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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022
 
 

memmove(3C)

Name

memory, memccpy, memchr, memcmp, memcpy, memmove, memset, explicit_memset, memmem, memcpy_s, memmove_s, memset_s - memory operations

Synopsis

#include <string.h>

void *memccpy(void *restrict s1, const void *restrict s2,
     int c, size_t n);
void *memchr(const void *s, int c, size_t n);
int memcmp(const void *s1, const void *s2, size_t n);
void *memcpy(void *restrict s1, const void *restrict s2, size_t n);
void *memmove(void *s1, const void *s2, size_t n);
void *memset(void *s, int c, size_t n);
void *explicit_memset(void *s, int c, size_t n);
void *memmem(const void *haystack, size_t haystacklen,
     const void *needle, size_t needlelen);

C11 Bounds Checking Interfaces

#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>

errno_t memcpy_s(void *restrict s1, rsize_t s1max,
    const void *restrict s2, rsize_t n);
errno_t memmove_s(void *s1, rsize_t s1max,
    const void *s2, rsize_t n);
errno_t memset_s(void *s, rsize_t smax, int c, rsize_t n);

ISO C++

#include <string.h>

const void *memchr(const void *s, int c, size_t n);
#include <cstring>

void *std::memchr(void *s, int c, size_t n);

Description

These functions operate as efficiently as possible on memory areas (arrays of bytes bounded by a count, not terminated by a null character). They do not check for the overflow of any receiving memory area. Except for the C11 Bounds Checking Interfaces, they do not check for null pointers, and programs may crash if passing null or otherwise invalid pointers to these functions. Use of adi(7) may help in detecting invalid pointer usage in code.

The memccpy() function copies bytes from memory area s2 into s1, stopping after the first occurrence of c (converted to an unsigned char) has been copied, or after n bytes have been copied, whichever comes first. It returns a pointer to the byte after the copy of c in s1, or a null pointer if c was not found in the first n bytes of s2. If copying takes place between objects that overlap, the behavior is undefined.

The memchr() function returns a pointer to the first occurrence of c (converted to an unsigned char) in the first n bytes (each interpreted as an unsigned char) of memory area s, or a null pointer if c does not occur.

The memcmp() function compares its arguments, looking at the first n bytes (each interpreted as an unsigned char), and returns an integer less than, equal to, or greater than 0, according to whether s1 is lexicographically less than, equal to, or greater than s2 when taken to be unsigned characters. The execution time of the memcmp() function may depend on the sequences being compared. For uses such as cryptography where invariant timing is desired, see timingsafe_memcmp(3C) instead.

The memcpy() function copies n bytes from memory area s2 to s1. It returns s1. If copying takes place between objects that overlap, the behavior is undefined - the memmove() function should be used instead for overlapping copies.

The memmove() function copies n bytes from memory area s2 to memory area s1. Copying between objects that overlap will take place correctly. It returns s1.

The memset() function sets the first n bytes in memory area s to the value of c (converted to an unsigned char). It returns s.

The explicit_memset() function performs the same operation as memset(). It differs from memset() in that it will not be removed by compiler dead store analysis. The explicit_memset() function is useful for clearing no longer needed sensitive data to ensure that it does not remain accessible in process memory.

The memmem() function locates the start of the first occurrence of the substring needle of length needlelen in the memory area haystack of length haystacklen. It returns a pointer to the start of the substring, or NULL if the substring is not found. If needlelen is zero, haystack is returned. If haystacklen is less than needlelen, NULL is returned.

C11 Bounds Checking Interfaces

The memcpy_s(), memmove_s(), and memset_s() functions are part of the C11 bounds checking interfaces specified in the C11 standard, Annex K. Each provide equivalent functionality to the respective memcpy(), memmove(), and memset() functions, except with differing parameters and return type in order to provide explicit runtime-constraints as defined in the C11 standard. See runtime_constraint_handler(3C) and INCITS/ISO/IEC 9899:2011.

If no runtime-constraint violation is detected, the memcpy_s(), memmove_s(), and memset_s() functions return zero. If a runtime constraint violation is detected and the handler returns, they return a non-zero value.

Errors

No errors are defined for the memccpy(), memchr(), memcmp(), memcpy(), memmove(), memset(), explicit_memset(), or memmem() functions. The behavior of these functions is undefined if null or invalid pointers are passed for any of their pointer arguments.

The memcpy_s() function will fail if:

EINVAL

NULL pointer passed or source and destination overlap.

ERANGE

size argument is invalid, that is, s1max or n is greater than RSIZE_MAX or n is greater than s1max.

The memmove_s() function will fail if:

EINVAL

NULL pointer passed.

ERANGE

size argument is invalid, that is, s1max or n is greater than RSIZE_MAX or n is greater than s1max.

The memset_s() function will fail if:

EINVAL

NULL pointer passed.

ERANGE

size argument is invalid, that is, smax or n is greater than RSIZE_MAX or n is greater than smax.

Usage

Using memcpy() might be faster than using memmove() if the application knows that the objects being copied do not overlap.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
See below
Standard
See below

MT-Level

The memccpy(), memchr(), memcmp(), memcpy(), memmove(), memset(), explicit_memset(), and memmem() functions are Async-Signal-Safe.

The memcpy_s(), memmove_s(), and memset_s() functions cannot be used safely in a multithreaded application due to the runtime constraint handler. For more information, see the runtime_constraint_handler(3C) man page.

Standard

See standards(7) for descriptions of the following standards:

INTERFACES
APPLICABLE STANDARDS
memchr(), memcmp(), memcpy(), memset()
  • C89 through C11,
  • POSIX.1-1990 through 2008,
  • SUS through SUSv4,
  • XPG1 through XPG7
memmove()
  • C89 through C11,
  • POSIX.1-1990 through 2008,
  • SUS through SUSv4,
  • XPG4 through XPG7
memccpy()
  • POSIX.1-2001 through 2008,
  • SUS through SUSv4,
  • XPG1 through XPG7
memcpy_s(), memmove_s(), memset_s()
C11 Annex K
explicit_memset(), memmem()
None

See Also

freezero(3C), string(3C), timingsafe_memcmp(3C), wmemchr(3C), wmemcmp(3C), wmemcpy(3C), wmemmove(3C), wmemset(3C), attributes(7), standards(7), runtime_constraint_handler(3C)

Notes

Overlap between objects being copied can arise even when their (virtual) address ranges appear to be disjoint; for example, as a result of memory-mapping overlapping portions of the same underlying file, or of attaching the same shared memory segment more than once.

History

The explicit_memset() function was added to Oracle Solaris in the Solaris 11.4.12 release.

The memcpy_s(), memmove_s(), and memset_s() functions were added to Oracle Solaris in the Solaris 11.4.0 release.

The memmem() function was added to Oracle Solaris in the Solaris 11.0.0 release.

The memccpy(), memchr(), memcmp(), memcpy(), memmove(), annd memset() functions have been included in Solaris since the Solaris 2.0 release.