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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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strtol(3) Library Functions Manual strtol(3)
strtol, strtoll, strtoq - convert a string to a long integer
Standard C library (libc, -lc)
#include <stdlib.h>
long strtol(const char *restrict nptr,
char **_Nullable restrict endptr, int base);
long long strtoll(const char *restrict nptr,
char **_Nullable restrict endptr, int base);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
strtoll():
_ISOC99_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
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"
or "0X" prefix, and the number will be read in base 16; if base is
zero or 2, the string may then include a "0b" or "0B" prefix, and
the number will be read in base 2; 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 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 uppercase or lowercase represents 10, 'B' represents 11,
and so forth, with 'Z' representing 35.)
If endptr is not NULL, and the base is supported, 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.
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, errno is set to ERANGE.
Precisely the same holds for strtoll() (with LLONG_MIN and
LLONG_MAX instead of LONG_MIN and LONG_MAX).
This function does not modify errno on success.
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).
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ strtol(), strtoll(), strtoq() │ Thread safety │ MT-Safe locale │
└───────────────────────────────┴───────────────┴────────────────┘
According to POSIX.1, in locales other than "C" and "POSIX", these
functions may accept other, implementation-defined numeric
strings.
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().
C23, POSIX.1-2024.
strtol()
POSIX.1-2001, C89, SVr4, 4.3BSD.
strtoll()
POSIX.1-2001, C99.
"0b", "0B"
C23. glibc 2.38. (Not in POSIX.)
Range checks
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 == ERANGE after the call.
errno = 0;
n = strtol(s, &end, base);
if (end == s)
goto no_number;
if ((errno == ERANGE && n == LONG_MIN) || n < min)
goto too_low;
if ((errno == ERANGE && n == LONG_MAX) || n > max)
goto too_high;
base
If the base needs to be tested, it should be tested in a call
where the string is known to succeed. Otherwise, it's impossible
to portably differentiate the errors.
errno = 0;
strtol("0", NULL, base);
if (errno == EINVAL)
goto unsupported_base;
White space
These functions silently accept leading white space. To reject
white space, call isspace(3) before strtol().
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
Program source
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
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]) : 0;
errno = 0; /* To distinguish success/failure after call */
strtol("0", NULL, base);
if (errno == EINVAL) {
perror("strtol");
exit(EXIT_FAILURE);
}
errno = 0; /* To distinguish success/failure after call */
val = strtol(str, &endptr, base);
/* Check for various possible errors. */
if (errno == ERANGE) {
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);
}
atof(3), atoi(3), atol(3), strtod(3), strtoimax(3), strtoul(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
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for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 strtol(3)
Pages that refer to this page: pmstore(1), pmtrace(1), atof(3), atoi(3), scanf(3), sscanf(3), strtod(3), strtoimax(3), strtoul(3), slapo-retcode(5), bpf-helpers(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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