Fopen Error Handling In C
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a minute: Sign up Error handling in file opening up vote 2 down vote favorite [Question 1] When I open a file into a function, generally I do something like this: int read_file (char *filename) { FILE *fin; if ( !(fin = fopen(filename, "r")) ) return 1; /* ... */ return fclose(fin); } int main () { char filename[100]; if ( exception handling in c sharp read_file(filename) ) { perror(filename); exit(1); } return 0; } Generally 0 return value is for errors (right?) then I can change the previous code into: int read_file (char *filename) { FILE *fin; if ( !(fin = fopen(filename, "r")) ) return 0; /* ... */ return !fclose(fin); } int main () { char filename[100]; if ( !read_file(filename) ) { perror(filename); exit(1); } return 0; } But I think that the first code is more clean. Another option is only change return 1; into return -1; (in the first code that I wrote). What's the best version? [Question 2] If I must handle more errors, is it correct a code like this? int read_file (char *filename, int **vet) { FILE *fin; if ( !(fin = fopen(filename, "r")) ) { perror(filename); return 1; } * vet = malloc (10 * sizeof(int)); if ( *vet == NULL ) { perror("Memory allocation error.\n"); return 1; } /* ... */ return fclose(fin); } int main () { char filename[100]; int *vet; if ( read_file(filename, &vet) ) exit(1); return 0; } c file error-handling fopen fclose share|improve th
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Business Learn more about hiring developers or posting ads with us Stack Overflow Questions c programming error codes Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, c stderr just like you, helping each other. Join them; it only takes a minute: Sign up Error handling in file opening up vote 2 down vote favorite [Question 1] When I open a file into a function, http://stackoverflow.com/questions/21267716/error-handling-in-file-opening generally I do something like this: int read_file (char *filename) { FILE *fin; if ( !(fin = fopen(filename, "r")) ) return 1; /* ... */ return fclose(fin); } int main () { char filename[100]; if ( read_file(filename) ) { perror(filename); exit(1); } return 0; } Generally 0 return value is for errors (right?) then I can change the previous code into: int read_file (char *filename) { FILE *fin; if ( !(fin = fopen(filename, "r")) http://stackoverflow.com/questions/21267716/error-handling-in-file-opening ) return 0; /* ... */ return !fclose(fin); } int main () { char filename[100]; if ( !read_file(filename) ) { perror(filename); exit(1); } return 0; } But I think that the first code is more clean. Another option is only change return 1; into return -1; (in the first code that I wrote). What's the best version? [Question 2] If I must handle more errors, is it correct a code like this? int read_file (char *filename, int **vet) { FILE *fin; if ( !(fin = fopen(filename, "r")) ) { perror(filename); return 1; } * vet = malloc (10 * sizeof(int)); if ( *vet == NULL ) { perror("Memory allocation error.\n"); return 1; } /* ... */ return fclose(fin); } int main () { char filename[100]; int *vet; if ( read_file(filename, &vet) ) exit(1); return 0; } c file error-handling fopen fclose share|improve this question asked Jan 21 '14 at 19:52 ᴜsᴇʀ 506314 will not (fin = fopen(filename, "r") be always true? –Dipto Jan 21 '14 at 19:59 2 In general we use 0 as the default error return as 0 refers to false and non null values refers to true. But when different errors needs to be handled we use negative values for errors. In your second question it's better to return different values for different e
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business Learn http://stackoverflow.com/questions/15753090/c-fopen-fails-for-write-with-errno-is-2 more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, http://www.cplusplus.com/reference/cstdio/fopen/ helping each other. Join them; it only takes a minute: Sign up C fopen fails for write with errno is 2 up vote 2 down vote favorite 1 I do not understand why this is seemingly failing with error handling errno of 2: char debugText [256]; sprintf (debugText, "C:\\List.txt"); dfile = fopen( debugText, "w"); fprintf ( dfile, " err %d \n", errno); I say seemingly because while dfile is NULL the file gets created and is filled with my output. so what is going on ? c fopen errno share|improve this question asked Apr 1 '13 at 22:32 JPM 1101112 What type is dfile? Where's the test for NULL? –David Schwartz Apr 1 '13 handling in c at 22:35 1 take a look at strerror() which return a string that explains errno for you. –Zaffy Apr 1 '13 at 22:53 (For the googlers)For me it was that the file had no write permissions. On Windows: Right click > Properties > Security > Edit. Then allow modify for all users –chuckleplant Jan 14 at 7:58 add a comment| 3 Answers 3 active oldest votes up vote 6 down vote All this tells you is that errno had the value 2 after your fopen call. You don't know that the call failed, because you didn't check whether dfile == NULL. If the output was actually written to the file, presumably the fopen call succeeded and the errno value was left over from some previous call, likely one you didn't make explicitly. Failing calls can set errno to some non-zero value, but successful calls don't set errno to 0. To check for errors, you need to Set errno to 0 before the call; Make the call and check the value it returned to see whether it succeeded or failed; and Check the value of errno after the call -- but only if you know it failed (otherwise the value of errno is meaningless). If defile == NULL, then the fprintf call has undefined behavior; it will probably fail. On the other hand,
identified in future operations by the FILE pointer returned. The operations that are allowed on the stream and how these are performed are defined by the mode parameter. The returned stream is fully buffered by default if it is known to not refer to an interactive device (see setbuf). The returned pointer can be disassociated from the file by calling fclose or freopen. All opened files are automatically closed on normal program termination. The running environment supports at least FOPEN_MAX files open simultaneously. Parameters filename C string containing the name of the file to be opened. Its value shall follow the file name specifications of the running environment and can include a path (if supported by the system). mode C string containing a file access mode. It can be: "r"read: Open file for input operations. The file must exist. "w"write: Create an empty file for output operations. If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file. "a"append: Open file for output at the end of a file. Output operations always write data at the end of the file, expanding it. Repositioning operations (fseek, fsetpos, rewind) are ignored. The file is created if it does not exist. "r+"read/update: Open a file for update (both for input and output). The file must exist. "w+"write/update: Create an empty file and open it for update (both for input and output). If a file with the same name already exists its contents are discarded and the file is treated as a new empty file. "a+"append/update: Open a file for update (both for input and output) with all output operations writing data at the end of the file. Repositioning operations (fseek, fsetpos, rewind) affects the next input operations, but output operations move the position back to the end of file. The file is created if it does not exist. With the mode specifiers above the file is open as a text file. In order to open a file as a binary file, a "b" character has to be included in the mode string. This additional "b" character can either be appended at the end of the string (thus making the following compound modes: "rb", "wb", "ab", "r+b", "w+b", "a+b") or be inserted between the letter and the "+" sign for the mixed modes ("rb+", "wb+", "ab+"). The new C standard (C2011, which is not part of C++) adds a new standard subspecifier ("x"), that can be appended to any "w" specifier (to form "wx", "