Why does a segmentation fault appear, but when the program is restarted, it does not?

Question

linux

I need to turn gpio38 on and off for this, I use C++ code, but once every other time a segmentation fault error pops up (that is, the first time I started everything normally, the second time I started the error, the third everything is fine, the fourth error, and so on, without changing anything in the code)

/* gpio38.c */
#include <stdio.h>
#include <stdlib.h>
#define GPIO_PATH   "/sys/class/gpio"
#define GPIO38_PATH “/sys/class/gpio/gpio38”
 
int main( int argc, char * argv[] )
{
   FILE * fp;
 
  int a=0;
 
  fp = fopen( GPIO_PATH"/export","w");
  
  if ( fp == NULL ) {
      printf(“can not open file\n”);
      return EXIT_FAILURE;
  }
  fprintf( fp, "38");
  fclose( fp );
 
  fp = fopen( GPIO38_PATH”/direction”, “w”);
 
  if ( fp == NULL ) {
      printf(“can not open file\n”);
      return EXIT_FAILURE;
  }
 if(a==1)
  fprintf( fp, “high”);
else fprintf( fp, “low”);
  fclose(fp);
 
  fp = fopen( GPIO38_PATH”/value”, “w”);
  
  if ( fp == NULL ) {
      printf(“can not open file\n”);
      return EXIT_FAILURE;
  }
 
  fclose(fp);
}

Tell me what is the error, I can not understand why such a strange behavior

Answer 1

I do not see a crime that could lead to a segfault in the above program.
Why behavior might differ between runs -- for example, because writing to a file /sys/class/gpio/exportchanges the state of the system and is remembered between runs.
In addition, according to https://www.kernel.org/doc/Documentation/gpio/sysfs.txt , either should be directionwritten in inor out, and highor lowshould be written in value. Segfolta from this should not be, however.
What happens if you build a program with debug information and run it so that it crashes under the debugger?
UPD: I looked a little more... In the given source text, there are crooked Unicode quotes. I recommend correcting for ASCII, or inserting the source text as is, without tricks.

Answer 2

Try using a different pointer for the second file, or add a sleep between closing and opening.

Answer 3

Why the program crashes with SIGSEGV is also not entirely clear to me.

In the debugger, when an error occurs at the line fp = fopen( GPIO38_PATH”/direction”, “w”);
fp is 0 because of this, everything falls down for another time the result is normal

Between the export and the use of (fopen(...direction)) it is worth using a short pause, the kernel needs some time to export the gpio.
Well, strange quotes, of course, as jcmvbkbc said . maybe you should compile like this:
You can also check if the files are closed, but it still does not give SIGSEGV.

Answer 4

Um, I could be wrong, but shouldn't the fp be cleaned before being reused. So you assign fp as a pointer to a new file, and leftovers from the past are left in the memory area. delete to help.