似乎您是说以下意思。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct extension
{
    char *ext;
    char *filetype;
};

char * getExtensions( const struct extension a[] )
{
    size_t n = 0;

    for ( size_t i = 0; a[i].ext != NULL; i++ )
    {
        n += strlen( a[i].ext ) + 1;
    }

    char *s = malloc( n + 1 );

    if ( s != NULL )
    {
        s[0] = '\0';

        for ( size_t i = 0; a[i].ext != NULL; i++ )
        {
            strcat( s,a[i].ext );
            strcat( s,"\n" );
        }
    }

    return s;
}

int main(void) 
{
    struct extension extensions [] = 
    {
        {"gif","image/gif" },{"jpg","image/jpg" },{"jpeg","image/jpeg"},{"png","image/png" },{ NULL,NULL }     
    };

    char *s = getExtensions( extensions ); 

    printf( "The following extensions are supported:\n%s",s );

    free( s );

    return 0;
}

程序输出为

The following extensions are supported:
gif
jpg
jpeg
png

#include <stdio.h>
#include <string.h>

// Your struct
struct extensionInfo {
   char *ext;
   char *filetype;
};

struct extensionInfo extensions [] = {
   {"gif",{0,0}
};

int main(int argc,char **args,char **env) {
   char buffer[1024];
   struct extensionInfo *ext;

   // Initialize the buffer
   memset(buffer,sizeof(buffer));

   // Insert your first text.
   strncat(buffer,"The following extensions are supported:",sizeof(buffer) - 1);

   // Loop through your array and append everything
   for (ext = extensions; ext->ext != 0; ext++) {
      strncat(buffer,"\n",sizeof(buffer) - 1);
      strncat(buffer,ext->ext,sizeof(buffer) - 1);
   }

   // Show your result
   printf("%s\n",buffer);

   return 0;
}

这是一个有效的注释示例。如有任何疑问，请随时提问。

我认为以下是您想要的。您可以阅读评论以了解其工作原理。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct {
    char *ext;
    char *filetype;
} extensions [] = {
        {"gif",0}
};

char *getExtensions(void) {
    size_t count = 0;
    // let's know how many characters are needed for all extensions
    for(int i = 0; extensions[i].ext != 0; ++i) {
        size_t j = 0;
        for (; extensions[i].ext[j] != 0; ++j) ;
        count += j + 1; // +1 for every newline
    }

    // +1 for null terminator
    char *str = calloc(count + 1,sizeof(char));

    // let's concatenate
    for(size_t i = 0; extensions[i].ext != 0; ++i) {
        strcat(str,extensions[i].ext);
        strcat(str,"\n");
    }

    return str;
}

int main(int argc,char **argv){
    char * const result = getExtensions();
    printf("\nThe following extensions are supported: \n%s",result);
    free(result);
}