c - How to write a macro that does malloc, formats a string and then returns the formatted string? -

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i have tried this:

#define format(f, ...) \                 int size = strlen(f) + (sizeof((int[]){__va_args__})/sizeof(int)) + 1); \                 char *buf = malloc(size); \                 snprintf(buf, size, f, __va_args__); \                 buf

but returns lot of syntactic errors. how do properly?

c macros not functions 1:1 substitutions. if want use macro this:

mystring = format("%d", 5);

you this:

mystring = int size = strlen(f) + (sizeof((int[]){5})/sizeof(int)) + 1); \             char *buf = malloc(size); \             snprintf(buf, size, f, 5); \             buf;

which not make sense. in case better off defining inline function should not worse in terms of performance on decent compiler.

if really has macro , on gcc, can use compound statement achieve goal. allows this: mystring = ({ statement1, statement2, ..., statementn}) execute statements in local scope , assign statementn mystring. make code non-portable , hell debug.

so here go, please don't use in real applications:

#define format(f, ...) \     ({ int size = snprintf(null, 0, f, __va_args__) + 1;\     char * buf = malloc(size);\     snprintf(buf, size, f, __va_args__); buf; })

i'm serious. don't use this. use inline function. can have variadic arguments in normal functions, using va_arg , va_start:

inline char * format(f, ...) {     va_list args;     va_start(args, f);     int size = vsnprintf(null, 0, f, args) + 1;     char * buf = malloc(size);     vsnprintf(buf, size, f, args);     return buf; }

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