Where is the implementation of strlen() in GCC?

Question

Can anyone point me to the definition of strlen() in GCC? I\'ve been grepping release 4.4.2 for about a half hour now (while Googling like crazy) and I can\'t seem

Answer 1

Is this what you are looking for? strlen() source. See the git repository for more information. The glibc resources page has links to the git repositories if you want to grab them rather than looking at the web view.

Answer 2

Here's the bsd implementation

size_t
strlen(const char *str)
{
        const char *s;

        for (s = str; *s; ++s)
                ;
        return (s - str);
}

Answer 3

You should be looking in glibc, not GCC -- it seems to be defined in strlen.c -- here's a link to strlen.c for glibc version 2.7... And here is a link to the glibc SVN repository online for strlen.c.

The reason you should be looking at glibc and not gcc is:

The GNU C library is used as the C library in the GNU system and most systems with the Linux kernel.

Answer 4

I realize this question is 4yrs old, but gcc will often include its own copy of strlen if you do not #include <string.h> and none of the answers (including the accepted answer) account for that. If you forget, you will get a warning:

file_name:line_number: warning: incompatible implicit declaration of built-in function 'strlen'

and gcc will inline its copy which on x86 is the repnz scasb asm variant unless you pass -Werror or -fno-builtin. The files related to this are in gcc/config/<platform>/<platform>.{c,md}

It is also controlled by gcc/builtins.c. In case you wondered if and how a strlen() was optimized to a constant, see the function defined as tree c_strlen(tree src, int only_value) in this file. It also controls how strlen (amongst others) is expanded and folded (based on the previously mentioned config/platform)

Answer 5

defined in glibc/string/strlen.c

#include <string.h>
#include <stdlib.h>

#undef strlen

#ifndef STRLEN
# define STRLEN strlen
#endif

/* Return the length of the null-terminated string STR.  Scan for
   the null terminator quickly by testing four bytes at a time.  */
size_t
STRLEN (const char *str)
{
  const char *char_ptr;
  const unsigned long int *longword_ptr;
  unsigned long int longword, himagic, lomagic;

  /* Handle the first few characters by reading one character at a time.
     Do this until CHAR_PTR is aligned on a longword boundary.  */
  for (char_ptr = str; ((unsigned long int) char_ptr
            & (sizeof (longword) - 1)) != 0;
       ++char_ptr)
    if (*char_ptr == '\0')
      return char_ptr - str;

  /* All these elucidatory comments refer to 4-byte longwords,
     but the theory applies equally well to 8-byte longwords.  */

  longword_ptr = (unsigned long int *) char_ptr;

  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
     the "holes."  Note that there is a hole just to the left of
     each byte, with an extra at the end:

     bits:  01111110 11111110 11111110 11111111
     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD

     The 1-bits make sure that carries propagate to the next 0-bit.
     The 0-bits provide holes for carries to fall into.  */
  himagic = 0x80808080L;
  lomagic = 0x01010101L;
  if (sizeof (longword) > 4)
    {
      /* 64-bit version of the magic.  */
      /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
      himagic = ((himagic << 16) << 16) | himagic;
      lomagic = ((lomagic << 16) << 16) | lomagic;
    }
  if (sizeof (longword) > 8)
    abort ();

  /* Instead of the traditional loop which tests each character,
     we will test a longword at a time.  The tricky part is testing
     if *any of the four* bytes in the longword in question are zero.  */
  for (;;)
    {
      longword = *longword_ptr++;

      if (((longword - lomagic) & ~longword & himagic) != 0)
    {
      /* Which of the bytes was the zero?  If none of them were, it was
         a misfire; continue the search.  */

      const char *cp = (const char *) (longword_ptr - 1);

      if (cp[0] == 0)
        return cp - str;
      if (cp[1] == 0)
        return cp - str + 1;
      if (cp[2] == 0)
        return cp - str + 2;
      if (cp[3] == 0)
        return cp - str + 3;
      if (sizeof (longword) > 4)
        {
          if (cp[4] == 0)
        return cp - str + 4;
          if (cp[5] == 0)
        return cp - str + 5;
          if (cp[6] == 0)
        return cp - str + 6;
          if (cp[7] == 0)
        return cp - str + 7;
        }
    }
    }
}
libc_hidden_builtin_def (strlen)

Answer 6

You can use this code, the simpler the better !

size_t Strlen ( const char * _str )
{
    size_t i = 0;
    while(_str[i++]);
    return i;
}