You need to subtract the ascii value of zero to get the integer value of the char before multiplying by 10.

#include #include unsigned int my_atoi(const char* str) { int num = 0; while (*str) { num *= 10; num += (*str++ - '0'); } return num; } int main(int argc, char* argv[]) { printf("atoi(%s)=%d\n",argv[1], my_atoi(argv[1]) ); return 0; }