when i'm tryin' to use malloc with string pointers to scan, it's givin' a segmentation fault
main(){ char *a; a = (char)malloc(50); printf("enter a string\t"); scanf("%s", a); printf("%s\n", a); } 
2 Answers
a = (char)malloc(50); Here, you meant to cast its type to char * instead of char.
Note that it's better not to cast the return type of malloc. See Do I cast the result of malloc?
1 Comment
pzaenger
Actually it is not better: you simply don't cast the result of
malloc. However, the OP should read the accepted answer in your given link.Your main problem, apart from your using scanf is that you're casting a pointer (which is size 4 or 8 on 32 or 64 bit systems respectively) to a char, which is guaranteed to be size 1 as per standard.
Do not ignore compiler warnings. You should've seen something along the lines of:
warning: cast from pointer to integer of different size [-Wpointer-to-int-cast] a = (char) malloc(10); warning: assignment makes pointer from integer without a cast [enabled by default] a = (char) malloc(50); Casting a pointer type to char, only to assign it to a pointer variable makes no sense. If you want to make it absolutely clear that you are allocating enough memory to accomodate N chars, then you could write:
a = malloc(50 * sizeof(char)); //or better still a = malloc(50 *sizeof *a);//50 times the size of whatever type a is pointing to... But a char is always size 1, like I said before. Anyway: I set about fixing the code you posted:
The following copy-pasted code works just fine, provided you don't enter more than 49 chars:
#include <stdio.h> #include <stdlib.h> int main( void ) { char *a = malloc(50); if (a == NULL) return EXIT_FAILURE;//<-- no memory available printf("enter a string\t"); scanf("%s", a); printf("%s\n", a); free(a);//<== be tidy, free your memory, though here, it's a bit redundant return 0; } But really, look into alternatives to scanf, and never cast the pointer returned by malloc in C. C++ is a different matter.
Also, check for NULL pointers, to be safe.
Lastly: Don't ignore compiler warnings.
copy-paste & test here
you can provide stdin input at the bottom right hand side of the page, it'll print the string you provided
answered Feb 7, 2014 at 16:45
Elias Van Ootegem
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main, 2. specify the argumentsmainexpects (ievoid) 3. Remove the cast! 4.return 0;should be the last statement inmain5. Consider using the stack, not the heap for something this trivialC. They have nothing to do with your problem.