memset, memset_explicit, memset_s

Defined in header `<string.h>`
void memset( void dest, int ch, size_t count );	(1)
void memset_explicit( void dest, int ch, size_t count );	(2)	(since C23)
errno_t memset_s( void *dest, rsize_t destsz, int ch, rsize_t count );	(3)	(since C11)

1) Copies the value (unsigned char)ch into each of the first count characters of the object pointed to by dest.

The behavior is undefined if access occurs beyond the end of the dest array. The behavior is undefined if dest is a null pointer.

2) Same as (1), except that is safe for sensitive information.

3) Same as (1), except that the following errors are detected at runtime and call the currently installed constraint handler function after storing ch in every location of the destination range [dest, dest+destsz) if dest and destsz are themselves valid:

dest is a null pointer
destsz or count is greater than RSIZE_MAX
count is greater than destsz (buffer overflow would occur)

The behavior is undefined if the size of the character array pointed to by dest < count <= destsz; in other words, an erroneous value of destsz does not expose the impending buffer overflow.

As with all bounds-checked functions, memset_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <string.h>.

Parameters

dest	-	pointer to the object to fill
ch	-	fill byte
count	-	number of bytes to fill
destsz	-	size of the destination array

Return value

1,2) A copy of dest

3) zero on success, non-zero on error. Also on error, if dest is not a null pointer and destsz is valid, writes destsz fill bytes ch to the destination array.

Notes

memset may be optimized away (under the as-if rules) if the object modified by this function is not accessed again for the rest of its lifetime (e.g., gcc bug 8537). For that reason, this function cannot be used to scrub memory (e.g., to fill an array that stored a password with zeroes).

This optimization is prohibited for memset_explicit and memset_s: they are guaranteed to perform the memory write.

Third-party solutions for that include FreeBSD explicit_bzero or Microsoft SecureZeroMemory.

Example

Run this code

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
 
int main(void)
{
    char str[] = "ghghghghghghghghghghgh";
    puts(str);
    memset(str,'a',5);
    puts(str);
 
#ifdef __STDC_LIB_EXT1__
    set_constraint_handler_s(ignore_handler_s);
    int r = memset_s(str, sizeof str, 'b', 5);
    printf("str = \"%s\", r = %d\n", str, r);
    r = memset_s(str, 5, 'c', 10);   // count is greater than destsz  
    printf("str = \"%s\", r = %d\n", str, r);
#endif
}

Possible output:

ghghghghghghghghghghgh
aaaaahghghghghghghghgh
str = "bbbbbhghghghghghghghgh", r = 0
str = "ccccchghghghghghghghgh", r = 22

References

C17 standard (ISO/IEC 9899:2018):

7.24.6.1 The memset function (p: 270)

K.3.7.4.1 The memset_s function (p: 451)

C11 standard (ISO/IEC 9899:2011):

7.24.6.1 The memset function (p: 371)

K.3.7.4.1 The memset_s function (p: 621-622)

C99 standard (ISO/IEC 9899:1999):

7.21.6.1 The memset function (p: 333)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.6.1 The memset function

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Technical Specifications
Symbol index

memcpymemcpy_s (C11)	copies one buffer to another (function)
wmemset (C95)	copies the given wide character to every position in a wide character array (function)
C++ documentation for memset