search / repalce

[Spedge]

i'm trying to make a function, which is searching a string in file. If the string exist it must be replaced with another one.
what is my idea...
I'm inserting string for searching
with fgets I'm reading all line-to-line content of the file
with strstr I'm checking if the searching string is exists in the file
if the string exist I'm inserting the new string
with freopen I'm closing the file and make new empty file with the same name
with fputs I'm writing the readed to this moment + the new string
And then I don't know how to continue
I'm posting my source code ( when I try to execute it it stops at while and says that fgets == NULL )

int update_field(char *phone)

{

	FILE *fp = fopen("names.txt", "a");
	char buff[60];
	char new_phone[20];
	
	if ( fp == NULL ) {
		printf("update_field/fp");
		exit(0);
	}

	else {
		while ( fgets(buff, sizeof buff, fp) != NULL) {
			if ( strstr(buff, phone) != NULL) {
				printf("Type the new phonenumber");
				gets(new_phone);
				fp = freopen("names.txt", "w", fp);
				fputs(buff, fp);
				fputs(new_phone, fp);
				fclose(fp);
			}
		}
	}
}

can u help me?

[Prelude]

>can u help me?
Try something more like this:

/* Return non-zero for success, zero for failure */
int update_field(char *phone, char *new_phone)
{
    FILE *file;
    FILE *temp;
    FILE *save;
    char buff[BUFSIZ];

    if ((file = fopen("names.txt", "r")) == NULL) {
        perror("update_field -- fopen");
        return 0;
    }

    if ((temp = tmpfile()) == NULL) {
        perror("update_field -- tmpfile");
        fclose(file);
        return 0;
    }

    while (fgets(buff, sizeof buff, file) != NULL) {
        if (strstr(buff, phone) != NULL)
            fputs(new_phone, temp);
        else
            fputs(buff, temp);
    }

    if ((save = freopen("names.txt", "w", file)) == NULL) {
        perror("update_field -- freopen");
        fclose(file);
        fclose(temp);
        return 0;
    }

    file = save;
    rewind(temp);

    while (fgets(buff, sizeof buff, temp) != NULL)
        fputs(buff, file);

    fclose(file);
    fclose(temp);

    return 1;
}

[C of Green]

void searchandreplace(char* s, char* r)
{
	char *pin,*pout,........tart;

	pin = &inline[0];
	pout = &outline[0];
	strcpy(pout,pin);

	sstart = strstr(outline,s);
	if (sstart == NULL) return;
	foundmatch = True;
	pin = r;
	pout = sstart;

	x = strlen(r);
	while (x > 0)
	{

	    *pout = *pin;
	     pin++; pout++;
	     x--;
	}

	pin = strstr(inline,s); 
	pin += strlen(s);
	strcpy(pout,pin);
}