Receiving warning "implicit declaration of function 'strlen'"

Question

I have a some simple code, but I am receiving a warning:

-bash-3.2$ gcc -Wall print_process_environ.c -o p_p print_process_environ.c: In function 'print_process_environ': print_process_environ.c:24: warning: implicit declaration of function 'strlen' print_process_environ.c:24: warning: incompatible implicit declaration of built-in function 'strlen' 

The following is the code:

#include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <strings.h> #include <sys/stat.h> #include <sys/types.h> #include <fcntl.h> #include <strings.h>      void     print_process_environ(pid_t pid)     {         int     fd;         char    filename[24];         char    environ[1024];         size_t  length;         char    *next_var;          snprintf(filename, sizeof(filename), "/proc/%d/environ", (int)pid);         printf("length of filename: %d\n", strlen(filename));          fd = open(filename, O_RDONLY); ...... 

The definition of strlen() is:

   #include <string.h>     size_t strlen(const char *s); 

how to get rid of this warning.

Answer 1

it's #include <string.h> . You are spelling it wrong in your code. Also if you ever get that warning in your compiler .. always do man function_name on terminal to see the header required for that function

 #include <string.h> // correct header  #include <strings.h> // incorrect header - change this in your code to string.h 

Answer 2

You were bitten by a easy to make mistake, you included the posix strings.h header:

#include <strings.h> 

instead of:

#include <string.h> 

the posix header includes support for:

int    bcmp(const void *, const void *, size_t); (LEGACY ) void   bcopy(const void *, void *, size_t); (LEGACY ) void   bzero(void *, size_t); (LEGACY ) int    ffs(int); char  *index(const char *, int); (LEGACY ) char  *rindex(const char *, int); (LEGACY ) int    strcasecmp(const char *, const char *); int    strncasecmp(const char *, const char *, size_t); 

which are all non-standard functions, this also explains the lack of error, I am having a hard time finding a good reference but BSD systems version of strings.h used to also include string.h.