I am trying to print logs into a file by writing macros. My macro looks like as shown below:
#define LOG(fmt,...){\
FILE *F;\
F = fopen("output.txt","a");\
fprintf(F,fmt " %s %d",__VA_ARGS__,__FILE__,__LINE__);}
And I plan to call LOG in the following format:
LOG("values are : %d %d",num1,num2);
But when I compile I get the error
error: expected expression before ‘,’ token
fprintf(F,fmt " %s %d",__VA_ARGS__,__FILE__,__LINE__);}
Can someone please explain where I am going wrong?
First of all you have to wrap you macro into a do-while loop, so it will be handled with expressions correctly.
#define LOG( fmt , ... ) do{ }while(0)
Then you have to make sure that fopen() call succeeds and that you close the file after usage.
FILE* f = fopen( "output.txt" , "a" ) ;
if( !f )
break ; //break works because you are in a loop
fclose( f ) ; //also flushes the stream
Then you include the print in the full macro.
#define LOG( fmt , ... ) \
do{ \
FILE* f = fopen( "output.txt" , "a" ) ; \
if( !f ) \
break ; \
fprintf(f, fmt" %s %d\n",__VA_ARGS__,__FILE__,__LINE__); \
fclose( f ) ; \
}while( 0 )
The call is in the form:
LOG("values are : %d %d",4444,55555);
where you have to input at least one correct optional parameter, with the corresponding flag in the string.
#define LOG(fmt,...){\
FILE *F;\
F = fopen("output.txt","a");\
fprintf(F,fmt " %d %d",__VA_ARGS__,__FILE__,__LINE__);}
Multiple issues
fclose
F. __FILE__
is a string. __VA_ARGS__
must go at the end... ... or use this little hack:
#define LOG(fmt,...){\\ FILE *F;\\ F = fopen("output.txt","a");\\ fprintf(F,fmt " %s %d", ##__VA_ARGS__, __FILE__,__LINE__);}
( ##__VA_ARGS__
is a GCC extension that removes the preceding comma if there are no args).
Also check out this answer about "custom" printf-like functions.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.