strtol函数详解

strtol(3) - Linux man page

Name

strtol, strtoll, strtoq - convert a string to a long integer

Synopsis

#include 

long int
strtol(const char *nptr, char **endptr, int base);

long long int
strtoll(const char *nptr, char **endptr, int base);

The  strtol () function converts the initial part of the string in  nptr  to a long integer value according to the given base , which must be between 2 and 36 inclusive, or be the special value 0.

The string may begin with an arbitrary amount of white space (as determined by isspace(3)) followed by a single optional '+' or '-' sign. If base is zero or 16, the string may then include a '0x' prefix, and the number will be read in base 16; otherwise, a zero base is taken as 10 (decimal) unless the next character is '0', in which case it is taken as 8 (octal).

The remainder of the string is converted to a long int value in the obvious manner, stopping at the first character which is not a valid digit in the given base. (In bases above 10, the letter 'A' in either upper or lower case represents 10, 'B' represents 11, and so forth, with 'Z' representing 35.)

If endptr is not NULL, strtol() stores the address of the first invalid character in *endptr. If there were no digits at all, strtol() stores the original value of nptr in *endptr (and returns 0). In particular, if *nptr is not '\0' but **endptr is '\0' on return, the entire string is valid.

The strtoll() function works just like the strtol() function but returns a long long integer value.

Return Value

The strtol() function returns the result of the conversion, unless the value would underflow or overflow. If an underflow occurs, strtol() returns LONG_MIN. If an overflow occurs, strtol() returns LONG_MAX. In both cases, errnois set to ERANGE. Precisely the same holds for strtoll() (with LLONG_MIN and LLONG_MAX instead of LONG_MIN and LONG_MAX).

Errors

EINVAL

(not in C99) The given base contains an unsupported value.

ERANGE

The resulting value was out of range.

The implementation may also set  errno to  EINVAL in case no conversion was performed (no digits seen, and 0 returned).

Notes

Since strtol() can legitimately return 0, LONG_MAX, or LONG_MIN (LLONG_MAX or LLONG_MIN for strtoll()) on both success and failure, the calling program should set errno to 0 before the call, and then determine if an error occurred by checking whether errno has a non-zero value after the call.

In locales other than the "C" locale, other strings may also be accepted. (For example, the thousands separator of the current locale may be supported.)

BSD also has

quad_t
strtoq(const char *nptr, char **endptr, int base);

with completely analogous definition. Depending on the wordsize of the current architecture, this may be equivalent to  strtoll() or to  strtol().

Example

The program shown below demonstrates the use of strtol(). The first command line argument specifies a string from which strtol() should parse a number. The second (optional) argument specifies the base to be used for the conversion. (This argument is converted to numeric form using atoi(3), a function that performs no error checking and has a simpler interface than strtol().) Some examples of the results produced by this program are the following:

$ ./a.out 123
strtol() returned 123
$ ./a.out '    123'
strtol() returned 123
$ ./a.out 123abc
strtol() returned 123
Further characters after number: abc
$ ./a.out 123abc 55
strtol: Invalid argument
$ ./a.out ''
No digits were found
$ ./a.out 4000000000
strtol: Numerical result out of range

The source code of the program is as follows:

#include 
#include 
#include 
#include 

int
main(int argc, char *argv[])
{
    int base;
    char *endptr, *str;
    long val;

   if (argc < 2) {
        fprintf(stderr, "Usage: %s str [base]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

   str = argv[1];
    base = (argc > 2) ? atoi(argv[2]) : 10;

   errno = 0; /* To distinguish success/failure after call */
    val = strtol(str, &endptr, base);

   /* Check for various possible errors */

   if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
            || (errno != 0 && val == 0)) {
        perror("strtol");
        exit(EXIT_FAILURE);
    }

   if (endptr == str) {
        fprintf(stderr, "No digits were found\n");
        exit(EXIT_FAILURE);
    }

   /* If we got here, strtol() successfully parsed a number */

   printf("strtol() returned %ld\n", val);

   if (*endptr != '\0') /* Not necessarily an error... */
        printf("Further characters after number: %s\n", endptr);

   exit(EXIT_SUCCESS);
}