CODEWITHSUNDEEP
clibpng
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Reputation: 780

Undefined reference when compiling libPNG example

I was trying out this libPNG example, but it failed at compilation. I used gcc -lm -lpng makePNG.c to compile it and got the following error:

/tmp/ccgGO8zw.o: In function `writeImage':
makePNG.c:(.text+0x360): undefined reference to `setRGB'
collect2: error: ld returned 1 exit status

I removed the function definitions and simply moved the functions so that they were in the following order:

void setRGB(png_byte *ptr, float val)
int writeImage(char* filename, int width, int height, float *buffer, char* title)
float *createMandelbrotImage(int width, int height, float xS, float yS, float rad, int maxIteration)
int main(int argc, char *argv[])

And it worked. My question is: why didn't it work before? SetRGB was defined prior to writeImage, so how could the 'setRGB' reference be undefined?

Edit:

I forgot to mention something that I now realize is very important. I removed the 'inline' keyword from the setRGB function. I tried compiling with the inline keyword and it had the same error message. So clearly my issue has to do with the inline keyword and not the forward declarations like I originally thought...

Upvotes: 1

Views: 1497

Answers (1)

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Reputation: 780

Thanks to David C. Rankin I have found a command that works to compile this. gcc -Ofast -lm -lpng makePNG.c works because it includes the -Ofast option. This is the description of that option in man gcc:

-Ofast

Disregard strict standards compliance.

-Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.

I suspect the way the author uses inline does not comply with the c specification, and that's why it won't compile without this option. Needless to say it is never OK to write code that doesn't comply with the c standards.

Edit:

Proof that the -Ofast option is the culprit:

$ gcc -Ofast -lm -lpng makePNG.c
$ gcc -lm -lpng makePNG.c
/tmp/cc2JlymP.o: In function `writeImage':
makePNG.c:(.text+0x360): undefined reference to `setRGB'
collect2: error: ld returned 1 exit status
$

Upvotes: 1

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