Thread: Cannot understand this syntax in png.h

Hello,

I'm having problems writing png files in c using libpng.
When in my code I write:

Code:

png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);

I'm told undefined reference to `png_create_write_struct'.

I look into the png.h file and the first occurence 'png_create_write_struct' and find it in:

/* Allocate and initialize png_ptr struct for writing, and any other memory */

Code:

extern PNG_EXPORT(png_structp,png_create_write_struct)
   PNGARG((png_const_charp user_png_ver, png_voidp error_ptr,
   png_error_ptr error_fn, png_error_ptr warn_fn));

What is this code?? Is the function here PNG_EXPORT or PNGARG? I never saw such syntax and don't know how to read it.

Maybe it's that I'm coding in ANSI C while that is some other C incompatible syntax?

> I'm told undefined reference to `png_create_write_struct'.
This usually means you're not linking with the library properly.
Is this the only one, or are there others?

Check your library search paths, and name of library (which compiler/IDE are you using?)

> What is this code?? Is the function here PNG_EXPORT or PNGARG?
They're just two clever macros which expand to a function declaration in K&R C, and a prototype in ANSI-C
I doubt you would need to worry about it if you're just interested in using the library.

I actually get the problem with other functions of the same file but not for variables defined in the same.

Code:

png_byte color_type;
png_byte bit_depth;

png_structp png_ptr;
png_infop info_ptr;

png_bytep * row_pointers;

All of these are recognized. I guess if there was a linking problem to the library they would also be undefined references, or not?

I found in the distribution of pnglib the c file that defines the function. However including it should not be required since I've the libpng.LIB in the lib folder.

I'm using Eclipse with MinGW.

Yep, sounds like a link problem to me.

No, those could be defined in the header itself. In other words, you might have something like:

Code:

typedef char png_byte;

I've tried adding it to the library paths in eclipse, as well as -l libpng, libpng.a, libpng.lib, from the linker options, but in vain.
I've also tried from the prompt:

C:\Documents and Settings\Simpatico\My Documents\ws\Test>make -I C:\MinGW\libpn
\include\libpng12 test test.c -l libpng
gcc test.c -o test
test.c:6:17: png.h: No such file or directory
test.c:20: error: syntax error before "color_type"
test.c:20: warning: data definition has no type or storage class
test.c:21: error: syntax error before "bit_depth"
test.c:21: warning: data definition has no type or storage class
test.c:22: error: syntax error before "png_ptr"
test.c:22: warning: data definition has no type or storage class
test.c:23: error: syntax error before "info_ptr"
test.c:23: warning: data definition has no type or storage class
test.c:25: error: syntax error before '*' token
test.c:25: warning: data definition has no type or storage class
test.c: In function `write_png_file':
test.c:36: error: `PNG_LIBPNG_VER_STRING' undeclared (first use in this functio
)
test.c:36: error: (Each undeclared identifier is reported only once
test.c:36: error: for each function it appears in.)
test.c:56: error: `width' undeclared (first use in this function)
test.c:56: error: `height' undeclared (first use in this function)
test.c:57: error: `PNG_INTERLACE_NONE' undeclared (first use in this function)
test.c:58: error: `PNG_COMPRESSION_TYPE_BASE' undeclared (first use in this fun
tion)
test.c:58: error: `PNG_FILTER_TYPE_BASE' undeclared (first use in this function

test.c:78: warning: passing arg 1 of `free' makes pointer from integer without
cast
make: *** [test] Error 1

C:\Documents and Settings\Simpatico\My Documents\ws\Test>
Please help me getting it to compile at least in the prompt.

-----

Solved it. I've specified in the linker the name libpng in the -l option and in the -L was specified the path.

although now eclipse generated the binaries, launching them they terminate before printing the test output (printf("test") . What else could be wrong here now? Launching the test.exe outside eclipse opens the prompt for .X seconds.
If I comment the code that should write the png, it works.