C problen with Files

Question

YoTaMiX 0 Light Poster

20 Years Ago

Hello to you all ,

Major problem in C , Files included

Summary of problem :
Program will not copy the feild "name" from struct "univ->std.name" properly. the rest of data is presented fairly.
do not touch function "output_university" .

Thank you ,

yotam , Israel .

c

input.txt (0.09 KB)

Nir 32251 99.80 11001
Ely 12347 77.89 01111
Moshe 45321 50.34 11111
Avi 31456  49.78 00011

TEST2.C (3.89 KB)

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>
#include <math.h>
#define MAX 20
static void forcefloat(float *p)
{
float f=*p;
forcefloat(&f);
}

typedef struct{
char* name;
long id;
float avrg;
char works[5];
}student;

typedef struct{
student *std;
}university;

void input_university(university *univ, FILE *in);
void output_university(university *univ, FILE *in);
void output_factor(university *univ, FILE *in);

int num_cells=0;
char* temp;

int main()
{
FILE *in, *out;
university univ;
char op='a';
static int flag=0;
printf("please enter your options:\n");
printf(" a:for reading\n b:for output of the student information\n c:for showing marks after the factor\n z:exit");
while(op!='z')
{
	scanf("%c",&op);
	switch(op)
	{
		case 'a':if((in=fopen("input.txt","r"))==NULL)
			 {
			 fprintf(stderr, "input file not available");
			 break;
			 }
		       else {
		     input_university(&univ,in);
				fclose(in);
				flag=1;
				break;  }
case 'b':if(flag==1)
				{
				if((out=fopen("output.txt","w"))==NULL)
					{
					fprintf(stderr, "output file not available");
					break;
					}
				else {
				output_university(&univ,out);
				fclose(out);
				flag=2; }
				}
				else
				{
				if((out=fopen("output.txt","w"))==NULL)
				{
				fprintf(stderr, "output file not available");
				break;
				}
				fprintf(out, "'b' before 'a'-not available");
				fclose(out);
				break;
				}
				break;
case 'c':			if(flag==2)
				{
				if((out=fopen("output.txt","a"))==NULL)
				{
				fprintf(stderr, "output file not available");
				break;
				}
				else {
				output_factor(&univ,out);
				fclose(out); }
				}
				else{
				if((out=fopen("output.txt","a"))==NULL)
				{
				fprintf(stderr, "output file not available\n");
				break;
				}
				fprintf(out, "'c' brfore 'a' or 'b' -not available\n");
				fclose(out);
				break;
				}
				break;
case 'z': { printf("end of program"); }
}
}
getch(); flushall();
return 0;
}



void input_university(university *univ, FILE *in)
{
int i,j,str_s;
char buffer[MAX];


while(fscanf(in,"%s",buffer)!=EOF)
{

   str_s=strlen(buffer);

   univ->std=(student*)realloc(univ->std,(num_cells+1)*sizeof(student));
     if (univ->std==NULL) exit(1);
   univ->std[num_cells].name=(char*)malloc(str_s+1);
     if (univ->std[num_cells].name==NULL) exit(1);
   temp=(char*)malloc(5*sizeof(char));
     if (temp==NULL) exit(1);

 fscanf(in,"%ld",&univ->std[num_cells].id);
 fscanf(in,"%f",&univ->std[num_cells].avrg);
 strcpy(temp,univ->std[num_cells].works);
 fscanf(in,"%s",temp);

 printf("%s %ld %.2f %s \n",univ->std[num_cells].name,univ->std[num_cells].id,univ->std[num_cells].avrg,univ->std->works);
 ++num_cells;

}
}


void output_university(university *univ, FILE *out)
{
 int i,j,count;
 char *temp;
fprintf(out,"BEFORE:\n");
for(i=0;i<num_cells;i++)
 {
 fprintf(out,"student %d : ",i);
 //fputs(univ->std[i].name,out);
 fprintf(out,"%s",univ->std[i].name);

 fprintf(out," %ld",univ->std[i].id);
 fprintf(out," %.2f",univ->std[i].avrg);
 count=0;
 temp=univ->std[i].works;
 while(temp!='\0'){
  if(*temp==1)
  count++;
  temp++;
  }
 if(count==3 || count>3)
 fprintf(out,"%s",'1');
 else
 fprintf(out,"%s",'0');


fprintf(out,"\n");
}
}
void output_factor(university *univ, FILE *out)
{
int i,j,max=univ->std[0].avrg,factor,count;
char *temp;
fprintf(out,"AFTER:\n");
for(i=1;i<num_cells;i++)
{
if(univ->std[i].avrg>max)
max=univ->std[i].avrg;
}
for(i=0;i<num_cells;i++)
{
fprintf(out,"student %d\n",i);
fputs(univ->std->name,out);
fprintf(out,"%d",univ->std->id);
factor=((univ->std[i].avrg) *100 )/max;
fprintf(out,"%f",factor);
}
temp=univ->std[i].works ;
count=0;
while(temp!='\0'){
  if(*temp==1)
  count++;
  temp++;
  }
 if(count==3 || count>3)
 fprintf(out,"%s",'1');
 else
 fprintf(out,"%s",'0');
}

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Latest Post 20 Years Ago Latest Post by YoTaMiX

Dani AI

Generated 8 Months Ago

In this thread reports that the name field isn’t being preserved and that adding a copy call fixed one item but then broke another. As observed, the missing copy is only the most obvious symptom. When adding the copy causes other strings to corrupt the real root cause is almost always a boundary or allocation error — common cases are (a) allocating the wrong number of bytes (forgetting the terminating NUL), (b) reading into a temporary buffer without a width limit, or (c) losing/overwriting pointers because realloc or pointer initialization is mishandled (the latter was noted by ).

Safer allocation/copy pattern (check sizes and failures): always allocate strlen(buffer) + 1, check the malloc return, and copy the bytes including the terminating NUL. strdup is convenient where available, but explicit allocation lets the program react to OOM:

size_t len = strlen(buffer);
char *name = malloc(len + 1);
if (!name) { /* handle OOM */ }
memcpy(name, buffer, len + 1);
univ->std[num_cells].name = name;

Realloc and pointer-safety: never assign the result of realloc directly back into the original pointer without checking. Use a temporary pointer and preserve the original on failure. Also avoid casting the result of malloc/realloc in C and include <stdlib.h> so prototypes are visible:

void *tmp = realloc(univ, new_bytes);
if (!tmp) { /* handle OOM; keep original univ intact */ }
univ = tmp;

Practical debugging and hardening: compile with -Wall -Wextra -g and use AddressSanitizer (-fsanitize=address) or Valgrind to catch overruns and use-after-free. Prefer fgets plus parsing (or width-limited scanning) for input, print pointer addresses and strlen() values after allocation, and free temporary buffers appropriately. Those checks will usually reveal the off-by-one, overflow, or realloc mistake that produces the cascading corruption seen here.

All 4 Replies

Ancient Dragon 5,243 Achieved Level 70

20 Years Ago

In function input_university() I see where scanf() is reading the string, and univ->std[num_cells].name is getting allocated, but I don't see where it is copying buffer to univ->std[num_cells].name after allocation. Add this and it will probably work ok.

strcpy(univ->std[num_cells].name,buffer);

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Salem 6,009 Posting Sage · Answer 1 · 2006-04-23T22:41:49+00:00

Apparently, I wasted my time :mad:
http://www.daniweb.com/techtalkforums/thread43435.html

Not only have you dropped the initialisation of univ in main(), but you've gone back to using fscanf to read the file.
Not only that, you're now using global variables like "temp", which get allocated on every iteration of the loop and never freed.
Don't forget the not using a temporary variable when calling realloc, nor the unnecessary casting of malloc and realloc.

Y'know what, if you're just going to ignore people, just say so - ok?

YoTaMiX 0 Light Poster · Answer 2 · 2006-04-23T22:49:16+00:00

Apparently, I wasted my time :mad:
http://www.daniweb.com/techtalkforums/thread43435.html
Not only have you dropped the initialisation of univ in main(), but you've gone back to using fscanf to read the file.
Not only that, you're now using global variables like "temp", which get allocated on every iteration of the loop and never freed.
Don't forget the not using a temporary variable when calling realloc, nor the unnecessary casting of malloc and realloc.
Y'know what, if you're just going to ignore people, just say so - ok?

I guess i didnt see it - sorry mate :confused:

YoTaMiX 0 Light Poster · Answer 3 · 2006-04-23T22:57:24+00:00

In function input_university() I see where scanf() is reading the string, and univ->std[num_cells].name is getting allocated, but I don't see where it is copying buffer to univ->std[num_cells].name after allocation. Add this and it will probably work ok.
strcpy(univ->std[num_cells].name,buffer);

I did just that . now it messes up another string - i'll figure it out
somehow , the thing is , we were never tought the subject BUFSIZ and
"sscanf" - so no offence there ;)
i got slapped on using it .... sorry

C problen with Files

Dani AI

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