I continue this month with another group, the functions that convert between text strings and encoded numeric data. Here are a few general comments on how to use these functions:

atof — The call atof(s) is equivalent to strtod(s, NULL), except that atof is not obliged to store ERANGE in errno to report a range error. You also get no indication with atof of how many characters from the string participate in the conversion. Use strtod instead.

atoi — Replace atoi(s) with (int)strtol(s, NULL, 10). Then consider altering the second argument so that you can determine how many characters participated in the conversion.

atol — Replace atol(s) with strtol(s, NULL, 10). Then consider altering the second argument.

strtod — This is the function called by the scan functions, to convert a sequence of characters to an encoded value of type double. You can call strtod directly to avoid the overhead of the scan functions. That also lets you determine more precisely what part of the string argument participates in the conversion.

Note that the behavior of strtod can change among locales. The function effectively calls isspace to skip leading white-space. It also uses the decimal point defined for the current locale. Beyond that, the valid text patterns are essentially those of the integer and floating constants (with no suffixes) in C. For example, the following are all valid ways to represent the value 12: 12, +12., and .12e2. An implementation can also recognize additional patterns in other than the "C" locale.

strtol — This is the function called by the scan functions, to convert a sequence of characters to an encoded value of type long. You can call strtol directly to avoid the overhead of the scan functions. That also lets you specify unusual bases and to determine more precisely what part of the string argument participates in the conversion.

Note that the behavior of strtol can change among locales. The function effectively calls isspace to skip leading white-space. Beyond that, the valid text patterns are essentially those of the integer constants (with no suffixes), modified as needed for various bases. For example, the following are all valid ways to represent the value 12 (assuming the third argument to strtol specifies a base of zero): 12, +014, and 0xC. An implementation can also recognize additional patterns in other than the "C" locale.

strtoul — Use this function instead of strtol when you need a result of type unsigned long. The function strtoul reports a range error only if the converted magnitude is greater than ULONG_MAX, defined in <limits.h>. (Negating the value cannot cause overflow.) strtol, on the other hand, reports a range error if the converted value is less than LONG_MIN or greater than LONG_MAX, both defined in <limits.h>. The same text patterns apply as for strtoul.

#include <stdlib.h>
double atof(const char *nptr);

strtod(nptr, (char **)NULL)

Forward references: the strtod function (7.10.1.4).

#include <stdlib.h>
int atoi(const char *nptr);

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

Forward references: the strtol function (7.10.1.5).

   #include <stdlib.h>
   long int atol(const char *nptr);

   strtol(nptr, (char **)NULL, 10)

Forward references: the strtol function (7.10.1.5).

#include <stdlib.h>
double strtod(const char *nptr, char **endptr);

The expected form of the subject sequence is an optional plus or minus sign, then a nonempty sequence of digits optionally containing a decimal-point character, then an optional exponent part as defined in 6.1.3.1, but no floating suffix. The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign, a digit, or a decimal-point character.

If the subject sequence has the expected form, the sequence of characters starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a floating constant according to the rules of 6.1.3.1, except that the decimal-point character is used in place of a period, and that if neither an exponent part nor a decimal-point character appears, a decimal point is assumed to follow the last digit in the string. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the "C" locale, additional implementation-defined subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

#include <stdlib.h>
long int strtol(const char *nptr, char **endptr, int base);

resembling an integer represented in some radix determined by the value of base, and a final string of one or more unrecognized characters, including the terminating null character of the input string. Then, it attempts to convert the subject sequence to an integer, and returns the result.

If the value of base is zero, the expected form of the subject sequence is that of an integer constant as described in 6.1.3.2, optionally preceded by a plus or minus sign, but not including an integer suffix. If the value of base is between 2 and 36, the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a plus or minus sign, but not including an integer suffix. The letters from a (or A) through z (or Z) are ascribed the values 10 to 35; only letters whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is zero, the sequence of characters starting with the first digit is interpreted as an integer constant according to the rules of 6.1.3.2. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the "C" locale, additional implementation-defined subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer

#include <stdlib.h>
unsigned long int strtoul
   (const char *nptr, char **endptr, int base);

If the value of base is zero, the expected form of the subject sequence is that of an integer constant as described in 6.1.3.2, optionally preceded by a plus or minus sign, but not including an integer suffix. If the value of base is between 2 and 36, the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a plus or minus sign, but not including an integer suffix. The letters from a (or A) through z (or Z) are ascribed the values 10 to 35; only letters whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is zero, the sequence of characters starting with the first digit is interpreted as an integer constant according to the rules of 6.1.3.2. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above.

If the subject sequence begins with a minus sign, the value resulting from the conversion is negated. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the "C" locale, additional implementation-defined subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.