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 berepresented, 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 <assert.h>#include <ctype.h>#include <errno.h>#include <limits.h>#include <stdio.h>#include <stdlib.h>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] == '\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 != '\0') 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;}
Don't use functions from ato...
group. These are broken and virtually useless. A moderately better solution would be to use sscanf
, although it is not perfect either.
To convert string to integer, functions from strto...
group should be used. In your specific case it would be strtol
function.