I'm getting this error:
warning: incompatible implicit declaration of built-in function ‘malloc’
I am trying to do this:
fileinfo_list* tempList = malloc(sizeof(fileinfo_list));
Just for the reference the struct used at hand is:
typedef struct {
fileinfo** filedata;
size_t nFiles;
size_t size;
size_t fileblock;
} fileinfo_list;
I don't see anything wrong with what I've done. I'm just creating a tempList with the size of 1 x fileinfo_list.
You likely forgot to #include <stdlib.h>
You need to #include <stdlib.h>. Otherwise it's defined as int malloc() which is incompatible with the built-in type void *malloc(size_t).
You're missing #include <stdlib.h>.
The stdlib.h file contains the header information or prototype of the malloc, calloc, realloc and free functions.
So to avoid this warning in ANSI C, you should include the stdlib header file.
The only solution for such warnings is to include stdlib.h in the program.
Related
This question already has answers here:
Message "warning: implicit declaration of function"
(10 answers)
Closed 6 years ago.
I have this very simple code written in C. They are in two separate files.
myFunction.c
#include <stdlib.h>
int *extFunc() {
int *a = (int *) calloc( 1, sizeof(int) );
*a = 12;
return a;
}
main.c
#include <stdio.h>
#include <stdlib.h>
int main()
{
int *p = (int *) extFunc();
int x = *p; // causes segmentation fault !
printf("%d\n", *p); // causes segmentation fault !
}
I compiled it with this command cc myFunction.c main.c and the output is like below
main.c:6:19: warning: implicit declaration of function 'extFunc' is invalid in C99 [-Wimplicit-function-declaration]
int *p = (int *) extFunc();
^
main.c:6:11: warning: cast to 'int *' from smaller integer type 'int' [-Wint-to-pointer-cast]
int *p = (int *) extFunc();
^
2 warnings generated.
and when I run it, it gives Segmentation fault: 11. What am I doing wrong?
I checked the function when it is in the main.c file and it worked. However, I need them to be separate like above.
In my original code there are struct pointers instead of integer pointers. I explained my problem here with a simplified example.
You haven't provided a prototype for extFunc(). Create a header file with:
int *extFunc(void);
and include it in both source files.
Since the compiler can't see a prototype it assumed int as return type for extFunc() which probably truncates the address returned by calloc() on your system (probably 64bit system).
But there's no implicit int rule since C99. Always include prototypes for functions and pay attention to compiler warnings.
The warnings are very important here. Especially the one saying
cast to 'int *' from smaller integer type 'int'
What is happening is that the compiler makes an implicit declaration of the function. One that doesn't return int * but int. On certain platform (most notably 64-bit systems) those are not the same which means the pointer p in your main function is not correct. Dereferencing it will lead to undefined behavior.
You need to add a declaration of your function for it to work:
int *extFunc(void);
int main(void) { ... }
Also note that I have changed the argument type for both your extFunc and for main. This is because in C when you don't explicitly say that a function takes void as argument then it can take any number of unspecified arguments.
Compiling both files is not enough, you need to create a header:
/* myFunction.h */
int *extFunc();
And then, include it in your main file:
#include <stdio.h>
#include <stdlib.h>
#include "myFunction.h" /* here */
int main()
{
...
}
The main cannot see the declaration of the function extFunc, so it's return type default to int.
As types int and int* are not of the same size, the function returns an invalid pointer, which when dereferenced caused a segmentation fault.
You will need to create a header called myFunction.h, which will contain the declaration of the function:
int* extFunc( void );
Include this header in main and also change the definition of extFunc to take no arguments in myFunction.c.
In xcode 5 I get this warning:
"implicit declaration of function free is invalid in c99"
How should I free my c structures if I can't use the function free()?
You should include <stdlib.h>.
You get that warning because you're calling a function without first declaring it, so the compiler doesn't know about the function.
All functions need to be declared before being called, there are no "built-in" functions in C.
It's true that free() is a function defined in the standard, but it's still not built-in, you must have a prototype for it.
To figure out which header has the prototype, try searching for "man free" and look for a Linux manual page. Close to the top, it says:
#include <stdlib.h>
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
This tells you that in order to use the listed functions, you should add:
#include <stdlib.h>
to your source code.
I have a code segment which goes something like this
typedef struct node* node_t;
struct node{
int value;
node_t link;
};
......
......
......
//now I want to allocate memory somewhere else in the program.
node_t x;
x=(node_t) malloc(sizeof *x);
Could you please tell me if the above statement is proper? Eclipse shows this warning
warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
../tree.c:22:9:
warning: incompatible implicit declaration of built-in function ‘malloc’
Can someone explain in detail about situations like this? What is actually wrong with this. I would really appreciate if you could list all the possible ways in which I can allocate memory in this program? Thank you in advance..
In order to use malloc you will need to include <stdlib.h>. Additionally, allocate memory like
node_t x = malloc(sizeof node_t);
You must not assign memory to stack-variables, but only to pointers. And never cast back to the allocated type, as malloc returns void* and this need not be casted explicitly.
You need an
#include <stdlib.h>
before you use malloc. You apparently declared the function wrong in your source. Never declare C standard library functions. Just include the headers that declare them.
Could you please tell me if the above statement is proper?
No, it isn't: you're casting the return value of malloc() and you're missing the <stdlib.h> header.
You're getting the warning because you don't have a declaration for malloc in scope; you need to include the stdlib.h header file:
#include <stdlib.h>
You should also drop the cast. It isn't necessary, and under other compilers will suppress a diagnostic that you're trying to assign an int value to a pointer type.
Please do not hide pointers behind typedefs. It's almost always a bad idea.
I know alot of similar questions were asked before but i couldn't find something that would fix this warning i get:
MyIntFunctions.c:19:2: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
Occurs here:
void IntPrint (const void *key)
{
printf("%d", *(int*)key); // line 19
printf("\t-->\t");
}
and a similar warning:
MyStringFunctions.c:22:2: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
void StringPrint (const void *key)
{
printf("%s",(char*)key); //line 22
printf("\t-->\t");
}
I really want to understand what is wrong so i won't do that again in the future.
You need to include the appropriate header
#include <stdio.h>
If you're not sure which header a standard function is defined in, the function's man page will state this.
You need to include a declaration of the printf() function.
#include <stdio.h>
the warning or error of kind IMPLICIT DECLARATION is that the compiler is expecting a Function Declaration/Prototype..
It might either be a header file or your own function Declaration..
warning: incompatible implicit declaration of built-in function 'printf'
warning: incompatible implicit declaration of built-in function 'scanf'
the above warnings of compiler says that there is need to include declaration of printf and scanf i.e. include appropriate header
#include <stdio.h>
For this code:
int i=0; char **mainp;
for(i=0;i<2;++i)
{
mainp[i]=malloc(sizeof(char)*200);
if(!scanf("%[^#],#",mainp[i]))
break;
if(i<2)
scanf("%[^#],#",mainp[i]);
}
GCC emits the warnings:
warning: implicit declaration of function ‘scanf’
warning: incompatible implicit declaration of built-in function ‘scanf’
warning: ‘mainp’ may be used uninitialized in this function
And I get a segmentation fault at runtime
input:(P>Q),(Q>R),-R#-P
output:
(P>Q),(Q>R),-R
(empt slot)
i expected to give me
(P>Q),(Q>R),-R
-P //where should i fix in my code such that it gives me expected
//output
Problem #1:
warning: ‘mainp’ may be used uninitialized in this function
You need to allocate memory for the array of arrays first.
char **mainp = malloc(sizeof(char*)*2);
Problem #2:
warning: implicit declaration of function ‘scanf’
warning: incompatible implicit declaration of built-in function ‘scanf’
You need to include stdio.h at the top of your file:
#include <stdio.h>
Problem #3: (Not included in your compiling warnings)
Remember to free both the allocated array members and also the array of array address.
gcc expects this line at the beginning of your file:
#include <stdio.h>
and a declaration of mainp like this one:
char *mainp[2];
You shouldn't use functions without declaring them; you used scanf, but at no point in your code is scanf declared. Since it's a standard library function it's declared in one of the standard headers, stdio.h, so you just need to include it:
#include <stdio.h>
Brian's answer is good for the other part