strncpy, strncpy_s - cppreference.com (2024)

1) Copies at most count characters of the character array pointed to by src (including the terminating null character, but not any of the characters that follow the null character) to character array pointed to by dest.

If count is reached before the entire array src was copied, the resulting character array is not null-terminated.

If, after copying the terminating null character from src, count is not reached, additional null characters are written to dest until the total of count characters have been written.

The behavior is undefined if the character arrays overlap, if either dest or src is not a pointer to a character array (including if dest or src is a null pointer), if the size of the array pointed to by dest is less than count, or if the size of the array pointed to by src is less than count and it does not contain a null character.

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[edit] Return value

1) returns a copy of dest

2) returns zero on success, returns non-zero on error. Also, on error, writes zero to dest[0] (unless dest is a null pointer or destsz is zero or greater than RSIZE_MAX) and may clobber the rest of the destination array with unspecified values.

[edit] Notes

As corrected by the post-C11 DR 468, strncpy_s, unlike strcpy_s, is only allowed to clobber the remainder of the destination array if an error occurs.

Unlike strncpy, strncpy_s does not pad the destination array with zeroes, This is a common source of errors when converting existing code to the bounds-checked version.

Although truncation to fit the destination buffer is a security risk and therefore a runtime constraints violation for strncpy_s, it is possible to get the truncating behavior by specifying count equal to the size of the destination array minus one: it will copy the first count bytes and append the null terminator as always: strncpy_s(dst, sizeof dst, src, (sizeof dst)-1);

[edit] Example

#define __STDC_WANT_LIB_EXT1__ 1#include <string.h>#include <stdio.h>#include <stdlib.h>#include <errno.h>int main(void){ char src[] = "hi"; char dest[6] = "abcdef"; // no null terminator strncpy(dest, src, 5); // writes five characters 'h', 'i', '\0', '\0', '\0' to dest printf("strncpy(dest, src, 5) to a 6-byte dest gives: "); for (size_t n = 0; n < sizeof dest; ++n) { char c = dest[n]; c ? printf("'%c' ", c) : printf("'\\0' "); } printf("\nstrncpy(dest2, src, 2) to a 2-byte dst gives: "); char dest2[2]; strncpy(dest2, src, 2); // truncation: writes two characters 'h', 'i', to dest2 for (size_t n = 0; n < sizeof dest2; ++n) { char c = dest2[n]; c ? printf("'%c' ", c) : printf("'\\0' "); } printf("\n");#ifdef __STDC_LIB_EXT1__ set_constraint_handler_s(ignore_handler_s); char dst1[6], src1[100] = "hello"; errno_t r1 = strncpy_s(dst1, 6, src1, 100); // writes 0 to r1, 6 characters to dst1 printf("dst1 = \"%s\", r1 =%d\n", dst1,r1); // 'h','e','l','l','o','\0' to dst1 char dst2[5], src2[7] = {'g','o','o','d','b','y','e'}; errno_t r2 = strncpy_s(dst2, 5, src2, 7); // copy overflows the destination array printf("dst2 = \"%s\", r2 =%d\n", dst2,r2); // writes nonzero to r2,'\0' to dst2[0] char dst3[5]; errno_t r3 = strncpy_s(dst3, 5, src2, 4); // writes 0 to r3, 5 characters to dst3 printf("dst3 = \"%s\", r3 =%d\n", dst3,r3); // 'g', 'o', 'o', 'd', '\0' to dst3#endif}

strncpy(dest, src, 5) to a 6-byte dst gives: 'h' 'i' '\0' '\0' '\0' 'f'strncpy(dest2, src, 2) to a 2-byte dst gives: 'h' 'i'dst1 = "hello", r1 = 0dst2 = "", r2 = 22dst3 = "good", r3 = 0

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