errno - perror vs fprintf + strerror

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Is0r Is0r is offline Offline Nov 11th, 2009, 11:23 am |
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Hi!

From the manpages I know, that perror evaluates the errno. So
using perror and fprintf + strerror, as coded in my example below, should result in the same error messages, but they do not.
The execution of the program leads to the following result:

bash> ./errno_test blub
fopen() failed: No such file or directory
fopen() failed
fopen() failed: Error code 29
fopen() failed: Error code Illegal seek

Can someone explain, where this behavior comes from? I think
fopen calls open, and open might invoke something such as a seek function. Is the errno set inappropriately? Or am I wrong about the
assumption, that perror and fprintf + strerror should cause the same output?

Thanks and Regards!
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  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <string.h>
  4. #include <errno.h>
  5.  
  6. int main (int argc, char** argv) {
  7.  
  8. 	if (argc != 2) {
  9. 		fprintf(stderr, "usage: ./error_test <file> \n");
  10. 		exit(1);
  11. 	}
  12.  
  13.  
  14. 	/* open any non-existing file to provoke an error message */
  15.  
  16.  
  17. 	if (NULL == fopen(argv[1], "r")) {
  18. 		perror ("fopen() failed");
  19. 		fprintf(stderr, "fopen() failed\n");
  20. 		fprintf(stderr, "fopen() failed: Error code %d\n", errno);
  21. 		fprintf(stderr, "fopen() failed: Error code %s\n", strerror(errno));
  22. 	} 
  23. 	return 0;
  24. }

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