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22 gru 2008 · Some functions return a valid value or an invalid value indicating an error requiring the additional step of fetching the error code by some means. For example the fopen() function returns either a pointer to a FILE object or it returns an invalid pointer value of NULL and sets errno to an error
2 lis 2023 · #define NULL 0: Using #define NULL 0 line in our program, we can solve the NULL undeclared error. Below code shows its implementation:
7 sie 2023 · A lot of C function calls return -1 or NULL or set an in case of an error code as the global variable errno, so quick tests on these values are easily done with an instance of ‘if statement’. What is errno?
You can handle errors based on return values of the function. In most of the cases functions return -1 or NULL in case of any errors. 1. By using errno. errno is set by system calls and some library functions in the event of an error to tell if something goes wrong. You need to include <errno.h> header to use the external variable "errno".
13 lut 2020 · NULL is not available by default: you need to include stdio.h to use it (or if you prefer, stddef.h: #include <stdio.h> int main ( void ) { int * p_some_variable = NULL ; } Otherwise the C compiler will give you an error:
4. Return NULL. If the function would normally return a pointer, then you can use NULL to indicate that something went wrong. Most functions that would be returning pointers will be doing heap allocation in order for that to be sound, so this scheme is likely not applicable when you want to avoid allocations.
To make use of errno you need to include errno.h and you need to call ‘extern int errno;’. Let us take a look at an example: FILE * fp; fp = fopen ("filedoesnotexist.txt", "rb"); if (fp == NULL) { fprintf(stderr, "Value of errno: %d\n", errno); } else { fclose (fp); return 0;
