Langue: en

Version: 2008-10-28 (ubuntu - 07/07/09)

Autres sections - même nom

Section: 3 (Bibliothèques de fonctions)


strcpy, strncpy - copy a string


 #include <string.h>
 char *strcpy(char *dest, const char *src);
 char *strncpy(char *dest, const char *src, size_t n);


The strcpy() function copies the string pointed to by src, including the terminating null byte ('\0'), to the buffer pointed to by dest. The strings may not overlap, and the destination string dest must be large enough to receive the copy.

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null terminated.

If the length of src is less than n, strncpy() pads the remainder of dest with null bytes.

A simple implementation of strncpy() might be:

 strncpy(char *dest, const char *src, size_t n){
     size_t i;
     for (i = 0 ; i < n && src[i] != '\0' ; i++)
         dest[i] = src[i];
     for ( ; i < n ; i++)
         dest[i] = '\0';
     return dest;


The strcpy() and strncpy() functions return a pointer to the destination string dest.


SVr4, 4.3BSD, C89, C99.


Some programmers consider strncpy() to be inefficient and error prone. If the programmer knows (i.e., includes code to test!) that the size of dest is greater than the length of src, then strcpy() can be used.

If there is no terminating null byte in the first n characters of src, strncpy() produces an unterminated string in dest. Programmers often prevent this mistake by forcing termination as follows:

 strncpy(buf, str, n);
 if (n > 0)
     buf[n - 1]= '\0';


If the destination string of a strcpy() is not large enough, then anything might happen. Overflowing fixed-length string buffers is a favorite cracker technique for taking complete control of the machine. Any time a program reads or copies data into a buffer, the program first needs to check that there's enough space. This may be unnecessary if you can show that overflow is impossible, but be careful: programs can get changed over time, in ways that may make the impossible possible.


bcopy(3), memccpy(3), memcpy(3), memmove(3), wcscpy(3), wcsncpy(3)


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