c compiler has a function stuck in its namespace (maybe?). mac - c

Question

A week ago I made a function called getline in a code that didn't work. Since then, whenever I name a function getline and try to compile it, it doesn't work. If I change the name of the function to something else, it works again. I have restarted my computer (though it really shouldn't require that). How can I get this namespace back?
example error message:
Numens-MBP:c examples mycotic$ cc testing.c
testing.c:9:5: error: conflicting types for 'getline'
int getline(void);
^
/usr/include/stdio.h:448:9: note: previous declaration is here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __lineca...
^
testing.c:13:1: warning: type specifier missing, defaults to 'int'
[-Wimplicit-int]
main()
^
testing.c:20:24: error: too few arguments to function call, expected 3, have 0
while ((len = getline()) > 0)
~~~~~~~ ^
/usr/include/stdio.h:448:1: note: 'getline' declared here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __lineca...
^
testing.c:36:5: error: conflicting types for 'getline'
int getline(void)
^
/usr/include/stdio.h:448:9: note: previous declaration is here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __lineca...
^
1 warning and 3 errors generated.

Answer 1

getline() is a standard C library function. Your function is conflicting with the definition in the standard library.
Pick another name for your function. This one is taken.

Answer 2

The getline() function is a part of POSIX 2008; it was not a part of POSIX 2001 or POSIX 1997. On a Mac, even when you specify -std=c11, the POSIX definitions are enabled by default.
You can work around it by specifying a POSIX version that did not support getline(). Consider this code — but ignore the fact that this 'getline()' outputs a line:
#include <stdio.h>
extern int getline(int);
int getline(int num)
{
printf("%s: %d\n", __func__, num);
return num + 1;
}
int main(void)
{
for (int i = 0; i < 5; i++)
printf("RV = %d\n", getline(i));
return 0;
}
When compiled with GCC 7.1.0, or with either the gcc or clang from XCode 8.3.2, generates all sorts of problems because of the conflicting type of getline() from POSIX and getline() as defined in this code unless you add arguments to prevent the problem. The source was in file gl23.c.
$ /usr/bin/clang -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes \
> -Wstrict-prototypes -Wold-style-definition gl23.c -o gl23
gl23.c:3:12: error: conflicting types for 'getline'
extern int getline(int);
^
/usr/include/stdio.h:448:9: note: previous declaration is here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __linecapp, FILE * __restrict __stream) __OSX_AVAILABLE_STARTIN...
^
gl23.c:5:5: error: conflicting types for 'getline'
int getline(int num)
^
/usr/include/stdio.h:448:9: note: previous declaration is here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __linecapp, FILE * __restrict __stream) __OSX_AVAILABLE_STARTIN...
^
gl23.c:14:38: error: too few arguments to function call, expected 3, have 1
printf("RV = %d\n", getline(i));
~~~~~~~ ^
/usr/include/stdio.h:448:1: note: 'getline' declared here
ssize_t getline(char ** __restrict __linep, size_t * __restrict __linecapp, FILE * __restrict __stream) __OSX_AVAILABLE_STARTIN...
^
3 errors generated.
$
However, if you specify conformance with the old versions of POSIX by adding -D_POSIX_C_SOURCE=200112L or -D_POSIX_C_SOURCE=199506L, then it compiles cleanly, and runs properly, producing:
getline: 0
RV = 1
getline: 1
RV = 2
getline: 2
RV = 3
getline: 3
RV = 4
getline: 4
RV = 5
Be aware that this is only a band-aid solution and it has ramifications — all the other nice new features of POSIX 2008 are no longer accessible.
The only long-term solution is to accept that getline() is now a part of the 'standard' C namespace — the standard is POSIX, of course, not ISO/IEC 9899:2011 — and rename the function. (Yes, it's a nuisance, and K&R 2nd Edition illustrates three versions of getline() that are no longer OK because of this. I had to rename my library function because of this. And the name was not in the reserved namespaces in prior versions of POSIX; there was no way to guess that it would be necessary.)