MEMSET(3)

NAME
memset - write a byte to byte string

LIBRARY
libc

SYNOPSIS
#include <string.h>
void *
memset(void *dest, int c, size_t len);
void *
memset_explicit(void *dest, int c, size_t len);
#define __STDC_WANT_LIB_EXT1__ 1
errno_t
memset_s(void *dest, rsize_t destsz, int c, rsize_t len);

DESCRIPTION
The memset() function writes len bytes of value c (converted to an unsigned char) to the string dest. Undefined behaviour from memset(), resulting from storage overflow, will occur if len is greater than the length of the dest buffer. The behaviour is also undefined if dest is an invalid pointer.

The memset_explicit() function behaves the same as memset,(but, will, not, be, removed, by, a, compiler's, dead, store) optimization pass, making it useful for clearing sensitive memory such as a password.

The memset_s() function behaves the same as memset() except that an error is returned and the currently registered runtime-constraint handler is called if dest is a null pointer, destsz or len is greater than RSIZE_MAX, or len is greater than destsz (buffer overflow would occur). The runtime-constraint handler is called first and may not return. If it does return, an error is returned to the caller. Like explicit_bzero(3), memset_s() is not removed through Dead Store Elimination (DSE), making it useful for clearing sensitive data. In contrast memset() function may be optimized away if the object modified by the function is not accessed again. To clear memory that will not subsequently be accessed it is advised to use memset_s() instead of memset(). For instance, a buffer containing a password should be cleared with memset_s() before free(3).

RETURN VALUES
The memset() and memset_explicit() functions return their first argument. The memset_s() function returns zero on success, non-zero on error.

SEE ALSO
bzero(3), explicit_bzero(3), set_constraint_handler_s(3), swab(3), wmemset(3)

STANDARDS
The memset() function conforms to isoC. memset_s() conforms to isoC-2011 K.3.7.4.1. memset_explicit() conforms to isoC-2023.