C语言中删去字符串头部空格的方法

C语言没有提供可删去字符串头部空格的标准库函数，但是，编写这样的一个函数是很方便的。请看下例:

#include <stdio.h>
#include <string.h>

char * ltrim (char * );
char * rtrim(char * );
void main (void)
{
    char * lead_str = " This string has leading spaces in it. " ;
    /* Show the status of the string before calling the Itrim() function. */
    printf("Before calling Itrim(), lead-str is '%s'\n", lead_str);
    printf("and has a length of %d. \n" , strlen(lead_str));
    /* Call the Itrim() function to remove the leading blanks. */
        ltrim(lead_str);
    /* Show the status of the string after calling the Itrim() function. */
    printf("After calling Itrim(), lead_str is '%s'\n", lead_str);
    printf("and has a length of %d. \n",strlen(lead_str)) ;
}
/* The Itrim() function removes leading spaces from a string. */

char * ltrim(char * str)
{
    strrev(str); /* Call strrevO to reverse the string. */
    rtrim(str); /* Call rtrimO to remvoe the "trailing" spaces. */
    strrev(str); /* Restore the string's original order. */
    return str ; /* Return a pointer to the string. */
}
/* The rtrim() function removes trailing spaces from a string. */

char* rtrim(char* str)
{
    int n = strlen (str)-1 ; /* Start at the character BEFORE the null character (\0). */
    while (n>0) /* Make sure we don't go out of bounds... */
    {
        if ( * (str+n)!=' ') /*If we find a nonspace character: */
        {
            * (str+n + 1) = '\0' ; /* Put the null character at one character past our current position. */
            break; /* Break out of the loop. */
        }
        else /* Otherwise, keep moving backward in the string. */
            n --;
    }
    return  str;    /*  Return a pointer tO the string．  */
}

在上例中，删去字符串头部空格的工作是由用户编写的ltrim()函数完成的，该函数调用了·6．2的例子中的rtrim()函数和标准C库函数strrev()。ltrim()函数首先调用strrev()函数将字符串颠倒一次，然后调用rtrim()函数删去字符串尾部的空格，最后调用strrev()函数将字符串再颠倒一次，其结果实际上就是删去原字符串头部的空格。