Myujth

In an LJ post, the --defsym flag was used for passing the build date into the source code:

#include <stdio.h>

extern char __BUILD_DATE;

void main(void) 
 printf("Build date: %un", (unsigned long) &__BUILD_DATE);

by linking with the following flags:

gcc example.c -Xlinker --defsym -Xlinker __BUILD_DATE=$(date +%Y%m%d)

According to the ld manual,

--defsym symbol=expression Create a global symbol in the output file, containing the absolute address given by expression.

I am trying to understand the following:

1. How is the 9-character string for the build date (YYYYmmdd+) stored in memory?


2. If --defsym creates a symbol containing an address, why __BUILD_DATE is defined as a char and not as a pointer or as an integral type?


3. Why __BUILD_DATE is defined as char and not unsigned long if it is eventually casted to unsigned long?

Linkers see globals as addresses (pointers to the "actual" global rather than the actual global -- even if the actual global doesn't exist at that address). -Xlinker --defsym -Xlinker __BUILD_DATE=$(date +%Y%m%d) sets the address of __BUILD_DATE not the value. When the __BUILD_DATE linker entity has an address but not a value, you can get the address by declaring the entity as anything and then taking the address of that.

In:

#include <stdio.h>

//extern long __BUILD_DATE[128];
//extern int __BUILD_DATE;
extern char __BUILD_DATE;

int main(void) 

 printf("Build date: %lun", (unsigned long)&__BUILD_DATE);

Any of the three declarations should work. Just don't try to use the value of that (pseudo) global. That would be like dereferencing an invalid pointer.

That should answer 2 and 3. To answer 1, -Xlinker --defsym -Xlinker __BUILD_DATE=$(date +%Y%m%d) stores the number returned (stdout) by $(date %Y%m%d) as the address of __BUILD_DATE. It doesn't store a string.