Trouble replacing line of file

hi again, i am having some dificulties replacing a certain line of a file.My problem in specific is writing to the file, a certain string, overwriting the line i want. However, my program just writes to the end of file ( as it should, according to the code ).
I will give you the code, along with examples:
file def:

Code:

---

im      man
this    strinks

---

the line i want to replace

Code:

---

this    stinks

---

i want to replace with chars ' '


my program, if he detects a certain string in the line, it is supposed to replace the line with chars ' ', if(name_esq == this), then replace the line that has this, like so:

Code:

---

im    man

---

however. it will do:

Code:

---

im      man
this      stinks

---

this is an excert of the program:

PHP Code:

---

#define MAXLINE[1024]
int main()
{
char line2[1024];
char *name_esq="this";
char *definitions="definitions.txt";
FILE * def
def=fopen(definitions, "r+");


while(fgets(line2, MAXLINE , def))
/* line2 - line of file def*/
{
printf("anything\n");        /* for debugging */
if(strstr(line2,name_esq))    /* if name_esq is in line2*/  
{
printf("if works\n");/* for debugging */
printf("%s\n", line2);    /* for debugging */
for(k=0; line2[k]!= '\n';k++)
    {
    line2[k]=' ';                   
    fprintf(def,"%s\n",line2);
    /*replaces line2's chars with ' ' 
    */  
    }
    
    }/* I expect to replace the line in def,
    which has the occurence of name_esq
    but instead, it writes to the next line
    as in the example.
    */
}
} 


---

any sugestions will be gratefully accepted.

P.S. : if you think i overexplained, please warn me and i will try to rearrange it.

Thanks in advance

Read lines into memory from one file, and write out all lines (with the relevant one amended). When you're done, delete the original input file, and rename the output file to the name of the first one.

Either that, or read and hold all lines in memory, then seek back to the beginning of the file, and write all lines out.

please give me a small example of how i can read lines into memory, and then read from memory, i am not sure i understand much about memory.
Sorry for being this noob.

Code:

---

char line2[1024]; <-- Memory

---

Just make an array that can hold all of the lines in the file:

Code:

---

char lines[5][1024]; /* Magic! */

---

ok that's all i can explain