Compilare: user.c: In function ‘main’: user.c:25:1: warning: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration] len = strlen(expression); ^ user.c:25:7: warning: incompatible implicit declaration of built-in function ‘strlen’ [enabled by default] len = strlen(expression); ^ user.c:24:7: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(file1, "%s", expression); ^