typedef - Why this C program complies and runs -

with curiosity of definition , scope of typedef have written below c code in 2 .c files:

main.c

#include <stdio.h>  int main() {     int = 5, b = 6;     printf("a = %d, b = %d\n", a, b);     swap(&a, &b);     printf("a = %d, b = %d\n", a, b); } 

swap.c

typedef t;  void swap(t* a, t* b) {     t t = *a;     *a = *b;     *b = t; } 

to surprise, code files compiled visual studio c compiler (cl.exe /tc main.c swap.c)

and program runs correctly! understanding, typedef requires 2 parameters, why code compiles , runs?

for further analysis, in main function, declared 2 float variables , try swap both after swapping 2 integers, time fails compile (with cl.exe). amazing code compiled , run tiny c (tcc.exe main.c swap.c), works template method!

typedef declaration (it creates aliases existing types), , in no way limited 2 'parameters'. see typedef declarations (c) , fundamental typedef operand syntax.

if write typedef t;, declaring t unspecified type (called "no specified type" in c89 spec). it's little (and little, conceptually might you) #define x defines x preprocessor replace empty string (i.e. remove x).

so, typedefing t unspecified, makes arguments swap function of unspecified type.

what seeing here in c89 (but not c99, results in undefined behaviour - contrast ansi 3.5.2 isoc99 6.7.2), unspecified types default int, why method works integers not floats in visual studio (presumably disallows implicit integer typing default). gcc compile floats, though, provided aren't using -werror, should be.

i suggest turning on warnings: -wall in gcc spit out following, among other things

swap.c:1:9: warning: type defaults ‘int’ in declaration of ‘t’ [-wimplicit-int] 

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the reason "works" on floats because floats , ints same size (32 bits) on machine. try double. or char. or short.

swap this:

void swap(int *a, int *b) {     int t = *a;     *a = *b;     *b = t; } 

with calling

swap((int*)&a, (int*)&b); 

try , compare results yourself.

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edit: tried in tcc. -wall not alert implicit int typing, sadly.