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.

Robust C89 strtol-based solution
With:

no undefined behavior (as could be had with the atoi family)
a stricter definition of integer than strtol (e.g. no leading whitespace nor trailing trash chars)
classification of the error case (e.g. to give useful error messages to users)
a “testsuite”

#include
#include
#include
#include #include
#include

typedef enum {
STR2INT_SUCCESS,
STR2INT_OVERFLOW,
STR2INT_UNDERFLOW,
STR2INT_INCONVERTIBLE
} str2int_errno;

/* Convert string s to int out.
*
* @param[out] out The converted int. Cannot be NULL.
*
* @param[in] s Input string to be converted.
*
* The format is the same as strtol,
* except that the following are inconvertible:
*
* – empty string
* – leading whitespace
* – any trailing characters that are not part of the number
*
* Cannot be NULL.
*
* @param[in] base Base to interpret string in. Same range as strtol (2 to 36).
*
* @return Indicates if the operation succeeded, or why it failed.
*/
str2int_errno str2int(int *out, char *s, int base) {
char *end;
if (s[0] == ‘’ || isspace(s[0]))
return STR2INT_INCONVERTIBLE;
errno = 0;
long l = strtol(s, &end, base);
/* Both checks are needed because INT_MAX == LONG_MAX is possible. */
if (l > INT_MAX || (errno == ERANGE && l == LONG_MAX))
return STR2INT_OVERFLOW;
if (l < INT_MIN || (errno == ERANGE && l == LONG_MIN)) return STR2INT_UNDERFLOW; if (*end != '') return STR2INT_INCONVERTIBLE; *out = l; return STR2INT_SUCCESS; } int main(void) { int i; /* Lazy to calculate this size properly. */ char s[256]; /* Simple case. */ assert(str2int(&i, "11", 10) == STR2INT_SUCCESS); assert(i == 11); /* Negative number . */ assert(str2int(&i, "-11", 10) == STR2INT_SUCCESS); assert(i == -11); /* Different base. */ assert(str2int(&i, "11", 16) == STR2INT_SUCCESS); assert(i == 17); /* 0 */ assert(str2int(&i, "0", 10) == STR2INT_SUCCESS); assert(i == 0); /* INT_MAX. */ sprintf(s, "%d", INT_MAX); assert(str2int(&i, s, 10) == STR2INT_SUCCESS); assert(i == INT_MAX); /* INT_MIN. */ sprintf(s, "%d", INT_MIN); assert(str2int(&i, s, 10) == STR2INT_SUCCESS); assert(i == INT_MIN); /* Leading and trailing space. */ assert(str2int(&i, " 1", 10) == STR2INT_INCONVERTIBLE); assert(str2int(&i, "1 ", 10) == STR2INT_INCONVERTIBLE); /* Trash characters. */ assert(str2int(&i, "a10", 10) == STR2INT_INCONVERTIBLE); assert(str2int(&i, "10a", 10) == STR2INT_INCONVERTIBLE); /* int overflow. * * `if` needed to avoid undefined behaviour * on `INT_MAX + 1` if INT_MAX == LONG_MAX. */ if (INT_MAX < LONG_MAX) { sprintf(s, "%ld", (long int)INT_MAX + 1L); assert(str2int(&i, s, 10) == STR2INT_OVERFLOW); } /* int underflow */ if (LONG_MIN < INT_MIN) { sprintf(s, "%ld", (long int)INT_MIN - 1L); assert(str2int(&i, s, 10) == STR2INT_UNDERFLOW); } /* long overflow */ sprintf(s, "%ld0", LONG_MAX); assert(str2int(&i, s, 10) == STR2INT_OVERFLOW); /* long underflow */ sprintf(s, "%ld0", LONG_MIN); assert(str2int(&i, s, 10) == STR2INT_UNDERFLOW); return EXIT_SUCCESS; } GitHub upstream. Based on: https://stackoverflow.com/a/6154614/895245