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fread: segementation fault(coredump) w/o stdlib.h

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# 1  
Old 02-11-2009
fread: segementation fault(coredump) w/o stdlib.h
Hello All,

I tried to test a sample fread example to read a complete file
and the code is

Code:
#include <stdio.h>
#include <stdlib.h>
int main () {
  FILE * pFile;
  long lSize;
  char * buffer;
  size_t result;

  pFile = fopen ( "test.xml" , "rb" );
  if (pFile==NULL) {fputs ("File error",stderr); exit (1);}

  fseek (pFile , 0 , SEEK_END);
  lSize = ftell (pFile);
  rewind (pFile);

  buffer = (char*) malloc (sizeof(char)*lSize);
  if (buffer == NULL) {fputs ("Memory error",stderr); exit (2);}

  result = fread (buffer,1,lSize,pFile);
  if (result != lSize) {fputs ("Reading error",stderr); exit (3);}

  /* the whole file is now loaded in the memory buffer. */

  fclose (pFile);
  printf(" sizeof char :%d \n", sizeof(char));
  printf( "File: %s ", buffer);
  free (buffer);
  return 0;
}

The program worked as expected and the got the correct output.

After i removed #include <stdlib.h> from the program, it got compiled successfully but failed with the following message: Memory fault(coredump)

It failed in the fread command (checked with dbx).

Can anyone explain this behaviour?

I use AIX ver 5.

rgrds,
# 2  
Old 02-11-2009
As the doctor would say, "don't do that". Smilie Either AIX has odd integer sizes, or you're on a 64-bit system. Or perhaps both.

When you use functions that you haven't included headers for, the compiler's forced to guess what parameters the function takes and returns, and will guess they are all integers. This is often a good guess on most 32-bit systems, where int, long, and all pointers are the same size -- 32 bits. But 64-bit systems will have 64-bit pointers while still perhaps having 32-bit integers, causing pointers to get balled up and crammed through the mail slot, so to speak.

Without stdlib.h, the value malloc() gives you is corrupted, causing fread to crash when it uses it.

So, always include the headers! Just because it works 'this time' when you don't doesn't mean it will in all situations.
# 3  
Old 02-11-2009
That's because malloc is declared in /usr/include/stdlib.h file and when you remove it from the code malloc is no longer defined and the program core dumps. Did your compiler not warn you of that.
# 4  
Old 02-11-2009
Quote:
Originally Posted by shamrock
That's because malloc is declared in /usr/include/stdlib.h file and when you remove it from the code malloc is no longer defined and the program core dumps. Did your compiler not warn you of that.
Nope. I did not receive any warnings from the compiler. Moreover when i debug the program it did go through the malloc part without any issue but failed in the fread command.
# 5  
Old 02-11-2009
Could you put some debug printf statements in your code so that we can see the values of lsize and buffer.
I thought malloc because if buffer is not allocated you cannot store anything there yet if (buffer == NULL) is never executed.
That is somewhat weird but I will try to reproduce it on my system running AIX ver 5.3
# 6  
Old 02-11-2009
I'm surprised the compiler does not complain about malloc, too. Unless the OP got a symbol not resolved error, the code will run, but it may be using some library calls that are confused about sizeof(int) - as Corona says. ftell returns a long int.

A long int in 64 bit is 64 bit. a long int in 32 bit is 32 bits. That may well be the issue. I dunno. compile with symbols and then run the dbeugger on the core. That will tell you what call caused a segfault.
# 7  
Old 02-12-2009
I have added the printf statement and yes, the program is failing because the buffer is not allocated. It fails in the strcpy function.

Code:
#include <stdio.h>
int main () {
  FILE * pFile;
  long lSize;
  char * buffer;
  size_t result;

  pFile = fopen ( "test.xml" , "rb" );
  if (pFile==NULL) {fputs ("File error",stderr); exit (1);}

  fseek (pFile , 0 , SEEK_END);
  lSize = ftell (pFile);
  rewind (pFile);

  buffer = (char*) malloc (sizeof(char)*lSize);
  if (buffer == NULL) {fputs ("Memory error",stderr); exit (2);}

  printf ("\n size of buffer %ld, length of file %d\n ", sizeof(buffer), lSize);

  strcpy (buffer, "test message");
  printf( "\n Buffer Data is : %s", buffer);
  result = fread (buffer,1,lSize,pFile);
  if (result != lSize) {fputs ("Reading error",stderr); exit (3);}

  /* the whole file is now loaded in the memory buffer. */

  fclose (pFile);
  printf(" sizeof char :%d \n", sizeof(char));
  printf( "File: %s ", buffer);
  free (buffer);
  return 0;
}

Output:

size of buffer 8, length of file 260
Memory fault(coredump)
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