How does strtod manage to parse?

I'm trying to understand how a well functioning piece of code actually does its work. In question is how the functions "strtod" and "strtol" manage to sequentially parse an input file or string of characters. I do understand that the functions in question do a conversion from characters to numbers; however, what I find confusing is how they manage to increment from one set of characters to the next in the input file. The file that is opened reads as follows, in part:

1
22050.0
4.0
/* and so on */


The following code opens that file and begins sequentially pulling and converting blocks of characters into the appropriate global variables. What I don't undersatnd is how each sucessive "strtod" ot "strtol" knows to skip the previously read characters and pull the next sucessive block. If these functions do this on their own somehow, I haven't found any documentation as to this capability in the function manuals.

Okay, here's the code in question (and it DOES work!):

Code:

---

int parseInputFile(const char *inputFile)
{
    int i;
    FILE *fopen(), *fp;
    char line[128];


    /*  OPEN THE INPUT FILE  */
    if ((fp = fopen(inputFile, "r")) == NULL) {
        fprintf(stderr, "Can't open input file \"%s\".\n", inputFile);
        return (ERROR);
    }


    /*  GET Global_Variable_0 */
    if (fgets(line, 128, fp) == NULL) {
        fprintf(stderr, "Can't read Global_Variable_0.\n");
        return(ERROR);
    }
    else
        Global_Variable_0  = strtol(line, NULL, 10);

    /*  GET Global_Variable_1 */
    if (fgets(line, 128, fp) == NULL) {
        fprintf(stderr, "Can't read Global_Variable_1.\n");
        return(ERROR);
    }
    else
        Global_Variable_1 = strtod(line, NULL);

    /*  GET Global_Variable_2 */
    if (fgets(line, 128, fp) == NULL) {
        fprintf(stderr, "Can't read Global_Variable_2.\n");
        return(ERROR);
    }
    else
        Global_Variable_2 = strtod(line, NULL);

/*  THE PROGRAM CONTINUES WITH A SERIES OF STATEMENTS
 IDENTICAL THE ABOVE THREE CHUNKS OF CODE UNTIL ALL THE
 INPUT FILE CHARACTERS (GLOBAL VARIABLE VALUES) HAVE BEEN PARSED */

                             

    /*  CLOSE THE INPUT FILE  */
    fclose(fp);

    /*  RETURN SUCCESS  */
    return (SUCCESS);
}

---

So, when it's done, the value "1" is assigned to Global_Variable_0, " 22050.0" to Global_Variable_1, "4.0" to Global_Variable_2, and so on. . .

Any ideas?

Attached is the GNU version of strtol.c which is called by all of the strto.... functions. glibc.2.2.3

The call to fgets reads one line at a time. The newline is not stripped away, but such a character that cannot be part of the target type will break out the call.

Code:

---

#include <stdio.h>
#include <stdlib.h>

int main()
{
  const char text[] = "123x5";
  long john_silver = strtol(text, NULL, 10);
  printf("john_silver = %ld\n", john_silver);
  return 0;
}

/* my output
john_silver = 123
*/

---

The parameters that here are NULL can be a pointer that will be set the the character that causes the break.

They aren't skipping anything, the array 'line' is being overwritten on each successive fgets call...

Okay, I think I understand it now: my problem was not with how strtod works, but with fgets. I didn't realize it reads one line at a time. The short man page I have on fgets doesn't explain that detail. I'll try to look into it better on the web.

Thanks again. You folks are GREAT!