How to find out which functions are present in which header file in C source code?

Question

I have the following code:

#include<stdio.h>
#include<stdlib.h>
int main()
{
     printf("Checking\n");
     exit(0);
}

Before source code, I have two functions one is printf() that is present in stdio.h header file. Second one is exit() function that is present in stdlib.h header file.

Now I want the output for this program is:

printf() function is present in stdio.h

exit() function is present in stdlib.h

Is there any way to find out this?

If you mean a magic function that do the job: NO there isn't. — LPs
You simply learn what header files are needed for some of these functions. And if you can't learn there's always references, and as a last resort you turn to your favorite search engine. — Some programmer dude
You you want some info by gcc you can use -aux-info option to print to file those infos. E.g. gcc test.c -aux-info=infos.txt -o test — LPs
There is no magic in header files. You can make your own containing just the prototypes for printf and exit, include that, and your program wil compile as usual. — Jongware

paxdiablo paxdiablo · Accepted Answer · 2017-01-13T08:24:03

There's really no portable way to do this since the header text doesn't even need to exist in a file at all! It's quite acceptable under the standard for the #include <system header file> to simply modify the environment without reference to a specific actual header file so, in that case, you don't have easy access to the information from a program.

In terms of finding out if the header text is available, this could range from a simple text search (likely to have false positives since there may be a comment about printf in the math.h header file) to a full blown C-aware parser (likely to be complicated).

Alternatively, you could just (manually) reference the actual standard since those details are available there, a la:

7.21.4.1 The remove function
    Synopsis
        #include <stdio.h>
        int remove(const char *filename);

This little snippet from C11 means that the remove function can be found in stdio.h.

And, if you still want a way to do this within a program, simply gather all that knowledge from the standard into a file or data structure (like an associative array) and write code to look up a given identifier, along the lines of:

#include <string.h>
#include <assert.h>
const char *lookFor (const char *ident) {
    // These should have ALL identifiers and their
    //   equivalent headers.

    static const char const *identifier[] = { "remove",  "strcpy",   ... };
    static const char const *header[] =     { "stdio.h", "string.h", ... };

    assert (sizeof(identifier) == sizeof(header));
    for (size_t i = 0; i < sizeof(header) / sizeof(*header); i++)
        if (strcmp (ident, identifier[i]) == 0)
            return header[i];

    return NULL;
}

How to find out which functions are present in which header file in C source code?

1 Answers