Warning(s):

source_file.c:4:1: warning: data definition has no type or storage class
 i;f(l){l=sizeof(a)/sizeof(int);char t;for(;i<l;i+=2){t=s[a[i]];s[a[i]]=s[a[i+1]];s[a[i+1]]=t;}puts(s);}
 ^
source_file.c:4:1: warning: type defaults to ‘int’ in declaration of ‘i’ [-Wimplicit-int]
source_file.c:4:3: warning: return type defaults to ‘int’ [-Wreturn-type]
 i;f(l){l=sizeof(a)/sizeof(int);char t;for(;i<l;i+=2){t=s[a[i]];s[a[i]]=s[a[i+1]];s[a[i+1]]=t;}puts(s);}
   ^
source_file.c: In function ‘f’:
source_file.c:4:3: warning: type of ‘l’ defaults to ‘int’ [-Wmissing-parameter-type]
source_file.c:4:1: warning: implicit declaration of function ‘puts’ [-Wimplicit-function-declaration]
 i;f(l){l=sizeof(a)/sizeof(int);char t;for(;i<l;i+=2){t=s[a[i]];s[a[i]]=s[a[i+1]];s[a[i+1]]=t;}puts(s);}
 ^
source_file.c:4:1: warning: control reaches end of non-void function [-Wreturn-type]
 i;f(l){l=sizeof(a)/sizeof(int);char t;for(;i<l;i+=2){t=s[a[i]];s[a[i]]=s[a[i+1]];s[a[i+1]]=t;}puts(s);}
 ^