Segmentation fault on vsprintf using macros in C - c

Question

I create a simple logger library in C.I have three files
My example file:
example.c
Library files:
loglib.h
loglib.c
My main logger function is called logme. I defined different macros as wrappers to indicate different log levels:
eg
WARN
ERROR
etc..
In example.c I call the macros:
int int_arg = 55;
WARN(1, "Warn message with level 1");
WARN(1, "Warn message with level %d", int_arg);
The WARN macro is defined in loglib.h:
#define WARN(LEVEL, ...) \
logme(LEVEL, 8, " <%s:%d> inside %s() -- "__VA_ARGS__, __FILE__, __LINE__, __func__);
And lastly here is the logme function:
void slog(int level, int flag, const char *msg, ...)
{
char string[10000];
bzero(string, sizeof(string));
va_list args;
va_start(args, msg);
vsprintf(string, msg, args);
va_end(args);
// .. do other things
}
When I run the example file this is what I get:
inside main() -- Warn message with level 1
Segmentation fault (core dumped)
I get segmentation fault when I call WARN with formated strings.
The segmentation fault appears in vsprintf(string, msg, args);
Is it something wrong with my macro?
Here is the make file of my lib:
CFLAGS = -g -O2 -Wall -lpthread
LIB = -lrt
OBJS = loglib.o
LIBINSTALL = /usr/local/lib
HEADINSTALL = /usr/local/include
.c.o:
$(CC) $(CFLAGS) -c $< $(LIB)
libslog.a: $(OBJS)
$(AR) rcs liblog.a $(OBJS)
#echo [-] Syncing static library
sync
install:
#test -d $(LIBINSTALL) || mkdir $(LIBINSTALL)
#test -d $(HEADINSTALL) || mkdir $(HEADINSTALL)
#install -m 0664 liblog.a $(LIBINSTALL)/
#install -m 0664 loglib.h $(HEADINSTALL)/
#echo [-] Done
loglib.o: loglib.h
.PHONY: clean
clean:
$(RM) liblog.a $(OBJS)

Answer 1

This can't work because you attempt string concatenation of __VA_ARGS__ here:
#define WARN(LEVEL, ...) \
logme(LEVEL, 8, " <%s:%d> inside %s() -- "__VA_ARGS__, __FILE__, __LINE__, __func__);
This messes up your sequence of parameters: __FILE__ will not go with the <%s, etc. You must redefine your WARN() macro so that __VA_ARGS__ is last. You'll have to write a proper variadic function if you want to prepend all that information. Note that __func__ is not a string literal, it references the appropriate string.

Answer 2

The ordering is incorrect when it hits the logme function.
You can do something like the following.
#include <strings.h>
#include <string.h>
#include <stdio.h>
#include <stdarg.h>
void logme(int level, int flag, const char *msg_preamble, const char* file, int line, const char* function, const char* msg, ... )
{
char buffer1[10000];
bzero(buffer1, sizeof(buffer1));
sprintf(buffer1, msg_preamble, file, line, function);
char buffer2[10000];
bzero(buffer2, sizeof(buffer2));
va_list args;
va_start(args, msg);
vsprintf(buffer2, msg, args);
va_end(args);
strcat(buffer1, buffer2);
printf("%s\n", buffer1);
}
#define WARN(LEVEL, ...) \
logme(LEVEL, 8, " <%s:%d> inside %s() -- ", __FILE__, __LINE__, __func__, __VA_ARGS__);
int main(int argc, char** argv)
{
int int_arg = 55;
WARN(1, "Warn message with level 1");
WARN(1, "Warn message with level %d", int_arg);
}

Answer 3

Here is the working solution:
/*
* SOURCE_THROW_LOCATION macro returns string which
* points to the file, as well as, the corresponding line
* of the caller function.
*/
#define LVL1(x) #x
#define LVL2(x) LVL1(x)
#define SOURCE_THROW_LOCATION "<"__FILE__":"LVL2(__LINE__)"> -- "
#define WARN(LEVEL, ...) \
logme(LEVEL, FWARN, SOURCE_THROW_LOCATION __VA_ARGS__);

Answer 4

The argument order is incorrect, hence the %s format does not correspond to the actual argument provided and the program invokes undefined behavior.
Here is a corrected version of the WARN macro:
#define WARN(LEVEL, ...) \
logme(LEVEL, 8, __FILE__, __LINE__, __func__, __VA_ARGS__);
And the corresponding logme function, using vsnprintf() instead of vsprintf() to avoid potential buffer overflows:
void slog(int level, int flag, const char *filename, in lineno,
const char *funcname, const char *msg, ...) {
char string[10000];
int prefix;
va_list args;
// check level, flag..
...
// format the message if needed
prefix = snprintf(string, sizeof string, " <%s:%d> inside %s() -- ",
filename, lineno, funcname);
va_start(args, msg);
vsnprintf(string + prefix, sizeof(string) - prefix, msg, args);
va_end(args);
// do other things
...
}