[SOLVED] C - Function only returns value if an irrelevant printf is in the function.

jmgibson1981 · 03-30-2023, 03:35 PM

Code:

char * file_search_func(FILE * file, const char * searchstr,
                      const char * delimiter, const int field)
{
  // declare and initialize
  char * token = NULL;
  char * retval = NULL;
  char buffer[256];
  int counter = 0;
  int keepgoing = 0;

  // loop until find identifier in string. typically first field
  // tokenize once found
  while (keepgoing == 0) {
    if (fgets(buffer, 256, file)) {
//      printf("indeed\n");
      if (strstr(buffer, searchstr)) {
        // tokenize and begin processing
        token = strtok(buffer, delimiter);
        while (token) {
          // if proper value copy to pointer
          if (counter == field) {
            retval = token;
            keepgoing = 1;
            break;
          } else {
          // else continue to next token until found
            counter++;
            token = strtok(NULL, delimiter);
          }
        }
      }
    }
  }

  // reset file to beginning
  rewind(file);
  return(retval);
}

I am so lost right now. I get my expected return value but only if the printf("indeed\n"); is uncommented. If it's commented nothing comes up on the terminal... I have a feeling its going to be staring me in the face but I simply cannot comprehend how a printf statement changes how everything else works :/

dugan · 03-30-2023, 03:38 PM

The newline in the printf flushes the standard output buffer. Maybe that has something to do with it?

dugan · 03-30-2023, 04:58 PM

Honestly, your returning of a char pointer just smells unsafe to me. My concern is: does the memory address pointed to by the pointer still contain the same thing after the function returns? It looks like the answer is "not necessarily", because it points to a stack-allocated buffer that gets destroyed at the return statement. I changed it to allocate a string for the return value and then return that, and that works without the stray printf.

Code:

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

char *file_search_func(FILE *file, const char *searchstr,
                       const char *delimiter, const int field)
{
    // declare and initialize
    char *token = NULL;
    char *retval = NULL;
    char buffer[256];
    int counter = 0;
    int keepgoing = 0;

    // loop until find identifier in string. typically first field
    // tokenize once found
    while (keepgoing == 0)
    {
        if (fgets(buffer, 256, file))
        {
            if (strstr(buffer, searchstr))
            {
                // tokenize and begin processing
                token = strtok(buffer, delimiter);
                while (token)
                {
                    // if proper value copy to pointer
                    if (counter == field)
                    {
                        retval = token;
                        keepgoing = 1;
                        break;
                    }
                    else
                    {
                        // else continue to next token until found
                        counter++;
                        token = strtok(NULL, delimiter);
                    }
                }
            }
        }
    }

    // reset file to beginning
    rewind(file);

    char *s = (char *)malloc(strlen(retval) * sizeof(char) + 1);
    strcpy(s, retval);
    return s;
}

int main()
{
    // a.txt contains one line:
    // b;
    FILE *f = fopen("a.txt", "r");
    char *s = file_search_func(f, "b", ";", 0);
    printf("%s\n", s);
    free(s);
    fclose(f);
}

jmgibson1981 · 03-30-2023, 06:48 PM

Thank you much. I was afraid of that. I had it working a similar way and was trying to eliminate having to call free outside the function. At least it now doesn't rely on a pass by reference.

ntubski · 04-01-2023, 11:07 AM

Additional minor suggestion: You can use while (retval == NULL) instead of while (keepgoing == 0), and then the keepgoing variable is redundant.

By the way, since 0 is usually considered false, your use of while (keepgoing == 0) looks very confusing to me (i.e., it reads like "if not keepgoing then do keep going, today is opposite day!").

jmgibson1981 · 04-01-2023, 11:45 AM

I always thought it was the other way around. I'll make those changes. Was thinking of a typical shell command return 0 if success.

NevemTeve · 04-01-2023, 11:56 AM

That's right, shell uses another logic: 0=success, nonzero=errorcode