foo.c: In function 'main':
foo.c:25:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &X);
     ^~~~~~~~~~~~~~~
/tmp/cc05D3zk.o: In function `main':
foo.c:(.text.startup+0x16): undefined reference to `ReadInput'
foo.c:(.text.startup+0x55): undefined reference to `PrintList'
collect2: error: ld returned 1 exit status

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