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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | CAVEATS | SEE ALSO | COLOPHON |
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memcmp(3) Library Functions Manual memcmp(3)
memcmp - memory compare
Standard C library (libc, -lc)
#include <string.h> // see memory.h(3head)
int memcmp(size_t n;
const void s1[n], const void s2[n], size_t n);
The memcmp() function compares the first n bytes (each interpreted
as unsigned char) of the memory areas s1 and s2.
The memcmp() function returns an integer less than, equal to, or
greater than zero if the first n bytes of s1 is found,
respectively, to be less than, to match, or be greater than the
first n bytes of s2.
For a nonzero return value, the sign is determined by the sign of
the difference between the first pair of bytes (interpreted as
unsigned char) that differ in s1 and s2.
If n is zero, the return value is zero.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ memcmp() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1 specifies only that:
The sign of a nonzero return value shall be determined by
the sign of the difference between the values of the first
pair of bytes (both interpreted as type unsigned char) that
differ in the strings being compared.
In glibc, as in most other implementations, the return value is
the arithmetic result of subtracting the last compared byte in s2
from the last compared byte in s1. (If the two characters are
equal, this difference is 0.)
C11, POSIX.1-2008.
POSIX.1-2001, C89, SVr4, 4.3BSD.
Do not use memcmp() to compare confidential data, such as
cryptographic secrets, because the CPU time required for the
comparison depends on the contents of the addresses compared, this
function is subject to timing-based side-channel attacks. In such
cases, a function that performs comparisons in deterministic time,
depending only on n (the quantity of bytes compared) is required.
Some operating systems provide such a function (e.g., NetBSD's
consttime_memequal()), but no such function is specified in POSIX.
On Linux, you may need to implement such a function yourself.
memory.h(3head), memeq(3), wmemcmp(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-6.19.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2026-09-09. 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 6.19 2026-08-10 memcmp(3)
Pages that refer to this page: bcmp(3), memory.h(3head), size_t(3type), strcmp(3), strcoll(3), strncmp(3), void(3type), wmemcmp(3), signal-safety(7)
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HTML rendering created 2026-09-09 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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