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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |NOTES |CAVEATS |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
string_copying(7) Miscellaneous Information Manualstring_copying(7)stpcpy, strcpy, strcat, stpecpy, strtcpy, strlcpy, strlcat, stpncpy, strncpy, strncat - copying strings and character sequences
Strings // Chain-copy a string.char *stpcpy(char *restrictdst, const char *restrictsrc); // Copy/catenate a string.char *strcpy(char *restrictdst, const char *restrictsrc);char *strcat(char *restrictdst, const char *restrictsrc); // Chain-copy a string with truncation.char *stpecpy(char *dst, charend[0], const char *restrictsrc); // Copy/catenate a string with truncation.ssize_t strtcpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);size_t strlcpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);size_t strlcat(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);Null-padded character sequences // Fill a fixed-size buffer with characters from a string // and pad with null bytes.char *strncpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);char *stpncpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize); // Chain-copy a null-padded character sequence into a character sequence.mempcpy(dst, src, strnlen(src, NITEMS(src))); // Chain-copy a null-padded character sequence into a string.stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), ""); // Catenate a null-padded character sequence into a string.char *strncat(size_t ssize;char *restrictdst, const charsrc[restrictssize],size_tssize); // Duplicate a null-padded character sequence into a string.char *strndup(size_t ssize;const charsrc[ssize], size_tssize);Length-bounded character sequences // Chain-copy a length-bounded character sequence.void *mempcpy(size_t len;voiddst[restrictlen], const voidsrc[restrictlen],size_tlen); // Chain-copy a length-bounded character sequence into a string.stpcpy(mempcpy(dst, src, len), "");
Terms (and abbreviations)string (str) is a sequence of zero or more non-null characters followed by a null character.character sequence is a sequence of zero or more non-null characters. A program should never use a character sequence where a string is required. However, with appropriate care, a string can be used in the place of a character sequence.null-padded character sequence Character sequences can be contained in fixed-size buffers, which contain padding null bytes after the character sequence, to fill the rest of the buffer without affecting the character sequence; however, those padding null bytes are not part of the character sequence. Don't confuse null-padded with null-terminated: null-padded means 0 or more padding null bytes, while null-terminated means exactly 1 terminating null character.length-bounded character sequence Character sequence delimited by its length. It may be a slice of a larger character sequence, or even of a string.length (len) is the number of non-null characters in a string or character sequence. It is the return value ofstrlen(str) and ofstrnlen(buf, size).size refers to the entire buffer where the string or character sequence is contained.end is the name of a pointer to one past the last element of a buffer. It is equivalent to&str[size]. It is used as a sentinel value, to be able to truncate strings or character sequences instead of overrunning the containing buffer.copy This term is used when the writing starts at the first element pointed to bydst.catenate This term is used when a function first finds the terminating null character indst, and then starts writing at that position.chain This term is used when it's the programmer who provides a pointer to the terminating null character in the stringdst (or one after the last character in a character sequence), and the function starts writing at that location. The function returns a pointer to the new location of the terminating null character (or one after the last character in a character sequence) after the call, so that the programmer can use it to chain such calls.duplicate Allocate a new buffer where a copy is placed.Copy, catenate, and chain-copy Originally, there was a distinction between functions that copy and those that catenate. However, newer functions that copy while allowing chaining cover both use cases with a single API. They are also algorithmically faster, since they don't need to search for the terminating null character of the existing string. However, functions that catenate have a much simpler use, so if performance is not important, it can make sense to use them for improving readability. The pointer returned by functions that allow chaining is a byproduct of the copy operation, so it has no performance costs. Functions that return such a pointer, and thus can be chained, have names of the form *stp*(), since it's common to name the pointer justp. Chain-copying functions that truncate should accept a pointer to the end of the destination buffer, and have names of the form *stpe*(). This allows not having to recalculate the remaining size after each call.Truncate or not? The first thing to note is that programmers should be careful with buffers, so they always have the correct size, and truncation is not necessary. In most cases, truncation is not desired, and it is simpler to just do the copy. Simpler code is safer code. Programming against programming mistakes by adding more code just adds more points where mistakes can be made. Nowadays, compilers can detect most programmer errors with features like compiler warnings, static analyzers, and_FORTIFY_SOURCE(seeftm(7)). Keeping the code simple helps these overflow-detection features be more precise. When validating user input, code should normally not truncate, but instead fail and prevent the copy at all. In some cases, however, it makes sense to truncate. Functions that truncate: •stpecpy() •strtcpy() •strlcpy(3bsd) andstrlcat(3bsd) are similar, but have important performance problems; see BUGS. •stpncpy(3) andstrncpy(3) also truncate, but they don't write strings, but rather null-padded character sequences.Null-padded character sequences For historic reasons, some standard APIs and file formats, such asutmpx(5) andtar(1), use null-padded character sequences in fixed- size buffers. To interface with them, specialized functions need to be used. To copy bytes from strings into these buffers, usestrncpy(3) orstpncpy(3). To read a null-padded character sequence, usestrnlen(src, NITEMS(src)), and then you can treat it as a length- bounded character sequence; or usestrncat(3) orstrndup(3) directly.Length-bounded character sequences The simplest character sequence copying function ismempcpy(3). It requires always knowing the length of your character sequences, for which structures can be used. It makes the code much faster, since you always know the length of your character sequences, and can do the minimal copies and length measurements.mempcpy(3) copies character sequences, so you need to explicitly set the terminating null character if you need a string. In programs that make considerable use of strings or character sequences, and need the best performance, using overlapping character sequences can make a big difference. It allows holding subsequences of a larger character sequence, while not duplicating memory nor using time to do a copy. However, this is delicate, since it requires using character sequences. C library APIs use strings, so programs that use character sequences will have to take care of differentiating strings from character sequences. To copy a length-bounded character sequence, usemempcpy(3). To copy a length-bounded character sequence into a string, usestpcpy(mempcpy(dst, src, len), ""). A string is also accepted as input, becausemempcpy(3) asks for the length, and a string is composed of a character sequence of the same length plus a terminating null character.String vs character sequence Some functions only operate on strings. Those require that the inputsrc is a string, and guarantee an output string (even when truncation occurs). Functions that catenate also require thatdst holds a string before the call. List of functions: •stpcpy(3) •strcpy(3),strcat(3) •stpecpy() •strtcpy() •strlcpy(3bsd),strlcat(3bsd) Other functions require an input string, but create a character sequence as output. These functions have confusing names, and have a long history of misuse. List of functions: •stpncpy(3) •strncpy(3) Other functions operate on an input character sequence, and create an output string. Functions that catenate also require thatdst holds a string before the call.strncat(3) has an even more misleading name than the functions above. List of functions: •strncat(3) •strndup(3) Other functions operate on an input character sequence to create an output character sequence. List of functions: •mempcpy(3)Functionsstpcpy(3) Copy the input string into a destination string. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.strcpy(3)strcat(3) Copy and catenate the input string into a destination string. The programmer is responsible for allocating a buffer large enough. The return value is useless.stpcpy(3) is a faster alternative to these functions.stpecpy() Chain-copy the input string into a destination string. If the destination buffer, limited by a pointer to its end, isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). It returns a pointer suitable for chaining. Truncation needs to be detected only once after the last chained call. This function is not provided by any library; see EXAMPLES for a reference implementation.strtcpy() Copy the input string into a destination string. If the destination buffer isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). It returns the length of the string, or -1 if it truncated. This function is not provided by any library; see EXAMPLES for a reference implementation.strlcpy(3bsd)strlcat(3bsd) Copy and catenate the input string into a destination string. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). They return the length of the total string they tried to create. Check BUGS before using these functions.strtcpy() andstpecpy() are better alternatives to these functions.stpncpy(3) Copy the input string into a destination null-padded character sequence in a fixed-size buffer. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting character sequence is truncated. Since it creates a character sequence, it doesn't need to write a terminating null character. It's impossible to distinguish truncation by the result of the call, from a character sequence that just fits the destination buffer; truncation should be detected by comparing the length of the input string with the size of the destination buffer.strncpy(3) This function is identical tostpncpy(3) except for the useless return value.stpncpy(3) is a more useful alternative to this function.strncat(3) Catenate the input character sequence, contained in a null- padded fixed-size buffer, into a destination string. The programmer is responsible for allocating a buffer large enough. The return value is useless. Do not confuse this function withstrncpy(3); they are not related at all.stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), "") is a faster alternative to this function.strndup(3) Duplicate the input character sequence, contained in a null-padded fixed-size buffer, into a newly allocated destination string. The string must be freed withfree(3).mempcpy(3) Copy the input character sequence, limited by its length, into a destination character sequence. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.
stpcpy(3) A pointer to the terminating null character in the destination string.stpecpy() A pointer to the terminating null character in the destination string, on success. On error, NULL is returned, anderrno is set to indicate the error.mempcpy(3)stpncpy(3) A pointer to one after the last character in the destination character sequence.strtcpy() The length of the string, on success. On error, -1 is returned, anderrno is set to indicate the error.strlcpy(3bsd)strlcat(3bsd) The length of the total string that they tried to create (as if truncation didn't occur).strcpy(3)strcat(3)strncpy(3)strncat(3) Thedst pointer, which is useless.strndup(3) The newly allocated string.
Most of these functions don't seterrno.stpecpy()strtcpy()ENOBUFSdsize was0.E2BIGThe string has been truncated.strndup(3)ENOMEMInsufficient memory available to allocate duplicate string.
The Linux kernel has an internal function for copying strings,strscpy(9), which is identical tostrtcpy(), except that it returns-E2BIGinstead of -1 and it doesn't seterrno.
Don't mix chain calls to truncating and non-truncating functions. It is conceptually wrong unless you know that the first part of a copy will always fit. Anyway, the performance difference will probably be negligible, so it will probably be more clear if you use consistent semantics: either truncating or non-truncating. Calling a non-truncating function after a truncating one is necessarily wrong.
All catenation functions share the same performance problem: Shlemiel the painter ⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩. As a mitigation, compilers are able to transform some calls to catenation functions into normal copy functions, sincestrlen(dst) is usually a byproduct of the previous copy.strlcpy(3) andstrlcat(3) need to read the entiresrc string, even if the destination buffer is small. This makes them vulnerable to Denial of Service (DoS) attacks if an attacker can control the length of thesrc string. And if not, they're still unnecessarily slow.
The following are examples of correct use of each of these functions.stpcpy(3) p = buf; p = stpcpy(p, "Hello "); p = stpcpy(p, "world"); p = stpcpy(p, "!"); len = p - buf; puts(buf);strcpy(3)strcat(3) strcpy(buf, "Hello "); strcat(buf, "world"); strcat(buf, "!"); len = strlen(buf); puts(buf);stpecpy() end = buf + NITEMS(buf); p = buf; p = stpecpy(p, end, "Hello "); p = stpecpy(p, end, "world"); p = stpecpy(p, end, "!"); if (p == NULL) { len = NITEMS(buf) - 1; goto toolong; } len = p - buf; puts(buf);strtcpy() len = strtcpy(buf, "Hello world!", NITEMS(buf)); if (len == -1) goto toolong; puts(buf);strlcpy(3bsd)strlcat(3bsd) if (strlcpy(buf, "Hello ", NITEMS(buf)) >= NITEMS(buf)) goto toolong; if (strlcat(buf, "world", NITEMS(buf)) >= NITEMS(buf)) goto toolong; len = strlcat(buf, "!", NITEMS(buf)); if (len >= NITEMS(buf)) goto toolong; puts(buf);stpncpy(3) p = stpncpy(u->ut_user, "alx", NITEMS(u->ut_user)); if (NITEMS(u->ut_user) < strlen("alx")) goto toolong; len = p - u->ut_user; fwrite(u->ut_user, 1, len, stdout);strncpy(3) strncpy(u->ut_user, "alx", NITEMS(u->ut_user)); if (NITEMS(u->ut_user) < strlen("alx")) goto toolong; len = strnlen(u->ut_user, NITEMS(u->ut_user)); fwrite(u->ut_user, 1, len, stdout);mempcpy(dst, src, strnlen(src, NITEMS(src))) char buf[NITEMS(u->ut_user)]; p = buf; p = mempcpy(p, u->ut_user, strnlen(u->ut_user, NITEMS(u->ut_user))); len = p - buf; fwrite(buf, 1, len, stdout);stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), "") char buf[NITEMS(u->ut_user) + 1]; p = buf; p = mempcpy(p, u->ut_user, strnlen(u->ut_user, NITEMS(u->ut_user))); p = stpcpy(p, ""); len = p - buf; puts(buf);strncat(3) char buf[NITEMS(u->ut_user) + 1]; strcpy(buf, ""); strncat(buf, u->ut_user, NITEMS(u->ut_user)); len = strlen(buf); puts(buf);strndup(3) buf = strndup(u->ut_user, NITEMS(u->ut_user)); len = strlen(buf); puts(buf); free(buf);mempcpy(3) p = buf; p = mempcpy(p, "Hello ", 6); p = mempcpy(p, "world", 5); p = mempcpy(p, "!", 1); len = p - buf; fwrite(buf, 1, len, stdout);stpcpy(mempcpy(dst, src, len), "") p = buf; p = mempcpy(p, "Hello ", 6); p = mempcpy(p, "world", 5); p = mempcpy(p, "!", 1); p = stpcpy(p, ""); len = p - buf; puts(buf);Implementations Here are reference implementations for functions not provided by libc. /* This code is in the public domain. */ char *stpecpy(char *dst, char end[0], const char *restrict src) { size_t dlen; if (dst == NULL) return NULL; dlen = strtcpy(dst, src, end - dst); return (dlen == -1) ? NULL : dst + dlen; } ssize_tstrtcpy(char *restrict dst, const char *restrict src, size_t dsize) { bool trunc; size_t dlen, slen; if (dsize == 0) { errno = ENOBUFS; return -1; } slen = strnlen(src, dsize); trunc = (slen == dsize); dlen = slen - trunc; stpcpy(mempcpy(dst, src, dlen), ""); if (trunc) errno = E2BIG; return trunc ? -1 : slen; }bzero(3),memcpy(3),memccpy(3),mempcpy(3),stpcpy(3),strlcpy(3bsd),strncat(3),stpncpy(3),string(3)
This page is part of theman-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-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. 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 isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28string_copying(7)Pages that refer to this page:stpncpy(3), strcpy(3), strncat(3)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |