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| NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON | |
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atoi(3) Library Functions Manual atoi(3) atoi, atol, atoll - convert a string to an integer
Standard C library (libc, -lc)
#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
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.
The converted value or 0 on error.
For an explanation of the terms used in this section, see attributes(7). ┌───────────────────────────────┬───────────────┬────────────────┐ │ Interface │ Attribute │ Value │ ├───────────────────────────────┼───────────────┼────────────────┤ │ atoi(), atol(), atoll() │ Thread safety │ MT-Safe locale │ └───────────────────────────────┴───────────────┴────────────────┘
POSIX.1 leaves the return value of atoi() on error unspecified. On glibc, musl libc, and uClibc, 0 is returned on error.
C11, POSIX.1-2008.
C99, POSIX.1-2001, SVr4, 4.3BSD. C89 and POSIX.1-1996 include the functions atoi() and atol() only.
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.
atof(3), strtod(3), strtol(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 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report 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 fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which is not part of the original manual page), send a mail to man-pages@man7.org 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)
| 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. | |