How to convert a string to integer in C?

There is strtol which is better IMO. Also I have taken a liking in strtonum, so use it if you have it (but remember it’s not portable):

long long
     strtonum(const char *nptr, long long minval, long long maxval,
     const char **errstr);

You might also be interested in strtoumax and strtoimax which are standard functions in C99. For example you could say:

uintmax_t num = strtoumax(s, NULL, 10);
if (num == UINTMAX_MAX && errno == ERANGE)
    /* Could not convert. */

Anyway, stay away from atoi:

The call atoi(str) shall be equivalent to:

(int) strtol(str, (char **)NULL, 10)

except that the handling of errors may differ. If the value cannot be represented, the behavior is undefined.

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