?) which is not avoided by using the #pragma directive.
Do you have any idea how to prevent this, without having to compile the whole file with -O0 option?
OR: Do you know of any other method to find the length of a function at runtime?
I prepare a small example for you, and the results with different compilation options, to highlight the case.
The Source:
// ===================================================================
// test.cpp
//
// Intention: To find the addr and length of a function at runtime
// Problem: The application output is correct when compiled with: "g++ -O0"
// but it's erroneous when compiled with "g++ -O2"
// (although a directive "#pragma GCC optimize 0" is present)
// ===================================================================
#include <stdio.h>
#include <math.h>
#pragma GCC optimize 0
static int test_01(int p1)
{
putchar('a');
putchar('\n');
return 1;
}
static int test_02(int p1)
{
putchar('b');
putchar('b');
putchar('\n');
return 2;
}
static int test_03(int p1)
{
putchar('c');
putchar('\n');
return 3;
}
static int test_04(int p1)
{
putchar('d');
putchar('\n');
return 4;
}
// Print a HexDump of a specific address and length
void HexDump(void *startAddr, long len)
{
unsigned char *buf = (unsigned char *)startAddr;
printf("addr:%ld, len:%ld\n", (long )startAddr, len);
len = (long )fabs(len);
while (len)
{
printf("%02x.", *buf);
buf++;
len--;
}
printf("\n");
}
int main(int argc, char *argv[])
{
printf("======================\n");
long fun_len = (long )test_02 - (long )test_01;
HexDump((void *)test_01, fun_len);
printf("======================\n");
fun_len = (long )test_03 - (long )test_02;
HexDump((void *)test_02, fun_len);
printf("======================\n");
fun_len = (long )test_04 - (long )test_03;
HexDump((void *)test_03, fun_len);
printf("Test End\n");
getchar();
// Just a trick to block optimizer from eliminating test_xx() functions as unused
if (argc > 1)
{
test_01(1);
test_02(2);
test_03(3);
test_04(4);
}
}
The (correct) Output when compiled with "g++ -O0":
[note the 'c3' byte (= assembly 'ret') at the end of all functions]
======================
addr:4199344, len:37
55.89.e5.83.ec.18.c7.04.24.61.00.00.00.e8.4e.62.00.00.c7.04.24.0a.00.00.00.e8.42
.62.00.00.b8.01.00.00.00.c9.c3.
======================
addr:4199381, len:49
55.89.e5.83.ec.18.c7.04.24.62.00.00.00.e8.29.62.00.00.c7.04.24.62.00.00.00.e8.1d
.62.00.00.c7.04.24.0a.00.00.00.e8.11.62.00.00.b8.02.00.00.00.c9.c3.
======================
addr:4199430, len:37
55.89.e5.83.ec.18.c7.04.24.63.00.00.00.e8.f8.61.00.00.c7.04.24.0a.00.00.00.e8.ec
.61.00.00.b8.03.00.00.00.c9.c3.
Test End
The erroneous Output when compiled with "g++ -O2":
(a) function test_01 addr & len seem correct
(b) functions test_02, test_03 have negative lengths,
and fun. test_02 length is also incorrect.
======================
addr:4199416, len:36
83.ec.1c.c7.04.24.61.00.00.00.e8.c5.61.00.00.c7.04.24.0a.00.00.00.e8.b9.61.00.00
.b8.01.00.00.00.83.c4.1c.c3.
======================
addr:4199452, len:-72
83.ec.1c.c7.04.24.62.00.00.00.e8.a1.61.00.00.c7.04.24.62.00.00.00.e8.95.61.00.00
.c7.04.24.0a.00.00.00.e8.89.61.00.00.b8.02.00.00.00.83.c4.1c.c3.57.56.53.83.ec.2
0.8b.5c.24.34.8b.7c.24.30.89.5c.24.08.89.7c.24.04.c7.04.
======================
addr:4199380, len:-36
83.ec.1c.c7.04.24.63.00.00.00.e8.e9.61.00.00.c7.04.24.0a.00.00.00.e8.dd.61.00.00
.b8.03.00.00.00.83.c4.1c.c3.
Test End
This is happening even if I put a "#pragma GCC optimize 0" line in the source file, which is supposed to be the equivalent of a "g++ -O0" command line option.
I don't believe this is true: it is supposed to be the equivalent of attaching __attribute__((optimize(0))) to subsequently defined functions, which causes those functions to be compiled with a different optimisation level. But this does not affect what goes on at the top level, whereas the command line option does.
If you really must do horrible things that rely on top level ordering, try the -fno-toplevel-reorder option. And I suspect that it would be a good idea to add __attribute__((noinline)) to the functions in question as well.