atoi(3) — Linux manual page

atoi(3)                  Library Functions Manual                 atoi(3)

NAME         top

       atoi, atol, atoll - convert a string to an integer

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdlib.h>

       int atoi(const char *nptr);
       long atol(const char *nptr);
       long long atoll(const char *nptr);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       atoll():
           _ISOC99_SOURCE
               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION         top

       The atoi() function converts the initial portion of the string
       pointed to by nptr to int.  The behavior is the same as

           strtol(nptr, NULL, 10);

       except that atoi() does not detect errors.

       The atol() and atoll() functions behave the same as atoi(), except
       that they convert the initial portion of the string to their
       return type of long or long long.

RETURN VALUE         top

       The converted value or 0 on error.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌───────────────────────────────┬───────────────┬────────────────┐
       │ Interface                     │ Attribute     │ Value          │
       ├───────────────────────────────┼───────────────┼────────────────┤
       │ atoi(), atol(), atoll()       │ Thread safety │ MT-Safe locale │
       └───────────────────────────────┴───────────────┴────────────────┘

VERSIONS         top

       POSIX.1 leaves the return value of atoi() on error unspecified.
       On glibc, musl libc, and uClibc, 0 is returned on error.

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       C99, POSIX.1-2001, SVr4, 4.3BSD.

       C89 and POSIX.1-1996 include the functions atoi() and atol() only.

BUGS         top

       errno is not set on error so there is no way to distinguish
       between 0 as an error and as the converted value.  No checks for
       overflow or underflow are done.  Only base-10 input can be
       converted.  It is recommended to instead use the strtol() and
       strtoul() family of functions in new programs.

SEE ALSO         top

       atof(3), strtod(3), strtol(3), strtoul(3)

COLOPHON         top

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Linux man-pages 6.15            2025-05-17                        atoi(3)

Pages that refer to this page: atof(3),  form_field_validation(3x),  strtod(3),  strtol(3),  strtoul(3)

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