Undefined reference to 'environ'? - c

Question

I am trying to use the 'environ' variable, but it keeps giving me an error. It seems to be a makefile/build error and I can't seem to fix it. I have searched fo answers, but still I am lost.
Here is my c file:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <dirent.h>
#include "cmd.h"
int cmdExec() {
...
extern char **environ;
...
printf("Enter a command\n");
//gets (input);
scanf("%s%*[^\n]", input);
if (...) {
...
}
else if (strcmp(input, "environ") == 0) {
int i;
for (i = 0; environ[i] != NULL; i++) {
printf("%s\n", environ[i]);
}
exit(0);
else
...
return 0;
}
and here is the makefile:
CC = gcc
CFLAGS = -c
CFLAGS-y = -std=c99
all: cmd
cmd.o: cmd.c cmd.h
$(CC) $(CFLAGS) $(CFLAGS-y) cmd.c
cmd.exe: cmd.o
$(CC) -o cmd.exe cmd.o
clean:
rm -rf *.o cmd.exe a.out
This is the output:
make all
gcc -c -std=c99 cmd.c
gcc cmd.o -o cmd
cmd.o:cmd.c:(.text+0x105): undefined reference to `environ'
cmd.o:cmd.c:(.text+0x127): undefined reference to `environ'
collect2: ld returned 1 exit status
make: *** [cmd] Error 1
From what I've searched this deals with linking libraries, but I don't know how to apply that to my specific situation. If someone could give me a hand I'd appreciate it.

Answer 1

Not all(if any) compilers on Windows provides access to environment variables through a global symbol named environ.
You can use e.g. getenv() to access environment variables.
The win32 API provides GetEnvironmentStrings() to access all the variables.
Some platforms allow you to access the environment through an additional argument to main(), you'd declare your main function as:
int main(int argc, char *argv[], char *environ[])

Answer 2

The environ global variable is defined by POSIX, and is not supported by Windows (unless you're using Cygwin, which is a POSIX-like layer implemented on top of Windows).
As far as I know, the non-standard definition
int main(int argc, char **argv, char **envp) { /* ... */ }
is also not supported on Windows.
But a quick Google search turned up this answer, which points to the documentation for the Windows-specific GetEnvironmentStrings function:
LPTCH WINAPI GetEnvironmentStrings(void);
If the function succeeds, the return value is a pointer to the
environment block of the current process.
If the function fails, the return value is NULL.
The result points to a long string with the environment variables separated by '\0' null characters, with the environment terminated by two consecutive null characters.
LPTCH is Microsoft's typedef for a pointer to either unsigned char or a 16-bit wchar_t. See the referenced documentation for more information.