Compilare: user.c:6:1: warning: data definition has no type or storage class [enabled by default] replace(char firstString[], char secondString[], char thirdString[]); ^ user.c:6:1: warning: type defaults to ‘int’ in declaration of ‘replace’ [enabled by default] user.c: In function ‘main’: user.c:42:22: warning: operation on ‘j’ may be undefined [-Wsequence-point] boolVar[letters[j++] - 'A'] = 1 - boolVar[letters[j] - 'A']; ^ user.c:14:16: warning: unused variable ‘k’ [-Wunused-variable] int i=-1, j=0, k=0, nChanges; ^ user.c: At top level: user.c:79:1: warning: return type defaults to ‘int’ [enabled by default] replace(char firstString[], char secondString[], char thirdString[]){ ^ user.c: In function ‘main’: user.c:27:7: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(file1, "%d", &nChanges); ^ user.c:28:7: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(file1, "%s", letters); ^ user.c: In function ‘replace’: user.c:135:1: warning: control reaches end of non-void function [-Wreturn-type] } ^