Compilare: user.c: In function ‘main’: user.c:24:5: warning: implicit declaration of function ‘strcat’ [-Wimplicit-function-declaration] strcat(op,"3"); ^ user.c:24:5: warning: incompatible implicit declaration of built-in function ‘strcat’ [enabled by default] user.c:31:5: warning: incompatible implicit declaration of built-in function ‘strcat’ [enabled by default] strcat(op,"2"); ^ user.c:9:8: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(f,"%d\n",&n); ^ user.c:14:9: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(f,"%d ",&v[i]); ^